Chemical milling or industrial etching is the
subtractive manufacturing
Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...
process of using baths of temperature-regulated etching chemicals to remove material to create an object with the desired shape.
Other names for chemical etching include photo etching, chemical etching, photo chemical etching and
photochemical machining
Photochemical machining (PCM), also known as photochemical milling or photo etching, is a chemical milling process used to fabricate sheet metal components using a photoresist and etchants to corrosively machine away selected areas. This process ...
. It is mostly used on metals, though other materials are increasingly important. It was developed from armor-decorating and printing
etching
Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other types ...
processes developed during the
Renaissance
The Renaissance ( , ) , from , with the same meanings. is a period in European history marking the transition from the Middle Ages to modernity and covering the 15th and 16th centuries, characterized by an effort to revive and surpass ideas ...
as alternatives to
engraving
Engraving is the practice of incising a design onto a hard, usually flat surface by cutting grooves into it with a Burin (engraving), burin. The result may be a decorated object in itself, as when silver, gold, steel, or Glass engraving, glass ...
on metal. The process essentially involves bathing the cutting areas in a corrosive chemical known as an etchant, which reacts with the material in the area to be cut and causes the solid material to be dissolved; inert substances known as maskants are used to protect specific areas of the material as
resist
A resist, used in many areas of manufacturing and art, is something that is added to parts of an object to create a pattern by protecting these parts from being affected by a subsequent stage in the process. Often the resist is then removed.
For ...
s.
History
![1600 Milan Half Armor](https://upload.wikimedia.org/wikipedia/commons/6/65/1600_Milan_Half_Armor.jpg)
Organic chemicals such as lactic acid and citric acid have been used to etch metals and create products as early as 400 BCE, when vinegar was used to corrode
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 ...
and create the pigment
ceruse
Venetian ceruse, also known as ''blanc de ceruse de Venise'' and Spirits of Saturn, was a 16th-century cosmetic used as a skin whitener. It was in great demand and considered the best available at the time, supposedly containing the best qualit ...
, also known as
white lead
White lead is the basic lead carbonate 2PbCO3·Pb(OH)2. It is a complex salt, containing both carbonate and hydroxide ions. White lead occurs naturally as a mineral, in which context it is known as hydrocerussite, a hydrate of cerussite. It was ...
. Most modern chemical milling methods involve alkaline etchants; these may have been used as early as the first century CE.
Armor etching, using strong mineral acids, was not developed until the fifteenth century. Etchants mixed from salt, charcoal, and vinegar were applied to plate armor that had been painted with a maskant of linseed-oil paint. The etchant would bite into the unprotected areas, causing the painted areas to be raised into
relief
Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term ''relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that the ...
. Etching in this manner allowed armor to be decorated as if with precise engraving, but without the existence of raised
burrs; it also prevented the necessity of the armor being softer than an engraving tool. Late in the seventeenth century, etching became used to produce the graduations on measuring instruments; the thinness of lines that etching could produce allowed for the production of more precise and accurate instruments than were possible before. Not long after, it became used to etch trajectory information plates for cannon and
artillery
Artillery is a class of heavy military ranged weapons that launch munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during siege ...
operators; paper would rarely survive the rigors of combat, but an etched plate could be quite durable. Often such information (normally ranging marks) was etched onto equipment such as
stiletto
A stiletto () is a knife or dagger with a long slender blade and needle-like point, primarily intended as a stabbing weapon.Limburg, Peter R., ''What's In The Names Of Antique Weapons'', Coward, McCann & Geoghegan, , (1973), pp. 77-78
The stile ...
daggers or shovels.
In 1782, the discovery was made by John Senebier that certain resins lost their solubility to turpentine when exposed to light; that is, they hardened. This allowed the development of
photochemical milling, where a liquid maskant is applied to the entire surface of a material, and the outline of the area to be masked created by exposing it to UV light. Photo-chemical milling was extensively used in the development of photography methods, allowing light to create impressions on metal plates.
One of the earliest uses of chemical etching to mill commercial parts was in 1927, when the Swedish company Aktiebolaget Separator patented a method of producing edge filters by chemically milling the gaps in the filters. Later, around the 1940s, it became widely used to machine thin samples of very hard metal; photo-etching from both sides was used to cut sheet metal, foil, and shim stock to create shims, recording heat frets, and other components.
Applications
Etching has applications in the
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 ...
and
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
fabrication industries. It is also used in the aerospace industry
to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes. Etching is used widely to manufacture
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s and
Microelectromechanical systems
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
.
In addition to the standard, liquid-based techniques, the semiconductor industry commonly uses
plasma etching
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch speci ...
.
Process
Chemical milling is normally performed in a series of five steps: cleaning, masking, scribing, etching, and demasking.
[Video of chemical milling processLearn more about the video]
Cleaning
Cleaning is the preparatory process of ensuring that the surface to be etched is free of contaminants which could negatively impact the quality of the finished part. An improperly cleaned surface could result in poor adhesion of the maskant, causing areas to be etched erroneously, or a non-uniform etch rate which could result in inaccurate final dimensions. The surface must be kept free from oils, grease, primer coatings, markings and other residue from the marking out Marking out or layout means the process of transferring a design or pattern to a workpiece, as the first step in the manufacturing process. It is performed in many industries or hobbies although in the repetition industries the machine's initial set ...
process, scale (oxidation), and any other foreign contaminants. For most metals, this step can be performed by applying a solvent substance to the surface to be etched, washing away foreign contaminants. The material may also be immersed in alkaline cleaners or specialized de-oxidizing solutions. It is common practice in modern industrial chemical etching facilities that the workpiece never be directly handled after this process, as oils from human skin could easily contaminate the surface.
Masking
Masking is the process of applying the maskant material to the surface to ensure that only desired areas are etched. Liquid maskants may be applied via dip-masking, in which the part is dipped into an open tank of maskant and then the maskant dried. Maskant may also be applied by flow coating: liquid maskant is flowed over the surface of the part. Certain conductive maskants may also be applied by electrostatic deposition
Electrostatic spray-assisted vapour deposition (ESAVD) is a technique (developed by a company called IMPT) to deposit both thin and thick layers of a coating onto various substrates. In simple terms chemical precursors are sprayed across an electr ...
, where electrical charge
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
s are applied to particles of maskant as it is sprayed onto the surface of the material. The charge causes the particles of maskant to adhere to the surface.
Maskant types
The maskant to be used is determined primarily by the chemical used to etch the material, and the material itself. The maskant must adhere to the surface of the material, and it must also be chemically inert enough with regard to the etchant to protect the workpiece. Most modern chemical milling processes use maskants with an adhesion around ; if the adhesion is too strong, the scribing process may be too difficult to perform. If the adhesion is too low, the etching area may be imprecisely defined. Most industrial chemical milling facilities use maskants based upon neoprene elastomers
An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic po ...
or isobutylene-isoprene copolymers.
Scribing
Scribing is the removal of maskant on the areas to be etched. For decorative applications, this is often done by hand through the use of a scribing knife, etching needle or similar tool; modern industrial applications may involve an operator scribing with the aid of a template or use computer numerical control
Numerical control (also computer numerical control, and commonly called CNC) is the automated control of machining tools (such as drills, lathes, mills, grinders, routers and 3D printers) by means of a computer. A CNC machine processes a pie ...
to automate the process. For parts involving multiple stages of etching, complex templates using colour codes and similar devices may be used.
Etching
Etching is the immersion of the part into the chemical bath, and the action of the chemical on the part to be milled. The time spent immersed in the chemical bath determines the depth of the resulting etch; this time is calculated via the formula:
:
where ''E'' is the rate of etching (usually abbreviated to ''etch rate''), ''s'' is the depth of the cut required, and ''t'' is the total immersion time. Etch rate varies based on factors such as the concentration and composition of the etchant, the material to be etched, and temperature conditions. Due to its inconstant nature, etch rate is often determined experimentally immediately prior to the etching process. A small sample of the material to be cut, of the same material specification, heat-treatment condition, and approximately the same thickness is etched for a certain time; after this time, the depth of the etch is measured and used with the time to calculate the etch rate. 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 ...
is commonly etched at rates around , and 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 ...
about
Demasking
Demasking is the process of clearing the part of etchant and maskant. Etchant is generally removed with a wash of clear, cold water. A de-oxidizing bath may also be required in the common case that the etching process left a film of oxide on the surface of the material. Various methods may be used to remove the maskant, the most common being hand removal using scraping tools. This is frequently time-consuming and laborious, and for large-scale processes may be automated.
Common etchants
;For 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 ...
* sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
* Keller's reagent Keller's reagent can refer to either of two different mixtures of acids.
In metallurgy, Keller's reagent is a mixture of nitric acid, hydrochloric acid, and hydrofluoric acid, used to etch aluminum alloys to reveal their grain boundaries and or ...
;For 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
* hydrochloric
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. It is a component of the gastric acid in the digestiv ...
and nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
s
* ferric chloride
Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The colo ...
for stainless steels
* Nital
Nital is a solution of nitric acid and alcohol commonly used for etching of metals. It is especially suitable for revealing the microstructure of carbon steels. The alcohol can be methanol, ethanol or methylated spirits.
Mixtures of ethanol and ni ...
(a mixture of nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
and ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
, methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
, or methylated spirits
Denatured alcohol (also called methylated spirits in Australia, Canada, Ireland, New Zealand, South Africa, and the United Kingdom; wood spirit; and denatured rectified spirit) is ethanol that has additives to make it poisonous, bad-tasting, fou ...
for mild steels.
2% Nital is common etchant for plain carbon steels.
;For copper
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 pinkis ...
* cupric chloride
Copper(II) chloride is the chemical compound with the chemical formula CuCl2. The anhydrous form is yellowish brown but slowly absorbs moisture to form a blue-green water of crystallization, dihydrate.
Both the anhydrous and the dihydrate forms ...
* ferric chloride
Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The colo ...
* ammonium persulfate
Ammonium persulfate (APS) is the inorganic compound with the formula (NH4)2S2O8. It is a colourless (white) salt that is highly soluble in water, much more so than the related potassium salt. It is a strong oxidizing agent that is used as a cataly ...
* ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
* 25-50% nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
.
* 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 ...
and hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%†...
;For silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
* hydrofluoric acid
Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...
See also
*Photochemical machining
Photochemical machining (PCM), also known as photochemical milling or photo etching, is a chemical milling process used to fabricate sheet metal components using a photoresist and etchants to corrosively machine away selected areas. This process ...
*Etching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete.
For many etch ...
*Electroetching
Electroetching is a metal etching process that involves the use of a solution of an electrolyte, an anode, and a cathode. The metal piece to be etched is connected to the positive pole of a source of direct electric current. A piece of the same m ...
Notes
References
*
{{Decorative arts
Etching
Metalworking
Chemical processes
Electrolysis
Metallurgical processes