A PRINTED CIRCUIT BOARD (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate . Components (e.g. capacitors , resistors or active devices) are generally soldered on the PCB. Advanced PCBs may contain components embedded in the substrate.
PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layer (outer and inner layers). Conductors on different layers are connected with vias . Multi-layer PCBs allow for much higher component density.
FR-4 glass epoxy is the primary insulating substrate. A basic building block of the PCB is an FR-4 panel with a thin layer of copper foil laminated to one or both sides. In multi-layer boards multiple layers of material are laminated together.
Printed circuit boards are used in all but the simplest electronic products. Alternatives to PCBs include wire wrap and point-to-point construction . PCBs require the additional design effort to lay out the circuit, but manufacturing and assembly can be automated. Manufacturing circuits with PCBs is cheaper and faster than with other wiring methods as components are mounted and wired with one single part.
A minimal PCB with a single component used for easier prototyping is called a breakout board.
When the board has no embedded components it is more correctly called a printed wiring board (PWB) or etched wiring board. However, the term printed wiring board has fallen into disuse. A PCB populated with electronic components is called a printed circuit assembly (PCA), printed circuit board assembly or PCB assembly (PCBA). The IPC preferred term for assembled boards is circuit card assembly (CCA), and for assembled backplanes it is backplane assemblies. The term PCB is used informally both for bare and assembled boards.
The world market for bare PCBs exceeded $60.2 billion in 2014.
* 1 Design
* 2 Manufacturing
* 2.1 PCB CAM * 2.2 Panelization
* 2.3.1 Large volume * 2.3.2 Small volume * 2.3.3 Hobbyist
* 2.4 Subtractive, additive and semi-additive processes
* 2.5 Chemical etching
* 2.6 Inner layer automated optical inspection (AOI)
* 2.7 Lamination
* 2.8 Drilling
Plating and coating
* 3 PCB characteristics
* 3.4 Materials
* 3.4.1 Laminates
* 3.4.2 Key substrate parameters
* 3.4.3 Common substrates
* 4 Multiwire boards * 5 Cordwood construction * 6 History * 7 See also * 8 References * 9 External links
A board designed in 1967; the sweeping curves in the traces are evidence of freehand design using adhesive tape
Initially PCBs were designed manually by creating a photomask on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. To fabricate the board, the finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards.
Modern PCBs are designed with dedicated layout software, generally in the following steps:
PCB manufacturing consists of many steps.
Manufacturing starts from the PCB fabrication data generated by
computer aided design , such as Gerber layer images, Gerber or
Excellon drill files, IPC-D-356 netlist and component information.
The Gerber or Excellon files in the fabrication data are never used
directly on the manufacturing equipment but always read into the CAM
* Input of the fabrication data. * Verification of the data; optionally DFM * Compensation for deviations in the manufacturing processes (e.g. scaling to compensate for distortions during lamination) * Panelization * Output of the digital tools (copper patterns , solder resist image , legend image , drill files , automated optical inspection data , electrical test files ,...)
Panelization is a procedure whereby a number of PCBs are grouped for manufacturing onto a larger board - the panel. Usually a panel consists of a single design but sometimes multiple designs are mixed on a single panel. There are two types of panels: assembly panels - often called arrays - and bare board manufacturing panels. The assemblers often mount components on panels rather than single PCBs because this is efficient. The bare board manufactures always uses panels, not only for efficiency, but because of the requirements of the plating process. Thus a manufacturing panel can consist of a grouping of individual PCBs or of arrays, depending on what must be delivered.
The panel is eventually broken apart into individual PCBs; this is
called depaneling. Separating the individual PCBs is frequently aided
by drilling or routing perforations along the boundaries of the
individual circuits, much like a sheet of postage stamps . Another
method, which takes less space, is to cut V-shaped grooves across the
full dimension of the panel. The individual PCBs can then be broken
apart along this line of weakness. Today depaneling is often done by
lasers which cut the board with no contact.
The first step is to replicate the pattern in the fabricator's CAM system on a protective mask on the copper foil PCB layers. Subsequent etching removes the unwanted copper. (Alternatively, a conductive ink can be ink-jetted on a blank (non-conductive) board. This technique is also used in the manufacture of hybrid circuits .)
* SILK SCREEN PRINTING uses etch-resistant inks to create the
* PHOTOENGRAVING uses a photomask and developer to selectively
remove a UV-sensitive photoresist coating and thus create a
photoresist mask. Direct imaging techniques are sometimes used for
high-resolution requirements. Experiments were made with thermal
* PCB MILLING uses a two or three-axis mechanical milling system to
mill away the copper foil from the substrate. A PCB milling machine
(referred to as a 'PCB Prototyper') operates in a similar way to a
plotter , receiving commands from the host software that control the
position of the milling head in the x, y, and (if relevant) z axis.
* LASER RESIST ABLATION Spray black paint onto copper clad laminate,
The method chosen depends on the number of boards to be produced and the required resolution.
* Print onto transparent film and use as photo mask along with
photo-sensitized boards (i.e., pre-sensitized boards), then etch.
(Alternatively, use a film photoplotter)
* Laser-printed resist: Laser-print onto toner transfer paper, heat-transfer with an iron or modified laminator onto bare laminate, soak in water bath, touch up with a marker, then etch. * Vinyl film and resist, non-washable marker, some other methods. Labor-intensive, only suitable for single boards.
SUBTRACTIVE, ADDITIVE AND SEMI-ADDITIVE PROCESSES
The two processing methods used to produce a double-sided PWB with plated-through holes
Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern. In additive methods the pattern is electroplated onto a bare substrate using a complex process. The advantage of the additive method is that less material is needed and less waste is produced. In the full additive process the bare laminate is covered with a photosensitive film which is imaged (exposed to light through a mask and then developed which removes the unexposed film). The exposed areas are sensitized in a chemical bath, usually containing palladium and similar to that used for through hole plating which makes the exposed area capable of bonding metal ions. The laminate is then plated with copper in the sensitized areas. When the mask is stripped, the PCB is finished.
Semi-additive is the most common process: The unpatterned board has a
thin layer of copper already on it. A reverse mask is then applied.
(Unlike a subtractive process mask, this mask exposes those parts of
the substrate that will eventually become the traces.) Additional
copper is then plated onto the board in the unmasked areas; copper may
be plated to any desired weight. Tin-lead or other surface platings
are then applied. The mask is stripped away and a brief etching step
removes the now-exposed bare original copper laminate from the board,
isolating the individual traces. Some single-sided boards which have
plated-through holes are made in this way.
The (semi-)additive process is commonly used for multi-layer boards as it facilitates the plating -through of the holes to produce conductive vias in the circuit board. PCB copper electroplating line in the process of pattern plating copper PCBs in process of having copper pattern plated (note the blue dry film resist)
Chemical etching is usually done with ammonium persulfate or ferric chloride . For PTH (plated-through holes), additional steps of electroless deposition are done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.
The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates.
As more copper is consumed from the boards, the etchant becomes saturated and less effective; different etchants have different capacities for copper, with some as high as 150 grams of copper per litre of solution. In commercial use, etchants can be regenerated to restore their activity, and the dissolved copper recovered and sold. Small-scale etching requires attention to disposal of used etchant, which is corrosive and toxic due to its metal content.
The etchant removes copper on all surfaces exposed by the resist. "Undercut" occurs when etchant attacks the thin edge of copper under the resist; this can reduce conductor widths and cause open-circuits. Careful control of etch time is required to prevent undercut. Where metallic plating is used as a resist, it can "overhang" which can cause short-circuits between adjacent traces when closely spaced. Overhang can be removed by wire-brushing the board after etching.
INNER LAYER AUTOMATED OPTICAL INSPECTION (AOI)
The inner layers are given a complete machine inspection before lamination because afterwards mistakes cannot be corrected. The automatic optical inspection system scans the board and compares it with the digital image generated from the original design data.
Cut through a SDRAM-module, a multi-layer PCB. Note the via , visible as a bright copper-colored band running between the top and bottom layers of the board.
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Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.
Holes through a PCB are typically drilled with small-diameter drill bits made of solid coated tungsten carbide . Coated tungsten carbide is recommended since many board materials are very abrasive and drilling must be high RPM and high feed to be cost effective. Drill bits must also remain sharp so as not to mar or tear the traces. Drilling with high-speed-steel is simply not feasible since the drill bits will dull quickly and thus tear the copper and ruin the boards. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or "Excellon files ". The drill file describes the location and size of each drilled hole.
Holes may be made conductive, by electroplating or inserting metal eyelets (hollow), to electrically and thermally connect board layers. Some conductive holes are intended for the insertion of through-hole-component leads. Others, typically smaller and used to connect board layers, are called vias .
When very small vias are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, the vias may be laser drilled —evaporated by lasers . Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.
It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers and no outer layers.
The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes. These holes electrically connect the conducting layers of the PCB. For multi-layer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper.
PLATING AND COATING
PCBs are plated with solder, tin, or gold over nickel and a resist for etching away the unneeded underlying copper.
After PCBs are etched and then rinsed with water, the solder mask is applied, and then any exposed copper is coated with solder, nickel/gold, or some other anti-corrosion coating.
Matte solder is usually fused to provide a better bonding surface for bare copper. Treatments, such as benzimidazolethiol , prevent surface oxidation of bare copper. The places to which components will be mounted are typically plated, because untreated bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was coated with solder by hot air solder levelling (HASL). The HASL finish prevents oxidation from the underlying copper, thereby guaranteeing a solderable surface. This solder was a tin -lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU , which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin, 0.7% copper, 0.05% nickel, and a nominal of 60 ppm germanium.
It is important to use solder compatible with both the PCB and the parts used. An example is ball grid array (BGA) using tin-lead solder balls for connections losing their balls on bare copper traces or using lead-free solder paste.
Other platings used are OSP (organic surface protectant), immersion
IAg ), immersion tin, electroless nickel with immersion gold
ENIG ), electroless nickel electroless palladium immersion
gold (ENEPIG ) and direct gold plating (over nickel). Edge connectors
, placed along one edge of some boards, are often nickel-plated then
gold-plated . Another coating consideration is rapid diffusion of
coating metal into
Electrochemical migration (ECM) is the growth of conductive metal
filaments on or in a printed circuit board (PCB) under the influence
of a DC voltage bias. Silver, zinc, and aluminum are known to grow
whiskers under the influence of an electric field. Silver also grows
conducting surface paths in the presence of halide and other ions,
making it a poor choice for electronics use.
SOLDER RESIST APPLICATION
Areas that should not be soldered may be covered with solder resist
(solder mask). One of the most common solder resists used today is
called "LPI" (liquid photoimageable solder mask ). A photo-sensitive
coating is applied to the surface of the PWB, then exposed to light
through the solder mask image film, and finally developed where the
unexposed areas are washed away. Dry film solder mask is similar to
the dry film used to image the PWB for plating or etching. After being
laminated to the PWB surface it is imaged and develop as LPI. Once
common but no longer commonly used because of its low accuracy and
resolution is to screen print epoxy ink.
A legend is often printed on one or both sides of the PCB. It contains the component designators , switch settings, test points and other indications helpful in assembling, testing and servicing the circuit board.
There are three methods to print the legend.
Unpopulated boards are usually bare-board tested for "shorts" and "opens". A short is a connection between two points that should not be connected. An open is a missing connection between points that should be connected. For high-volume production a fixture or a rigid needle adapter makes contact with copper lands on the board. The fixture or adapter is a significant fixed cost and this method is only economical for high-volume or high-value production. For small or medium volume production flying probe testers are used where test probes are moved over the board by an XY drive to make contact with the copper lands. There is no need for a fixture and hence the fixed costs are much lower. The CAM system instructs the electrical tester to apply a voltage to each contact point as required and to check that this voltage appears on the appropriate contact points and only on these.
PCB with test connection pads
In assembly the bare board is populated (or "stuffed") with electronic components to form a functional printed circuit assembly (PCA), sometimes called a "printed circuit board assembly" (PCBA). In through-hole technology , the component leads are inserted in holes surrounded by conductive pads; the holes keep the components in place. In surface-mount technology (SMT), the component is placed on the PCB so that the pins line up with the conductive pads or lands on the surfaces of the PCB; solder paste, which was previously applied to the pads, holds the components in place; if surface-mount components are applied to both sides of the board, the bottom-side components are glued to the board. In both through hole and surface mount, the components are then soldered .
There are a variety of soldering techniques used to attach components to a PCB. High volume production is usually done with a "Pick and place machine" or SMT placement machine and bulk wave soldering or reflow ovens , but skilled technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02 in. by 0.01 in.) by hand under a microscope , using tweezers and a fine tip soldering iron for small volume prototypes. Some parts cannot be soldered by hand, such as BGA packages.
Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques. For further comparison, see the SMT page .
After the board has been populated it may be tested in a variety of ways:
* While the power is off, visual inspection , automated optical
To facilitate these tests, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise boundary scan test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.
In boundary scan testing, test circuits integrated into various ICs
on the board form temporary connections between the PCB traces to test
that the ICs are mounted correctly.
Boundary scan testing requires
that all the ICs to be tested use a standard test configuration
procedure, the most common one being the Joint Test Action Group (JTAG
) standard. The
When boards fail the test, technicians may desolder and replace failed components, a task known as rework .
PROTECTION AND PACKAGING
PCBs intended for extreme environments often have a conformal coating , which is applied by dipping or spraying after the components have been soldered. The coat prevents corrosion and leakage currents or shorting due to condensation. The earliest conformal coats were wax ; modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another technique for applying a conformal coating is for plastic to be sputtered onto the PCB in a vacuum chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered extremely difficult.
Many assembled PCBs are static sensitive, and therefore must be placed in antistatic bags during transport. When handling these boards, the user must be grounded (earthed) . Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components. Even bare boards are sometimes static sensitive. Traces have become so fine that it's quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is especially true on non-traditional PCBs such as MCMs and microwave PCBs.
Much of the electronics industry's PCB design, assembly, and quality control follows standards published by the IPC organization.
The first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides. Horizontal installation of through-hole parts with two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction, inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength), soldering the leads, and trimming off the ends. Leads may be soldered either manually or by a wave soldering machine.
Through-hole PCB technology almost completely replaced earlier electronics assembly techniques such as point-to-point construction . From the second generation of computers in the 1950s until surface-mount technology became popular in the late 1980s, every component on a typical PCB was a through-hole component.
Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and limits the available routing area for signal traces on layers immediately below the top layer on multi-layer boards since the holes must pass through all layers to the opposite side. Once surface-mounting came into use, small-sized SMD components were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB.
Through-hole devices mounted on the circuit board of a mid-1980s home computer
A box of drill bits used for making holes in printed circuit boards. While tungsten-carbide bits are very hard, they eventually wear out or break. Drilling is a considerable part of the cost of a through-hole printed circuit board.
Main article: Surface-mount technology Surface mount components, including resistors, transistors and an integrated circuit
Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper; prices of semiconductor surface mount devices (SMDs) are determined more by the chip itself than the package, with little price advantage over larger packages. Some wire-ended components, such as 1N4148 small-signal switch diodes, are actually significantly cheaper than SMD equivalents. A PCB in a computer mouse : the component side (left) and the printed side (right)
CIRCUIT PROPERTIES OF THE PCB
Each trace consists of a flat, narrow part of the copper foil that remains after etching. Its resistance , determined by its width, thickness, and length, must be sufficiently low for the current the conductor will carry. Power and ground traces may need to be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return. For microwave circuits, transmission lines can be laid out in a planar form such as stripline or microstrip with carefully controlled dimensions to assure a consistent impedance . In radio-frequency and fast switching circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; but they can be used as a deliberate part of the circuit design, obviating the need for additional discrete components.
Excluding exotic products using special materials or processes all printed circuit boards manufactured today can be built using the following four materials:
Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermoset resin to form an integral final piece of uniform thickness. The size can be up to 4 by 8 feet (1.2 by 2.4 m) in width and length. Varying cloth weaves (threads per inch or cm), cloth thickness, and resin percentage are used to achieve the desired final thickness and dielectric characteristics. Available standard laminate thickness are listed in
TABLE 1 Standard laminate thickness per ANSI/IPC-D-275 IPC laminate number Thickness in inches Thickness in millimeters IPC laminate number Thickness in inches Thickness in millimeters
L1 0.002 0.05 L9 0.028 0.70
L2 0.004 0.10 L10 0.035 0.90
L3 0.006 0.15 L11 0.043 1.10
L4 0.008 0.20 L12 0.055 1.40
L5 0.010 0.25 L13 0.059 1.50
L6 0.012 0.30 L14 0.075 1.90
L7 0.016 0.40 L15 0.090 2.30
L8 0.020 0.50 L16 0.122 3.10
* ^ Although this specification has been superseded and the new specification does not list standard sizes, these are still the most common sizes stocked and ordered for manufacturer.
The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate's type designation (FR-4, CEM-1, G-10, etc.) and therefore the characteristics of the laminate produced. Important characteristics are the level to which the laminate is fire retardant , the dielectric constant (er), the loss factor (tδ), the tensile strength , the shear strength , the glass transition temperature (Tg), and the Z-axis expansion coefficient (how much the thickness changes with temperature).
There are quite a few different dielectrics that can be chosen to
provide different insulating values depending on the requirements of
the circuit. Some of these dielectrics are polytetrafluoroethylene
(Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known pre-preg materials
used in the PCB industry are
FR-2 (phenolic cotton paper), FR-3
(cotton paper and epoxy),
FR-4 (woven glass and epoxy), FR-5 (woven
glass and epoxy), FR-6 (matte glass and polyester), G-10 (woven glass
and epoxy), CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and
epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven glass and
epoxy), CEM-5 (woven glass and polyester).
FR-4 is by far the most common material used today. The board with copper on it is called "copper-clad laminate".
With decreasing size of board features and increasing frequencies, small nonhomogeneities like uneven distribution of fiberglass or other filler, thickness variations, and bubbles in the resin matrix, and the associated local variations in the dielectric constant, are gaining importance.
Key Substrate Parameters
The circuitboard substrates are usually dielectric composite materials. The composites contain a matrix (usually an epoxy resin), a reinforcement (usually a woven, sometimes nonwoven, glass fibers, sometimes even paper), and in some cases a filler is added to the resin (e.g. ceramics; titanate ceramics can be used to increase the dielectric constant).
The reinforcement type defines two major classes of materials - woven and non-woven. Woven reinforcements are cheaper, but the high dielectric constant of glass may not be favorable for many higher-frequency applications. The spatially nonhomogeneous structure also introduces local variations in electrical parameters, due to different resin/glass ratio at different areas of the weave pattern. Nonwoven reinforcements, or materials with low or no reinforcement, are more expensive but more suitable for some RF/analog applications.
The substrates are characterized by several key parameters, chiefly thermomechanical (glass transition temperature , tensile strength , shear strength , thermal expansion ), electrical (dielectric constant , loss tangent , dielectric breakdown voltage , leakage current , tracking resistance ...), and others (e.g. moisture absorption ).
At the glass transition temperature the resin in the composite softens and significantly increases thermal expansion; exceeding Tg then exerts mechanical overload on the board components - e.g. the joints and the vias. Below Tg the thermal expansion of the resin roughly matches copper and glass, above it gets significantly higher. As the reinforcement and copper confine the board along the plane, virtually all volume expansion projects to the thickness and stresses the plated-through holes. Repeated soldering or other exposition to higher temperatures can cause failure of the plating, especially with thicker boards; thick boards therefore require high Tg matrix.
The materials used determine the substrate's dielectric constant. This constant is also dependent on frequency, usually decreasing with frequency. As this constant determines the signal propagation speed , frequency dependence introduces phase distortion in wideband applications; as flat dielectric constant vs frequency characteristics as achievable is important here. The impedance of transmission lines decreases with frequency, therefore faster edges of signals reflect more than slower ones.
Tracking resistance determines how the material resists high voltage electrical discharges creeping over the board surface.
Loss tangent determines how much of the electromagnetic energy from the signals in the conductors is absorbed in the board material. This factor is important for high frequencies. Low-loss materials are more expensive. Choosing unnecessarily low-loss material is a common error in high-frequency digital design; it increases the cost of the boards without a corresponding benefit. Signal degradation by loss tangent and dielectric constant can be easily assessed by an eye pattern .
Moisture absorption occurs when the material is exposed to high humidity or water. Both the resin and the reinforcement may absorb water; water may be also soaked by capillary forces through voids in the materials and along the reinforcement. Epoxies of the FR-4 materials aren't too susceptible, with absorption of only 0.15%. Teflon has very low absorption of 0.01%. Polyimides and cyanate esters, on the other side, suffer from high water absorption. Absorbed water can lead to significant degradation of key parameters; it impairs tracking resistance, breakdown voltage, and dielectric parameters. Relative dielectric constant of water is about 73, compared to about 4 for common circuitboard materials. Absorbed moisture can also vaporize on heating and cause cracking and delamination, the same effect responsible for "popcorning" damage on wet packaging of electronic parts. Careful baking of the substrates may be required.
Often encountered materials:
FR-2 (Flame Retardant 2), phenolic paper or phenolic cotton paper,
paper impregnated with a phenol formaldehyde resin . Cheap, common in
low-end consumer electronics with single-sided boards. Electrical
properties inferior to FR-4. Poor arc resistance. Generally rated to
* Flexible substrates - can be a standalone copper-clad foil or can be laminated to a thin stiffener, e.g. 50-130 µm
* KAPTON , a polyimide foil. Used for flexible printed circuits, in
this form common in small form-factor consumer electronics or for
flexible interconnects. Resistant to high temperatures.
* PYRALUX , a polyimide-fluoropolymer composite foil.
Less-often encountered materials:
* FR-1 (Flame Retardant 1), like FR-2, typically specified to 105
°C, some grades rated to 130 °C. Room-temperature punchable. Similar
to cardboard. Poor moisture resistance. Low arc resistance.
* FR-3 (Flame Retardant 3), cotton paper impregnated with epoxy.
Typically rated to 105 °C.
* FR-5 (Flame Retardant 5), woven fiberglass and epoxy, high
strength at higher temperatures, typically specified to 170 °C.
* FR-6 (Flame Retardant 6), matte glass and polyester
* G-10, woven glass and epoxy - high insulation resistance, low
moisture absorption, very high bond strength. Typically rated to 130
* G-11, woven glass and epoxy - high resistance to solvents, high
flexural strength retention at high temperatures. Typically rated to
* CEM-1, cotton paper and epoxy
* CEM-2, cotton paper and epoxy
* CEM-3, non-woven glass and epoxy
* CEM-4, woven glass and epoxy
* CEM-5, woven glass and polyester
The printed circuit board industry defines heavy copper as layers exceeding three ounces of copper, or approximately 0.0042 inches (4.2 mils, 105 μm) thick. PCB designers and fabricators often use heavy copper when design and manufacturing circuit boards in order to increase current-carrying capacity as well as resistance to thermal strains. Heavy copper-plated vias transfer heat to external heat sinks. IPC 2152 is a standard for determining current-carrying capacity of printed circuit board traces.
On the common
FR-4 substrates, 1 oz copper (35 µm) is the usual,
most common thickness; 2 oz (70 µm) and 0.5 oz (18 µm) thickness is
often an option. Less common are 12 and 105 µm, 9 µm is sometimes
available on some substrates. Flexible substrates typically have
thinner metalization; 18 and 35 µm seem to be common, with 9 and 70
µm sometimes available.
Safety Certification (US)
Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as components in devices or appliances. Testing analyzes characteristics such as flammability, maximum operating temperature , electrical tracking, heat deflection, and direct support of live electrical parts.
Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting matrix (often plastic resin). It was used during the 1980s and 1990s. (Kollmorgen Technologies Corp, U.S. Patent 4,175,816 filed 1978) Multiwire is still available in 2010 through Hitachi. There are other competitive discrete wiring technologies that have been developed (Jumatech , layered sheets).
Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach allowed designers to forget completely about the routing of wires (usually a time-consuming operation of PCB design): Anywhere the designer needs a connection, the machine will draw a wire in straight line from one location/pin to another. This led to very short design times (no complex algorithms to use even for high density designs) as well as reduced crosstalk (which is worse when wires run parallel to each other—which almost never happens in Multiwire), though the cost is too high to compete with cheaper PCB technologies when large quantities are needed.
Corrections can be made to a Multiwire board more easily than to a PCB.
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A cordwood module Cordwood construction was used in proximity fuzes .
Cordwood construction can save significant space and was often used with wire-ended components in applications where space was at a premium (such as fuzes, missile guidance, and telemetry systems) and in high-speed computers , where short traces were important. In cordwood construction, axial-leaded components were mounted between two parallel planes. The components were either soldered together with jumper wire, or they were connected to other components by thin nickel ribbon welded at right angles onto the component leads. To avoid shorting together different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next interconnection layer. One disadvantage of this system was that special nickel -leaded components had to be used to allow the interconnecting welds to be made. Differential thermal expansion of the component could put pressure on the leads of the components and the PCB traces and cause physical damage (as was seen in several modules on the Apollo program). Additionally, components located in the interior are difficult to replace. Some versions of cordwood construction used soldered single-sided PCBs as the interconnection method (as pictured), allowing the use of normal-leaded components.
Before the advent of integrated circuits , this method allowed the highest possible component packing density; because of this, it was used by a number of computer vendors including Control Data Corporation . The cordwood method of construction was used only rarely once semiconductor electronics and PCBs became widespread.
Development of the methods used in modern printed circuit boards
started early in the 20th century. In 1903, a German inventor, Albert
Hanson, described flat foil conductors laminated to an insulating
board, in multiple layers.
The Austrian engineer
Paul Eisler invented the printed circuit as
part of a radio set while working in the UK around 1936. In 1941 a
multi-layer printed circuit was used in German magnetic influence
naval mines. Around 1943 the USA began to use the technology on a
large scale to make proximity fuses for use in
World War II
Before printed circuits (and for a while after their invention),
point-to-point construction was used. For prototypes, or small
production runs, wire wrap or turret board can be more efficient.
Predating the printed circuit invention, and similar in spirit, was
John Sargrove 's 1936–1947 Electronic Circuit Making Equipment
(ECME) which sprayed metal onto a
During World War II, the development of the anti-aircraft proximity fuse required an electronic circuit that could withstand being fired from a gun, and could be produced in quantity. The Centralab Division of Globe Union submitted a proposal which met the requirements: a ceramic plate would be screenprinted with metallic paint for conductors and carbon material for resistors , with ceramic disc capacitors and subminiature vacuum tubes soldered in place. The technique proved viable, and the resulting patent on the process, which was classified by the U.S. Army, was assigned to Globe Union. It was not until 1984 that the Institute of Electrical and Electronics Engineers (IEEE) awarded Mr. Harry W. Rubinstein, the former head of Globe Union's Centralab Division, its coveted Cledo Brunetti Award for early key contributions to the development of printed components and conductors on a common insulating substrate. As well, Mr. Rubinstein was honored in 1984 by his alma mater, the University of Wisconsin-Madison, for his innovations in the technology of printed electronic circuits and the fabrication of capacitors. A PCB as a design on a computer (left) and realized as a board assembly populated with components (right). The board is double sided, with through-hole plating, green solder resist and a white legend. Both surface mount and through-hole components have been used.
Originally, every electronic component had wire leads, and the PCB
had holes drilled for each wire of each component. The components'
leads were then passed through the holes and soldered to the PCB
trace. This method of assembly is called through-hole construction .
In 1949, Moe Abramson and Stanislaus F. Danko of the United States
Army Signal Corps developed the Auto-Sembly process in which component
leads were inserted into a copper foil interconnection pattern and dip
soldered . The patent they obtained in 1956 was assigned to the U.S.
Army. With the development of board lamination and etching
techniques, this concept evolved into the standard printed circuit
board fabrication process in use today.
From the 1980s small surface mount parts have been used increasingly instead of through-hole components; this has led to smaller boards for a given functionality and lower production costs, but with some additional difficulty in servicing faulty boards.
Historically, many PCB measurements were in multiples of a thousandth of an inch , also called "mils". For example, the Dual In-line Package (DIP) and most other through-hole components have pins located on a grid spacing of 100 mils (0.1 inch). Surface-mount SOIC components have a pin pitch of 50 mils. SOP components have a pin pitch of 25 mils. Level B technology recommends a minimum trace width of 8 mils, which allows "double-track" – two traces between DIP pins.
* Electronics portal
Design for manufacturability (PCB)
Composite epoxy material , CEM-1,5
FR-4 , the most common PCB material
PCB LAYOUT SOFTWARE
* ^ "What is a Breakout Board for Arduino?".
* ^ IPC-14.38
* ^ "iconnect007 :: Article". www.iconnect007.com. Retrieved
* ^ Printed Circuit Board Design Flow Methodology
* ^ "See appendix D of IPC-2251" (PDF).
* ^ A B C D E Tavernier, Karel. "PCB Fabrication Data - A Guide".
Ucamco. Retrieved 8 January 2015.
* ^ Vermeire, Filip. "PCB Fabrication Data Example 1". Ucamco.
Ucamco. Retrieved 7 January 2015.
* ^ Vermeire, Filip. "PCB Fabrication Data Example 2". Ucamco.
Ucamco. Retrieved 7 January 2015.
* ^ A B "The Gerber