Photoetching is a chemical process that removes material from a substrate, and the chemical used is generally an acid (such as ferric chloride or hydrofluoric acid). To selectively remove material from a substrate, a mask is required that protects areas of material that are needed while unprotected areas are etched by a chemical solution.
The first step to photoetching is the application of a layer of photosensitive material on top of a substrate. A mask of the design is then placed on top of the photosensitive layer, and light (typically UV), is shone onto the mask to expose areas not covered by the mask. Once properly exposed, the mask is removed, and the photosensitive layer is developed to remove areas that were not exposed to UV light.
The substrate is then bathed in a chemical etchant which attacks areas uncovered by the mask. Once fully etched, the remaining material then has the photosensitive layer removed using a strong base (such as sodium hydroxide).
Any metal that works with common etchants can be used with the photoetching process. One of the most commonly photoetched materials is copper, and it is easily etched with ferric chloride. A nice advantage to ferric chloride is that the chemical itself does not attack organic tissue in the same way as it attacks copper meaning that ferric chloride spills are nowhere near as dangerous as other acids (such as sulfuric and hydrofluoric). Ponoko offers steel, copper, aluminum, and brass for photoetching in varying thicknesses and sizes.
Photoetching can be used to cut right through metal, but there are some factors that need to be considered. The biggest factor is a phenomenon known as undercutting which doesn’t occur with other mechanical methods.
As the chemical etchant eats the material, it starts to expose the metal directly underneath the mask through the sides, and the etchant can start to eat at these sides. The result is that the etchant can slowly attack the sides of the mask resulting in a design that is narrower than the mask. For thin pieces of metal, this is usually not a problem, but can be a major challenge for thick pieces of material. Generally speaking, photoetched metals are rarely thicker than 2mm due to undercutting.
Undercutting is a major problem for thick pieces of metal as the etchant attacks the sides of a design that are eventually exposed. But photoetching also suffers from developing process mistakes that can be tricky to get right.
Simply put, the photosensitive layer must be exposed for a specific amount of time for it to work correctly. If the mask is exposed for too long, UV light can penetrate the covered areas and partly develop the photosensitive layer underneath.
Photoetching is also a monochromatic process that does not handle greyscale images well. Tricks such as bit patterns can be used to give the impression of shade, but at the end of the day, areas are either etched or not.
Photoetching can be used for a wide range of applications that require precision and accuracy. PCBs use photoetching to create wires that connect components together (this copper is typically 65um in thickness).
Photoetching can be used to create very small mechanical parts such as cogs, gears, levers, and arms used in clocks. But photoetching can also be used to create signs, letters, and metallic shapes of great detail.
The photoetching process is entirely chemical meaning that there are no mechanical components involved. As such, there are no mechanical parts that wear down, nor are there any mechanical consumables that need replacing. The process is also far faster than mechanical processes as layer imaging generally takes a few minutes, and the etching process can take anywhere between one hour to two minutes (depending on the thickness of the metal).
This depends on the level of accuracy and precision needed in the final parts, but undercutting generally limits this thickness to around 2mm. Ponoko offers PCM materials up to 0.8mm in thickness.
Photoetched parts are found in a wide range of different industries due to the versatility of photoetching. For one, the printed circuit board industry is heavily dependent on photoetching methods as the same mask can be used many thousands of times while producing high-quality circuit traces with trace widths down to 5mil (0.005 inches).
The semiconductor industry is another sector that uses photoetching as its primary means for creating designs. However, the semiconductor industry has extremely specialized equipment to create transistors whose overall length is in the 10s of nanometers.
Artistic engravings can take advantage of photoetched materials by either fully etching material to create gaps in the final product, or just partly etch the material to create a shallow engraving. Artistic etchings can also take advantage of bit patterns to try and create shading to give the appearance of a full grayscale image.
Motor laminations are made from thin pieces of steel which are then sandwiched to other laminations. These thin pieces of metal are an essential component of motors, and the use of laminations helps to reduce the presence of Eddie currents. The extreme thinness of the metal makes photoetching an ideal process for creating precision lamination pieces.
Gaskets can also be used with the photoetching process. Not all gaskets are made from rubber or material, and copper gaskets can be useful for high-end vacuums and other scientific equipment. Such gaskets are often made from thin metal, and this makes photoetching ideal.