![Desoldering](https://upload.wikimedia.org/wikipedia/commons/8/8f/Desoldering.jpg)
Soldering (; ) is a process in which two or more items are joined by melting and putting a
filler metal A filler metal is a metal added in the making of a joint through welding, brazing, or soldering.
Soldering
Soldering and brazing processes rely on a filler metal added to the joint to form the junction between the base metal parts. Soft solderin ...
(
solder
Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...
) into the joint, the filler metal having a lower
melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...
than the adjoining metal. Unlike
welding
Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...
, soldering does not involve melting the work pieces. In
brazing
Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.
Brazing differs from we ...
, the work piece metal also does not melt, but the filler metal is one that melts at a higher temperature than in soldering. In the past, nearly all solders contained
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
, but environmental and health concerns have increasingly dictated use of
lead-free alloys for electronics and plumbing purposes.
Origins
![Solder sugarlabs](https://upload.wikimedia.org/wikipedia/commons/0/0a/Solder_sugarlabs.jpg)
There is evidence that soldering was employed as early as 5,000 years ago in Mesopotamia. Soldering and
brazing
Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.
Brazing differs from we ...
are thought to have originated very early in the history of metal-working, probably before 4000 BC. Sumerian swords from were assembled using hard soldering.
Soldering was historically used to make jewelry, cookware and cooking tools, assembling
stained glass
Stained glass is coloured glass as a material or works created from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant religious buildings. Although tradition ...
, as well as other uses.
Applications
Soldering is used in plumbing, electronics, and metalwork from
flashing to jewelry and musical instruments.
Soldering provides reasonably permanent but reversible connections between copper pipes in
plumbing
Plumbing is any system that conveys fluids for a wide range of applications. Plumbing uses pipes, valves, plumbing fixtures, tanks, and other apparatuses to convey fluids. Heating and cooling (HVAC), waste removal, and potable water delivery ...
systems as well as joints in sheet metal objects such as food cans,
roof flashing,
rain gutter
A rain gutter, eavestrough, eaves-shoot or surface water collection channel is a component of a water discharge system for a building. It is necessary to prevent water dripping or flowing off roofs in an uncontrolled manner for several reasons ...
s and automobile
radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics.
A radiator is always a ...
s.
Jewelry
Jewellery ( UK) or jewelry (U.S.) consists of decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a western ...
components, machine tools and some refrigeration and plumbing components are often assembled and repaired by the higher temperature silver soldering process. Small mechanical parts are often soldered or brazed as well. Soldering is also used to join lead
came
A came is a divider bar used between small pieces of glass to make a larger glazing panel.
There are two kinds of came: the H-shaped sections that hold two pieces together and the U-shaped sections that are used for the borders. Cames are mostl ...
and
copper foil
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-o ...
in
stained glass
Stained glass is coloured glass as a material or works created from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant religious buildings. Although tradition ...
work.
Electronic soldering connects
electrical wiring
Electrical wiring is an electrical installation of cabling and associated devices such as switches, distribution boards, sockets, and light fittings in a structure.
Wiring is subject to safety standards for design and installation. Allowable ...
to devices, and
electronic component
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not ...
s to
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
s. Electronic connections may be hand-soldered with a soldering iron. Automated methods such as
wave soldering
Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the ci ...
or use of ovens can make many joints on a complex circuit board in one operation, vastly reducing production cost of electronic devices.
Musical instruments, especially brass and woodwind instruments, use a combination of soldering and brazing in their assembly. Brass bodies are often soldered together, while keywork and braces are most often brazed.
Solderability
The solderability of a substrate is a measure of the ease with which a soldered joint can be made to that material.
Some metals are easier to solder than others. Copper, zinc, brass, silver and gold are easy. Iron,
mild steel
Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:
* no minimum content is specified or required for chromium, cobalt ...
and
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
are next in difficulty. Because of their thin, strong oxide films,
stainless steel
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...
and some aluminium alloys are even more difficult to solder.
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
,
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
,
cast iron
Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
s, some high-carbon
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
s,
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s, and
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
can be soldered but it involves a process similar to joining carbides: they are first plated with a suitable metallic element that induces interfacial bonding.
Solders
Soldering filler materials are available in many different
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s for differing applications. In electronics assembly, the
eutectic alloy with 63% tin and 37% lead (or 60/40, which is almost identical in melting point) has been the alloy of choice. Other alloys are used for plumbing, mechanical assembly, and other applications. Some examples of soft-solder are tin-lead for general purposes, tin-zinc for joining
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, lead-silver for strength at higher than room temperature, cadmium-silver for strength at high temperatures, zinc-aluminium for aluminium and corrosion resistance, and tin-silver and tin-bismuth for electronics.
A eutectic formulation has advantages when applied to soldering: the
liquidus and
solidus
Solidus (Latin for "solid") may refer to:
* Solidus (coin), a Roman coin of nearly solid gold
* Solidus (punctuation), or slash, a punctuation mark
* Solidus (chemistry), the line on a phase diagram below which a substance is completely solid
* S ...
temperatures are the same, so there is no plastic phase, and it has the lowest possible melting point. Having the lowest possible melting point minimizes heat stress on electronic components during soldering. And, having no plastic phase allows for quicker wetting as the solder heats up, and quicker setup as the solder cools. A non-eutectic formulation must remain still as the temperature drops through the liquidus and solidus temperatures. Any movement during the plastic phase may result in cracks, resulting in an unreliable joint.
Common
solder formulations based on tin and lead are listed below. The fraction represent percentage of tin first, then lead, totaling 100%:
* 63/37: melts at (eutectic: the only mixture that melts at a ''point'', instead of over a range)
* 60/40: melts between
* 50/50: melts between
For environmental reasons and the introduction of regulations such as the European RoHS (
Restriction of Hazardous Substances Directive
The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Unio ...
), lead-free solders are becoming more widely used. They are also suggested anywhere young children may come into contact with (since young children are likely to place things into their mouths), or for outdoor use where rain and other precipitation may wash the lead into the groundwater. Unfortunately, common lead-free solders are not eutectic formulations, melting at around , making it more difficult to create reliable joints with them.
Other common solders include low-temperature formulations (often containing
bismuth
Bismuth is a chemical element with the Symbol (chemistry), symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental ...
), which are often used to join previously soldered assemblies without unsoldering earlier connections, and high-temperature formulations (usually containing
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
) which are used for high-temperature operation or for first assembly of items which must not become unsoldered during subsequent operations. Alloying silver with other metals changes the melting point, adhesion and wetting characteristics, and tensile strength. Of all the brazing alloys, silver solders have the greatest strength and the broadest applications. Specialty alloys are available with properties such as higher strength, the ability to solder aluminum, better electrical conductivity, and higher corrosion resistance.
Flux
The purpose of
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
is to facilitate the soldering process. One of the obstacles to a successful solder joint is an impurity at the site of the joint; for example, dirt, oil or
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
. The impurities can be removed by mechanical cleaning or by chemical means, but the elevated temperatures required to melt the filler metal (the solder) encourages the work piece (and the solder) to re-oxidize. This effect is accelerated as the soldering temperatures increase and can completely prevent the solder from joining to the workpiece. One of the earliest forms of flux was
charcoal
Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, cal ...
, which acts as a
reducing agent
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are commonly reducing agents include the Earth meta ...
and helps prevent oxidation during the soldering process. Some fluxes go beyond the simple prevention of oxidation and also provide some form of chemical cleaning (corrosion). Many fluxes also act as a
wetting agent
Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming ...
in the soldering process, reducing the
surface tension
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to f ...
of the molten solder and causing it to flow and wet the workpieces more easily.
For many years, the most common type of flux used in electronics (soft soldering) was
rosin
Rosin (), also called colophony or Greek pitch ( la, links=no, pix graeca), is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene comp ...
-based, using the rosin from selected
pine tree
A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accepts ...
s. It was nearly ideal in that it was non-corrosive and non-conductive at normal temperatures but became mildly reactive (corrosive) at elevated soldering temperatures. Plumbing and automotive applications, among others, typically use an acid-based (
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
) flux which provides rather aggressive cleaning of the joint. These fluxes cannot be used in electronics because their residues are conductive leading to unintended electrical connections, and because they will eventually dissolve small diameter wires.
Citric acid
Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...
is an excellent water-soluble acid-type flux for copper and electronics but must be washed off afterwards.
Fluxes for soft solder are currently available in three basic formulations:
* Water-soluble fluxes – higher activity fluxes which can be removed with water after soldering (no
VOC
VOC, VoC or voc may refer to:
Science and technology
* Open-circuit voltage (VOC), the voltage between two terminals when there is no external load connected
* Variant of concern, a category used during the assessment of a new variant of a virus
...
s required for removal).
* No-clean fluxes – mild enough to not "require" removal due to their non-conductive and non-corrosive residues.
These fluxes are called "no-clean" because the residue left after the solder operation is non-conductive and won't cause electrical shorts; nevertheless they leave a plainly visible white residue. No-clean flux residue is acceptable on all 3 classes of PCBs as defined by IPC-610 provided it does not inhibit visual inspection, access to test points, or have a wet, tacky or excessive residue that may spread onto other areas. Connector mating surfaces must also be free of flux residue. Fingerprints in no-clean residue are a class 3 defect
* Traditional
rosin
Rosin (), also called colophony or Greek pitch ( la, links=no, pix graeca), is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene comp ...
fluxes – available in non-activated (R), mildly activated (RMA) and activated (RA) formulations. RA and RMA fluxes contain rosin combined with an activating agent, typically an acid, which increases the wettability of metals to which it is applied by removing existing oxides. The residue resulting from the use of RA flux is
corrosive
A corrosive substance is one that will damage or destroy other substances with which it comes into contact by means of a chemical reaction.
Etymology
The word ''corrosive'' is derived from the Latin verb ''corrodere'', which means ''to gnaw'', ...
and must be cleaned. RMA flux is formulated to result in a residue which is less corrosive, so that cleaning becomes optional, though usually preferred. R flux is still less active and even less corrosive.
Flux performance must be carefully evaluated for best results; a very mild 'no-clean' flux might be perfectly acceptable for production equipment, but not give adequate performance for more variable hand-soldering operations.
Heating methods
Different types of soldering tools are made for specific applications. The required heat can be generated from burning fuel or from an electrically operated heating element. Another method for soldering is to place solder at the locations of joints in the object to be soldered, then heat the entire object in an oven to melt the solder.
The electric
soldering iron
A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip (the ''bit'') and an insulated handle. Heating i ...
is widely used for hand-soldering. It can be fitted with a variety of tips, ranging from blunt, to very fine, to chisel heads for hot-cutting plastics rather than soldering. The simplest irons do not have temperature regulation. Small irons rapidly cool when used to solder to, say, a metal chassis, while large irons have tips too cumbersome for working on
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
s (PCBs) and similar fine work. A 25-watt iron will not provide enough heat for large electrical connectors, joining copper roof flashing, or large stained-glass lead came. On the other hand, a 100-watt iron may provide too much heat for PCBs. Temperature-controlled irons have a reserve of power and can maintain temperature over a wide range of work.
The
soldering gun
A soldering gun is an approximately pistol-shaped, electrically powered tool for soldering metals using tin-based solder to achieve a strong mechanical bond with good electrical contact. The tool has a trigger-style switch so it can be easily ope ...
heats faster but has a larger and heavier body. Gas-powered irons using a catalytic tip to heat a bit, without flame, are used for portable applications. Hot-air guns and pencils allow
rework of component packages which cannot easily be performed with electric irons and guns.
For non-electronic applications, soldering torches use a flame rather than a soldering tip to heat solder. Soldering torches are often powered by butane and are available in sizes ranging from very small butane/oxygen units suitable for very fine but high-temperature jewelry work, to full-size oxy-fuel torches suitable for much larger work such as copper piping. Common multipurpose propane torches, the same kind used for heat-stripping paint and thawing pipes, can be used for soldering pipes and other fairly large objects either with or without a soldering tip attachment; pipes are generally soldered with a torch by directly applying the open flame.
A soldering copper is a tool with a large copper head and a long handle which is heated in a
blacksmith
A blacksmith is a metalsmith who creates objects primarily from wrought iron or steel, but sometimes from #Other metals, other metals, by forging the metal, using tools to hammer, bend, and cut (cf. tinsmith). Blacksmiths produce objects such ...
's forge fire and used to apply heat to
sheet metal
Sheet metal is metal formed into thin, flat pieces, usually by an industrial process. Sheet metal is one of the fundamental forms used in metalworking, and it can be cut and bent into a variety of shapes.
Thicknesses can vary significantly; ex ...
for soldering. Typical soldering coppers have heads weighing between one and four pounds. The head provides a large
thermal mass
In building design, thermal mass is a property of the mass of a building that enables it to store heat and provide inertia against temperature fluctuations. It is sometimes known as the thermal flywheel effect. The thermal mass of heavy structura ...
to store enough heat for soldering large areas before needing re-heating in the fire; the larger the head, the longer the working time. Historically, soldering coppers were standard tools used in auto bodywork, although
body solder has been mostly superseded by
spot welding
A spot welder
Spot welding (or resistance spot welding) is a type of electric resistance welding used to weld various sheet metal products, through a process in which contacting metal surface points are joined by the heat obtained from resistance ...
for mechanical connection, and non-metallic fillers for contouring.
Toaster ovens and hand-held infrared lights have been used by hobbyists to replicate production soldering processes on a much smaller scale.
During WW2 and for some time afterwards
SOE forces used small pyrotechnic self-soldering joints to make connections for the remote detonation of demolition and sabotage explosives. These consisted of a small copper tube partially filled with solder and a slow-burning pyrotechnic composition wrapped around the tube. The wires to be joined would be inserted into the tube and a small blob of ignition compound allowed the device to be struck like a match to ignite the pyrotechnic and heat the tube for long enough to melt the solder and make the joint.
Laser soldering
''Laser soldering'' is a technique where a 30–50
W laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
is used to melt and solder an electrical connection joint.
Diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
laser systems based on semiconductor junctions are used for this purpose.
Suzanne Jenniches patented laser soldering in 1980.
Wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s are typically 808 nm through 980 nm. The beam is delivered via an optical fiber to the workpiece, with fiber diameters 800 µm and smaller. Since the beam out of the end of the fiber diverges rapidly,
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
es are used to create a suitable spot size on the workpiece at a suitable working distance. A wire feeder is used to supply solder.
Both lead-tin and silver-tin material can be soldered. Process recipes will differ depending on the
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
composition. For soldering 44-pin chip carriers to a board using soldering preforms, power levels were on the order of 10 watts and solder times approximately 1 second. Low power levels can lead to incomplete wetting and the formation of voids, both of which can weaken the joint.
Induction soldering
Induction soldering uses
induction heating
Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...
by high-frequency
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
in a surrounding copper coil. This induces currents in the part being soldered, which generates heat because of the higher resistance of a joint versus its surrounding metal (
resistive heating
Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current through a conductor (material), conductor produces heat.
Joule's first law (also just Joule's law), also known in c ...
). These copper coils can be shaped to fit the joint more precisely. A filler metal (solder) is placed between the facing surfaces, and this solder melts at a fairly low temperature. Fluxes are commonly used in induction soldering. This technique is particularly suited to continuously soldering, in which case these coils wrap around a cylinder or a pipe that needs to be soldered.
Fiber focus infrared soldering
Fiber focus infrared soldering is technique where many
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
sources are led through
fibers
Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
, then focused onto a single spot at which the connection is soldered.
[ 070927 vincenc.com.tw (mentioned as a technique)]
Resistance soldering
Resistance soldering is soldering in which the heat required to flow the solder is created by passing an electric current through the solder. When current is conducted through a resistive material a certain level of heat is generated. By regulating the amount of current conducted and the level of resistance encountered, the amount of heat produced can be predetermined and controlled.
Electrical resistance (usually described as a material's opposition to the flow of an electric current) is used to convert electric energy into thermal energy as an electric current (''I'') conducted through a material with resistance (''R'') releases power (''P'') equal to ''P'' = ''I'R'', where ''P'' is the power measured in watts, ''I'' is the current measured in amperes and ''R'' is the resistance measured in ohms.
Resistance soldering is unlike using a conduction iron, where heat is produced within an element and then passed through a thermally conductive tip into the joint area. A cold soldering iron requires time to reach working temperature and must be kept hot between solder joints. Thermal transfer may be inhibited if the tip is not kept properly wetted during use. With resistance soldering an intense heat can be rapidly developed directly within the joint area and in a tightly controlled manner. This allows a faster ramp up to the required solder melt temperature and minimizes thermal travel away from the solder joint, which helps to minimize the potential for thermal damage to materials or components in the surrounding area. Heat is only produced while each joint is being made, making resistance soldering more energy efficient. Resistance soldering equipment, unlike conduction irons, can be used for difficult soldering and brazing applications where significantly higher temperatures may be required. This makes resistance comparable to flame soldering in some situations. When the required temperature can be achieved by either flame or resistance methods the resistance heat is more localized because of direct contact, whereas the flame will spread thus heating a potentially larger area.
Active soldering
Flux-less soldering with aid of conventional
soldering iron
A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip (the ''bit'') and an insulated handle. Heating i ...
,
ultrasonic soldering iron or specialized solder pot and active solder that contains an active element, most often
titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
,
zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
or
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
.
The active elements, owing to mechanical activation, react with the surface of the materials generally considered difficult to solder without premetallization. The active solders can be protected against excessive oxidation of their active element by addition of
rare-earth elements
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
with higher affinity to oxygen (typically
cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 o ...
or
lanthanum
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...
). Another common additive is
gallium
Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
– usually introduced as a wetting promoter. Mechanical activation, needed for active soldering, can be performed by brushing (for example with use of stainless wire brush or steel spatula) or ultrasonic vibration (20–60 kHz). Active soldering has been shown to effectively bond ceramics,
aluminium, titanium, silicon, graphite and carbon nanotube based structures at temperatures lower than 450 °C or use of protective atmosphere.
Soldering vs. brazing
![Brazing and soldering processes - AWS A3](https://upload.wikimedia.org/wikipedia/commons/7/77/Brazing_and_soldering_processes_-_AWS_A3.0_2001.svg)
There are three forms of soldering, each requiring progressively higher temperatures and producing an increasingly stronger joint strength:
# soft soldering, which originally used a tin-lead
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
as the filler metal
# silver soldering, which uses an alloy containing
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
#
brazing
Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.
Brazing differs from we ...
which uses a
brass
Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other with ...
alloy for the filler
The alloy of the filler metal for each type of soldering can be adjusted to modify the melting temperature of the filler. Soldering differs from gluing significantly in that the filler metals directly bond with the surfaces of the workpieces at the junction to form a bond that is both electrically conductive and gas- and liquid-tight.
Soft soldering is characterized by having a melting point of the filler metal below approximately , whereas silver soldering and brazing use higher temperatures, typically requiring a flame or carbon arc torch to achieve the melting of the filler. Soft solder filler metals are typically alloys (often containing
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
) that have
liquidus temperatures below .
In this soldering process, heat is applied to the parts to be joined, causing the solder to melt and to bond to the workpieces in a surface alloying process called
wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
. In stranded wire, the solder is drawn up into the wire between the strands by
capillary action
Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
in a process called 'wicking'. Capillary action also takes place when the workpieces are very close together or touching. The joint's
tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
is dependent on the filler metal used; in electrical soldering little tensile strength comes from the added solder which is why it is advised that wires be twisted or folded together before soldering to provide some mechanical strength for a joint. A good solder joint produces an electrically conductive, water- and gas-tight join.
Each type of solder offers advantages and disadvantages. Soft solder is so called because of the soft lead that is its primary ingredient. Soft soldering uses the lowest temperatures (and so thermally stresses components the least) but does not make a strong joint and is unsuitable for mechanical load-bearing applications. It is also unsuitable for high-temperature applications as it loses strength, and eventually melts. Silver soldering, as used by jewelers, machinists and in some plumbing applications, requires the use of a torch or other high-temperature source, and is much stronger than soft soldering. Brazing provides the strongest of the non-welded joints but also requires the hottest temperatures to melt the filler metal, requiring a torch or other high temperature source and darkened goggles to protect the eyes from the bright light produced by the white-hot work. It is often used to repair cast-iron objects, wrought-iron furniture, etc.
Soldering operations can be performed with hand tools, one joint at a time, or ''en masse'' on a production line. Hand soldering is typically performed with a
soldering iron
A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip (the ''bit'') and an insulated handle. Heating i ...
,
soldering gun
A soldering gun is an approximately pistol-shaped, electrically powered tool for soldering metals using tin-based solder to achieve a strong mechanical bond with good electrical contact. The tool has a trigger-style switch so it can be easily ope ...
, or a torch, or occasionally a
hot-air pencil. Sheetmetal work was traditionally done with "soldering coppers" directly heated by a flame, with sufficient stored heat in the mass of the soldering copper to complete a joint; gas torches (e.g. butane or propane) or electrically heated soldering irons are more convenient. All soldered joints require the same elements of cleaning of the metal parts to be joined, fitting up the joint, heating the parts, applying flux, applying the filler, removing heat and holding the assembly still until the filler metal has completely solidified. Depending on the nature of flux material used and the application, cleaning of the joint may be required after it has cooled.
Each solder alloy has characteristics that work best for certain applications, notably strength and conductivity, and each type of solder and alloy has different melting temperatures. The term ''silver solder'' denotes the type of solder that is used. Some soft solders are "silver-bearing" alloys used to solder silver-plated items. Lead-based solders should not be used on precious metals because the lead dissolves the metal and disfigures it.
Soldering and brazing
The distinction between soldering and brazing is based on the melting temperature of the filler alloy. A temperature of 450 °C is usually used as a practical demarcation between soldering and brazing. Soft soldering can be done with a heated iron whereas the other methods typically require a higher temperature torch or a furnace to melt the filler metal.
Different equipment is usually required since a
soldering iron
A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip (the ''bit'') and an insulated handle. Heating i ...
cannot achieve high enough temperatures for hard soldering or brazing. Brazing filler metal is stronger than silver solder, which is stronger than lead-based soft solder. Brazing solders are formulated primarily for strength, silver solder is used by jewelers to protect the precious metal and by machinists and refrigeration technicians for its tensile strength but lower melting temperature than brazing, and the primary benefit of soft solder is the low temperature used (to prevent heat damage to electronic components and insulation).
Since the joint is produced using a metal with a lower melting temperature than the workpiece, the joint will weaken as the ambient temperature approaches the melting point of the filler metal. For that reason, the higher temperature processes produce joints which are effective at higher temperatures. Brazed connections can be as strong or nearly as strong as the parts they connect, even at elevated temperatures.
[Properties of gold-nickel alloy brazed joints in high temperature materials]
in ''Gold Bulletin'', June 1974, Volume 7, Issue 2, pp 42–49; by Jakob Colbus and Karl Friedrich Zimmermann; https://doi.org/10.1007/BF03215037
Silver soldering
"Hard soldering" or "silver soldering" is used to join precious and semi-precious metals such as gold, silver, brass, and copper. The solder is usually described as easy, medium, or hard in reference to its melting temperature, not the strength of the joint. Extra-easy solder contains 56% silver and has a melting point of . Extra-hard solder has 80% silver and melts at . If multiple joints are needed, then the jeweler will start with hard or extra-hard solder and switch to lower-temperature solders for later joints.
Silver solder is somewhat absorbed by the surrounding metal, resulting in a joint that is actually stronger than the metal being joined. The metal being joined must be perfectly flush, as silver solder cannot normally be used as a filler and will not fill gaps.
Another difference between brazing and soldering is how the solder is applied. In brazing, one generally uses rods that are touched to the joint while being heated. With silver soldering, small pieces of solder wire are placed onto the metal prior to heating. A flux, often made of boric acid and denatured alcohol, is used to keep the metal and solder clean and to prevent the solder from moving before it melts.
When silver solder melts, it tends to flow towards the area of greatest heat. Jewelers can somewhat control the direction the solder moves by leading it with a torch; it will even sometimes run straight up along a seam.
Mechanical and aluminium soldering
A number of solder materials, primarily
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
alloys, are used for soldering aluminium metal and alloys and to a lesser extent steel and zinc. This mechanical soldering is similar to a low temperature brazing operation, in that the mechanical characteristics of the joint are reasonably good and it can be used for structural repairs of those materials.
The
American Welding Society
The American Welding Society (AWS) was founded in 1919 as a non-profit organization to advance the science, technology and application of welding and allied joining and cutting processes, including brazing, soldering and thermal spraying.
Headqu ...
defines brazing as using filler metals with melting points over — or, by the traditional definition in the United States, above . Aluminium soldering alloys generally have melting temperatures around .
[Alumaloy](_blank)
, accessed 2009-04-03 This soldering / brazing operation can use a propane torch heat source.
, accessed 2009-04-03
These materials are often advertised as "aluminium welding", but the process does not involve melting the base metal, and therefore is not properly a weld.
United States Military Standard
A United States defense standard, often called a military standard, "MIL-STD", "MIL-SPEC", or (informally) "MilSpecs", is used to help achieve standardization objectives by the U.S. Department of Defense.
Standardization is beneficial in achievin ...
or MIL-SPEC specification MIL-R-4208 defines one standard for these zinc-based brazing/soldering alloys.
[MIL-R-4208](_blank)
, accessed 2009-04-03 A number of products meet this specification.
, accessed 2009-04-03[HTS-2000](_blank)
, accessed 2009-03-09 or very similar performance standards.
Pipe soldering
![LeadFree CadmiumFree solder](https://upload.wikimedia.org/wikipedia/commons/3/3c/LeadFree_CadmiumFree_solder.jpg)
Copper pipe, or 'tube', is commonly joined by soldering. When applied in a plumbing trade context in the United States, soldering is often referred to as ''sweating'', and a tubing connection so made is referred to as a ''sweated joint''.
Outside the United States, "sweating" refers to the joining of flat metallic surfaces by a two step process by which solder is first applied to one surface, then this first piece is placed in position against the second surface and both are re-heated to achieve the desired joint.
Copper tubing conducts heat away much faster than a conventional hand-held soldering iron or gun can provide, so a
propane torch
A propane torch is a tool normally used for the application of flame or heat which uses propane, a hydrocarbon gas, for its fuel and ambient air as its combustion medium. Propane is one of a group of by-products of the natural gas and petroleum ...
is most commonly used to deliver the necessary power; for large tubing sizes and fittings a
MAPP-fueled,
acetylene
Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
-fueled, or
propylene
Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petrole ...
-fueled torch is used with atmospheric air as the oxidizer; MAPP/oxygen or acetylene/oxygen are rarely used because the flame temperature is much higher than the melting point of copper. Too much heat destroys the temper of hard-tempered copper tubing, and can burn the flux out of a joint before the solder is added, resulting in a faulty joint. For larger tubing sizes, a torch fitted with various sizes of interchangeable ''swirl tips'' is employed to deliver the needed heating power. In the hands of a skilled
tradesman
A tradesman, tradeswoman, or tradesperson is a skilled worker that specializes in a particular trade (occupation or field of work). Tradesmen usually have work experience, on-the-job training, and often formal vocational education in contrast to ...
, the hotter flame of acetylene, MAPP, or propylene allows more joints to be completed per hour without damage to copper tempering.
However, it is possible to use an electrical tool to solder joints in copper pipe sized from . For example, the Antex Pipemaster is recommended for use in tight spaces, when
open flame
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
At a certain point in the combustion reaction, called the ignition point, flames are pr ...
s are hazardous, or by
do-it-yourself
"Do it yourself" ("DIY") is the method of building, modifying, or repairing things by oneself without the direct aid of professionals or certified experts. Academic research has described DIY as behaviors where "individuals use raw and semi ...
users. The
pliers
Pliers are a hand tool used to hold objects firmly, possibly developed from tongs used to handle hot metal in Bronze Age Europe. They are also useful for bending and physically compressing a wide range of materials. Generally, pliers consist ...
-like tool uses heated fitted jaws that completely encircle the pipe, allowing a joint to be melted in as little as 10 seconds.
Solder fittings, also known as 'capillary fittings', are usually used for copper joints. These fittings are short sections of smooth pipe designed to slide over the outside of the mating tube. Commonly used fittings include for straight connectors, reducers, bends, and tees. There are two types of solder fittings: 'end feed fittings' which contain no solder, and '
solder ring fitting A solder ring fitting, also known by the trademarked name Yorkshire fitting, is a pre-soldered capillary connector for joining copper pipes used in plumbing.
Operation
To obtain perfect joins, the inside of the fitting and the outside of the copper ...
s' (also known as Yorkshire fittings), in which there is a ring of solder in a small circular recess inside the fitting.
As with all solder joints, all parts to be joined must be clean and oxide free. Internal and external wire brushes are available for the common pipe and fitting sizes;
emery cloth
upright=1.35, Sheets of sandpaper with different grit sizes (40 (coarse), 80, 150, 240, 600 (fine)).
Sandpaper and glasspaper are names used for a type of coated abrasive that consists of sheets of paper or cloth with abrasive material glued to ...
and wire-wool are frequently used as well, although metal wool products are discouraged, as they can contain oil, which would contaminate the joint.
Because of the size of the parts involved, and the high activity and contaminating tendency of the flame, plumbing fluxes are typically much more chemically active, and often more acidic, than electronic fluxes. Because plumbing joints may be done at any angle, even upside down, plumbing fluxes are generally formulated as pastes which stay in place better than liquids. Flux is applied to all surfaces of the joint, inside and out. Flux residues are removed after the joint is complete to prevent erosion and failure of the joint.
Many plumbing solder formulations are available, with different characteristics, such as higher or lower melting temperature, depending on the specific requirements of the job.
Building code
A building code (also building control or building regulations) is a set of rules that specify the standards for constructed objects such as buildings and non-building structures. Buildings must conform to the code to obtain planning permission ...
s currently almost universally require the use of lead-free solder for
drinking water
Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
piping (and also flux must be approved for drinking water applications), though traditional tin-lead solder is still available. Studies have shown that lead-soldered plumbing pipes can result in elevated levels of lead in drinking water.
Since copper pipe quickly conducts heat away from a joint, great care must be taken to ensure that the joint is properly heated through to obtain a good bond. After the joint is properly cleaned, fluxed and fitted, the torch flame is applied to the thickest part of the joint, typically the fitting with the pipe inside it, with the solder applied at the gap between the tube and the fitting. When all the parts are heated through, the solder will melt and flow into the joint by capillary action. The torch may need to be moved around the joint to ensure all areas are wetted out. However, the installer must take care to not overheat the areas being soldered. If the tube begins to discolor it means that the tube has been over-heated and is beginning to oxidize, stopping the flow of the solder and causing the soldered joint not to seal properly. Before oxidation the molten solder will follow the heat of the torch around the joint. When the joint is properly wetted out, the solder and then the heat are removed, and while the joint is still very hot, it is usually wiped with a dry rag. This removes excess solder as well as flux residue before it cools down and hardens. With a solder ring joint, the joint is heated until a ring of molten solder is visible around the edge of the fitting and allowed to cool.
Of the three methods of connecting copper tubing, solder connections require the most skill, but soldering copper is a very reliable process, provided some basic conditions are met:
* The tubing and fittings must be cleaned to bare metal with no tarnish
* Any pressure which is formed by heating of the tubing must have an outlet
* The joint must be dry (which can be challenging when repairing water pipes)
Copper is only one material that is joined in this manner. Brass fittings are often used for valves or as a connection fitting between copper and other metals. Brass piping is soldered in this manner in the making of
brass instruments
A brass instrument is a musical instrument that produces sound by sympathetic vibration of air in a tubular resonator in sympathy with the vibration of the player's lips. Brass instruments are also called labrosones or labrophones, from Latin ...
and some
woodwind
Woodwind instruments are a family of musical instruments within the greater category of wind instruments. Common examples include flute, clarinet, oboe, bassoon, and saxophone. There are two main types of woodwind instruments: flutes and reed ...
(saxophone and flute) musical instruments
Wire brush
A wire brush is a tool consisting of a brush whose bristles are made of wire, most often steel wire. The steel used is generally a medium- to high-carbon variety and very hard and springy. Other wire brushes feature bristles made from brass ...
,
wire wool
Wire wool is a generic term for products made of filaments entwined, woven or otherwise agglomerated into a sort of felt pad or belt that is often rolled for convenience. Typical dimensions for most domestic wire-wool might be about 1cm/¼" thick ...
and emery cloth are commonly used to prepare plumbing joints for connection. Bristle brushes are usually used to apply plumbing paste flux. A heavy rag is usually used to remove flux from a plumbing joint before it cools and hardens. A fiberglass brush can also be used.
When soldering pipes closely connected to valves such as in refrigeration systems it may be necessary to protect the valve from heat that could damage rubber or plastic components within, in this case a wet cloth wrapped around the valve can often sink sufficient heat through the boiling of the water to protect the valve.
Copper tube soldering defects
In the joining of copper tube, failure to properly heat and fill a joint may lead to a 'void' being formed. This is usually a result of improper placement of the flame. If the heat of the flame is not directed at the back of the fitting cup, and the solder wire applied degrees opposite the flame, then solder will quickly fill the opening of the fitting, trapping some flux inside the joint. This bubble of trapped flux is the void; an area inside a soldered joint where solder is unable to completely fill the fittings' cup, because flux has become sealed inside the joint, preventing solder from occupying that space.
Stained glass soldering
Historically,
stained glass
Stained glass is coloured glass as a material or works created from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant religious buildings. Although tradition ...
soldering tips were copper, heated by being placed in a
charcoal
Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, cal ...
-burning
brazier
A brazier () is a container used to burn charcoal or other solid fuel for cooking, heating or cultural rituals. It often takes the form of a metal box or bowl with feet. Its elevation helps circulate air, feeding oxygen to the fire. Braziers h ...
. Multiple tips were used; when one tip cooled down from use, it was placed back in the brazier of charcoal and the next tip was used.
More recently, electrically heated soldering irons are used. These are heated by a coil or ceramic heating element inside the tip of the iron. Different power ratings are available, and temperature can be controlled electronically. These characteristics allow longer beads to be run without interrupting the work to change tips. Soldering irons designed for electronic use are often effective though they are sometimes underpowered for the heavy copper and lead came used in stained glass work.
Oleic acid is the classic flux material that has been used to improve solderability.
Tiffany-type stained glass is made by gluing copper foil around the edges of the pieces of glass and then soldering them together. This method makes it possible to create three-dimensional stained glass pieces.
Electronics soldering
Hand soldering
![Ersin Multicore Solder Tube](https://upload.wikimedia.org/wikipedia/commons/d/db/Ersin_Multicore_Solder_Tube.jpg)
For attachment of electronic components to a
PCB
PCB may refer to:
Science and technology
* Polychlorinated biphenyl, an organic chlorine compound, now recognized as an environmental toxin and classified as a persistent organic pollutant
* Printed circuit board, a board used in electronics
* ...
, proper selection and use of flux helps prevent
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
during soldering; it is essential for good wetting and heat transfer. The
soldering iron
A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip (the ''bit'') and an insulated handle. Heating i ...
tip must be clean and pre-tinned with solder to ensure rapid heat transfer.
Electronic joints are usually made between surfaces that have been tinned and rarely require mechanical cleaning, though tarnished component leads and copper traces with a dark layer of oxide passivation (due to aging), as on a new prototyping board that has been on the shelf for about a year or more, may need to be mechanically cleaned.
To simplify soldering, beginners are usually advised to apply the soldering iron and the solder separately to the joint, rather than the solder being applied directly to the iron. When sufficient solder is applied, the solder wire is removed. When the surfaces are adequately heated, the solder will flow around the workpieces. The iron is then removed from the joint.
If all metal surfaces have not been properly cleaned ("fluxed") or brought entirely above the melting temperature of the solder used, the result will be an unreliable ("cold solder") joint, even though its appearance may suggest otherwise.
Excess solder, unconsumed flux and residue is sometimes wiped from the soldering iron tip between joints. The tip of the bit (commonly iron plated to reduce erosion) is kept wetted with solder ("tinned") when hot to assist soldering, and to minimize oxidation and corrosion of the tip itself.
After inserting a
through-hole
In electronics, through-hole technology (also spelled "thru-hole") is a manufacturing scheme in which leads on the components are inserted through holes drilled in printed circuit boards (PCB) and soldered to pads on the opposite side, either by ...
mounted component, the excess lead is cut off, leaving a length of about the radius of the pad.
Hand-soldering techniques require a great deal of skill for the fine-pitch soldering of
surface-mount chip packages. In particular
ball grid array (BGA) devices are notoriously difficult, if not impossible, to rework by hand.
Defects
Cold joints
![Gebrochene loetstellen](https://upload.wikimedia.org/wikipedia/commons/7/72/Gebrochene_loetstellen.jpg)
Various problems may arise in the soldering process which lead to joints which are nonfunctional either immediately or after a period of use.
The most common defect when hand-soldering results from the parts being joined not exceeding the solder's liquidus temperature, resulting in a "cold solder" joint. This is usually the result of the soldering iron being used to heat the solder directly, rather than the parts themselves. Properly done, the iron heats the parts to be connected, which in turn melt the solder, guaranteeing adequate heat in the joined parts for thorough wetting. If using solder wire with an embedded flux core, heating the solder first may cause the flux to evaporate before it cleans the surfaces being soldered.
A cold-soldered joint may not conduct at all, or may conduct only intermittently. Cold-soldered joints also happen in mass production, and are a common cause of equipment which passes testing, but malfunctions after sometimes years of operation.
Dry joints
A "dry joint" occurs when the cooling solder is moved. Since
non-eutectic solder alloys have a small plastic range, the joint must not be moved until the solder has cooled down through both the liquidus and solidus temperatures. Dry joints often occur because the joint moves when the soldering iron is removed from the joint. They are weak mechanically and poor conductors electrically.
Avoiding overheating of components
For hand soldering, the heat source tool is selected to provide adequate heat for the size of joint to be completed. A 100-watt
soldering iron
A soldering iron is a hand tool used in soldering. It supplies heat to melt solder so that it can flow into the joint between two workpieces.
A soldering iron is composed of a heated metal tip (the ''bit'') and an insulated handle. Heating i ...
may provide too much heat for
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
s (PCBs), while a 25-watt iron will not provide enough heat for large electrical connectors.
Using a tool with too high a temperature can damage sensitive components, but protracted heating by a tool that is too cool or under powered can also cause heat damage. Excessive heating of a PCB may result in delamination — the copper traces may actually lift off the substrate, particularly on single sided PCBs without
through hole plating.
While hand-soldering, a
heat sink
A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...
, such as a crocodile clip, may be used on the leads of heat-sensitive components to reduce heat transfer to the components and avoid damaging them. This is especially applicable to
germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
parts.
The heat sink limits the temperature of the component body by absorbing and dissipating heat, by reducing the thermal resistance between the component and the air. Meanwhile, the thermal resistance of the leads maintains the temperature difference between the part of the leads being soldered and the component body. Thus, the leads become hot enough to melt the solder while the component body remains cooler. The heat sink will mean the use of more heat to complete the joint, since heat taken up by the heat sink will not heat the work pieces.
Components which dissipate large amounts of heat during operation are sometimes elevated above the PCB to avoid PCB overheating. Plastic or metal mounting clips or holders may be used with large devices to aid heat dissipation and reduce joint stresses.
Visual inspection of joints
When visually inspected, a good solder joint will appear smooth, bright and shiny, with the outline of the soldered wire clearly visible. In general a good-looking soldered joint is a good joint.
A matte gray surface is a good indicator of a joint that was moved during soldering. A dry joint has a characteristically dull or grainy appearance immediately after the joint is made. This appearance is caused by crystallization of the liquid solder. Too little solder will result in a dry and unreliable joint.
Cold solder joints are dull and sometimes cracked or pock-marked. If the joint has lumps or balls of otherwise shiny solder, the metal has not wetted properly. Too much solder (the familiar 'solder blob' to beginners) is not necessarily unsound, but tends to mean poor wetting.
A
concave
Concave or concavity may refer to:
Science and technology
* Concave lens
* Concave mirror
Mathematics
* Concave function, the negative of a convex function
* Concave polygon, a polygon which is not convex
* Concave set
* The concavity
In ca ...
fillet is ideal. The boundary between the solder and the workpiece in a good joint will have a low angle. This indicates good wetting and minimal use of solder, and therefore minimal heating of heat sensitive components. A joint may be good, but if a large amount of unnecessary solder is used, then excess heating was obviously required.
Lead-free solder formulations may cool to a dull surface even if the joint is good. The solder looks shiny while molten, and suddenly hazes over as it solidifies even though it has not been disturbed during cooling.
Flux use and residue
An improperly selected or applied flux can cause joint failure. Without flux the joint may not be clean, or may be oxidized, resulting in an unsound joint.
For electronic work, flux-core solder wire is generally used, but additional flux may be used from a
flux pen
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
or dispensed from a small bottle with a syringe-like needle.
Some fluxes are designed to be stable and inactive when cool and do not need to be cleaned off, though they can if desired. If such fluxes are used, cleaning may merely be a matter of aesthetics or to make visual inspection of joints easier in specialised 'mission critical' applications such as medical devices, military and aerospace. For satellites, this will also reduce weight, slightly but usefully. In high humidity, since even non-corrosive flux might remain slightly active, the flux may be removed to reduce corrosion over time.
Some fluxes are corrosive and flux residue must be removed after soldering. If not properly cleaned, the flux may corrode the joint or the PCB. Water, alcohol, acetone, or other solvents compatible with the flux and the parts involved are commonly used with cotton swabs or bristle brushes.
In some applications, the PCB might also be coated in some form of protective material such as a
lacquer
Lacquer is a type of hard and usually shiny coating or finish applied to materials such as wood or metal. It is most often made from resin extracted from trees and waxes and has been in use since antiquity.
Asian lacquerware, which may be ca ...
to protect it and exposed solder joints from the environment.
Desoldering and resoldering
![Soldering a 0805](https://upload.wikimedia.org/wikipedia/commons/f/f8/Soldering_a_0805.jpg)
Used solder contains some of the dissolved base metals and is unsuitable for reuse in making new joints. Once the solder's capacity for the base metal has been reached, it will no longer properly bond with the base metal, usually resulting in a brittle cold solder joint with a crystalline appearance.
It is good practice to remove solder from a joint prior to resoldering —
desoldering braid
In electronics, desoldering is the removal of solder and components from a circuit board for troubleshooting, repair, replacement, and salvage.
Tools
Desoldering tools and materials include the following:
*Solder wick
*Heat guns, also called hot ...
s (or wicks) or vacuum desoldering equipment (
solder sucker
In electronics, desoldering is the removal of solder and components from a circuit board for troubleshooting, repair, replacement, and salvage.
Tools
Desoldering tools and materials include the following:
*Solder wick
*Heat guns, also called hot ...
s) can be used. Desoldering wicks contain plenty of flux which will remove the oxidation from the copper trace and any device leads that are present. This will leave a bright, shiny, clean junction to be resoldered.
The lower melting point of solder means it can be melted away from the base metal, leaving it mostly intact, though the outer layer will be "tinned" with solder. Flux will remain which can easily be removed by abrasive or chemical processes. This tinned layer will allow solder to flow onto a new joint, resulting in a new joint, as well as making the new solder flow very quickly and easily.
Wave soldering and reflow soldering
Currently, mass-production printed circuit boards (PCBs) are mostly
wave soldered or
reflow soldered, though hand soldering of production electronics is also still widely used.
In wave soldering, components are prepped (trimmed or modified) and installed on the PCB. Sometimes, to prevent movement they are temporarily kept in place with small dabs of adhesive or secured with a fixture, then the assembly is passed over flowing solder in a bulk container. This solder flow is forced to produce a
standing wave
In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...
so the whole PCB is not submerged in solder, but rather just touched. The result is that solder stays on pins and pads, but not on the PCB itself.
Reflow soldering is a process in which a
solder paste
Solder paste is used in the manufacture of printed circuit boards to connect surface mount components to pads on the board. It is also possible to solder through-hole pin in paste components by printing solder paste in and over the holes. The ...
(a mixture of prealloyed solder powder and a flux-vehicle that has a peanut butter-like consistency
) is used to stick the components to their attachment pads, after which the assembly is heated by an infrared lamp, a hot air pencil, or, more commonly, by passing it through a carefully controlled oven.
Since different components can be best assembled by different techniques, it is common to use two or more processes for a given PCB. For example,
surface mounted parts may be reflow soldered first, with a wave soldering process for the
through-hole
In electronics, through-hole technology (also spelled "thru-hole") is a manufacturing scheme in which leads on the components are inserted through holes drilled in printed circuit boards (PCB) and soldered to pads on the opposite side, either by ...
mounted components coming next, and bulkier parts hand-soldered last.
Hot-bar reflow
''Hot-bar reflow'' is a selective soldering process where two pre-fluxed, solder coated parts are heated with a heating element (called a thermode) to a temperature sufficient to melt the solder.
Pressure is applied through the entire process (usually 15 seconds) to ensure that components stay in place during cooling. The heating element is heated and cooled for each connection. Up to 4000
W can be used in the heating element, allowing fast soldering, good results with connections requiring high energy.
Environmental regulation and RoHS
Environmental legislation in many countries has led to a change in formulation of both solders and fluxes.
The
RoHS
The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Unio ...
directives in the
European Community
The European Economic Community (EEC) was a regional organization created by the Treaty of Rome of 1957,Today the largely rewritten treaty continues in force as the ''Treaty on the functioning of the European Union'', as renamed by the Lisbo ...
required many new electronic circuit boards to be lead-free by 1 July 2006, mostly in the consumer goods industry, but in some others as well. In Japan, lead was phased out prior to legislation by manufacturers, due to the additional expense in recycling products containing lead.
[FACT AND FICTION IN LEAD FREE SOLDERING](_blank)
from ''www.dkmetals.co.uk''
Water-soluble non-rosin-based fluxes have been increasingly used since the 1980s so that soldered boards can be cleaned with water or water-based cleaners. This eliminates hazardous solvents from the production environment, and from factory effluents.
Even without the presence of lead, soldering can release fumes that are harmful and/or toxic to humans. It is highly recommended to use a device that can remove the fumes from the work area either by ventilating outside or filtering the air.
Lead-free
Lead free soldering requires higher soldering temperatures than lead/tin soldering.
Sn Pb 63/37
eutectic solder melts at .
SAC lead-free solder melts at .
Nevertheless, many new technical challenges have arisen with this endeavor. To reduce the melting point of tin-based solder alloys, various new alloys have had to be researched, with additives of copper, silver, bismuth as typical minor additives to reduce the melting point and control other properties. Additionally, tin is a more corrosive metal, and can eventually lead to the failure of solder baths.
Lead-free construction has also extended to components, pins, and connectors. Most of these pins used copper frames, and either lead, tin, gold or other finishes. Tin finishes are the most popular of lead-free finishes. Nevertheless, this brings up the issue of how to deal with
tin whiskers
Metal whiskering is a phenomenon which occurs in electrical devices when metals form long whisker-like projections over time. Tin whiskers were noticed and documented in the vacuum tube era of electronics early in the 20th century in equipment th ...
. The current movement brings the electronics industry back to the problems solved in the 1960s by adding lead.
JEDEC
The JEDEC Solid State Technology Association is an independent semiconductor engineering trade organization and standardization body headquartered in Arlington County, Virginia, United States.
JEDEC has over 300 members, including some of the w ...
has created a classification system to help lead-free electronic manufacturers decide what provisions to take against whiskers, depending upon their application.
See also
*
Dip soldering
Dip soldering is a small-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The solder wets to the exposed metallic areas of the board (those not protected with s ...
*
Electronics technician
Electronics technicians help design, develop, test, manufacture, install, and repair electrical and electronic equipment such as communication equipment, medical monitoring devices, navigational equipment, and computers. They may be employed i ...
*
Solder fatigue
Solder fatigue is the mechanical degradation of solder due to deformation under cyclic loading. This can often occur at stress levels below the yield stress of solder as a result of repeated temperature fluctuations, mechanical vibrations, or mec ...
*
Solder mask
Solder mask, solder stop mask or solder resist is a thin lacquer-like layer of polymer that is usually applied to the copper traces of a printed circuit board (PCB) for protection against oxidation and to prevent solder bridges from forming b ...
References
External links
Desoldering guideA short video explanation of how solder worksSoldering 101 (Archived)
{{Authority control
Electronics manufacturing
Laser applications
Metalworking