The Photochemical Machining Process
The photochemical machining (PCM) process consists of several important steps. Each step is a critical prerequisite to the next. Although companies that produce parts using the PCM process have unique and sometimes proprietary methods to achieve the final goal, the basic operations are similar.
Once the material has been cut to the specified sheet size, it’s chemically cleaned to remove all foreign debris, dust, grease, oil and other contaminants from the surface so that the photoresist can adhere to the metal. Many years ago, the common cleaning method for metal sheets in the PCM industry was solvent degreasing with solvents such as trichloroethylene. The environmental hazards of using these solvents became quite apparent, and, in the late 1980s, PEI became the first PCM manufacturer to have a solvent-free facility. Coming up with alternative degreasing procedures was a huge undertaking, but it put PEI on the path to becoming the first company in the photo chemical machining industry to “go green.”
Today, most metals are cleaned using mild soaps. Sheets of metal are fed through a large four-chamber/stage conveyor spray machine, with each chamber containing a different concentration of cleaning agents, as well as three stages of rinsing after the cleaning. Using high pressure, and rinsing the sheets with two chambers of city water before a final rinse of deionized (DI) water, insures that the sheets are truly clean and ready for the next step in the PCM process.
The next step in the process is the application of photoresist. Because PEI uses an aqueous dry-film photoresist, we refer to it as “the coating operation.” The coating operation is exactly what it sounds like - dry-film photoresist is rolled onto the cleaned metal sheets using a hot-roll lamination system to sensitize the metal. At this point in the process, the sheets of metal are processed in a yellow safe-light environment.
The photoresist is protected by a plastic layer to prevent damage prior to the next two stages. Depending on the application, different photoresists are used. PEI only uses dry film photoresists (rather than liquid types) because dry film makes more sense for volume applications, tends to be more controllable, and in most cases can be easily removed during the stripping process. Dry film can also be used to mask for selective plating or other types of finishing operations.
While PEI typically coats both sides of a metal sheet previously cut to size, in some cases, metals in thinner gauges can bypass the prep process and the resist can be applied to the entire coil. This is because thinner gauge material is generally clean of contamination, and will not be exposed to the harsh chemistry long enough for the photoresist to fail. Once the sheet or coil is coated with photoresist all edges are trimmed of the excess. At this point the material is ready for exposure.
Developing/Creating the Phototool
Photochemical etching uses Mylar film to make a working tool, as opposed to the “hard” tools used in metal stamping. When a new part is ordered, a phototool has to be created to profile the part. PEI can create a phototool either from a fully-dimensional drawing supplied by the customer, or by working with a customer-supplied CAD file with detailed dimensions. Once the CAD file has been received, PEI’s tooling engineers will redraw the part in its entirety, modifying the dimensions in order to compensate for the unique characteristics of the PCM etching process. (All features must compensate for surface and lateral etching, which happen simultaneously during the etching process.)
Once the image is completed, it is arrayed over an area specified by the tooling engineer to maximize the yield per sheet of metal. After that, the image is sent to a laser photo-plotter and is created on a piece of Mylar film, which is then developed. If all features on both sides of the part are identical, a duplicate film is created; if not, a second piece of film is developed and aligned with the first. Holes are punched in the two pieces of film and they are aligned top to bottom. Registration holes are punched in both pieces of film. This is critical to prevent misalignment of the film during subsequent processing.
In the printing area, the phototool image is transferred to the metal sheet by activating the polymer within the photoresist. The metal sheet is sandwiched between two pieces of film (the phototool), the operator creates a vacuum to prevent air bubbles and ensure that there are no gaps between the phototool and the metal, and the panel is exposed to UV light.
The areas on the phototool that prevented the UV light from seeing the photoresist will not harden or cure, so an image is formed in the photoresist. PEI has different types of exposure units for all types of parts.
Once the phototool has been printed onto the metal sheet, it is ready to move into the wet process area for developing.
All sheets that have been coated and exposed must go through a conveyorized developing system. The unexposed areas of the photoresist are rinsed off in a high-pressure alkaline solution, exposing the bare metal and leaving resist on those areas that have to be protected from the etchant. The developing process forms an adherent, durable image on both sides of the metal, which is now ready for the etching process itself.
PEI uses several types of etchants, depending on the base metal being etched. Here are some examples:
|Base Material |
Ferrous alloys (stainless steel, kovar, etc.)
|Ferric Chloride |
|Non-ferrous alloys (brass, nickel-silver, beryllium copper, etc.) ||Cupric Chloride |
|Titanium and zinc and their alloys ||Copper Based Proprietary Etchant |
|Molybdenum and silver ||Ferric Nitrate |
|Gold and gold alloys ||Iodine-Based Etchant |
PEI segregates the acids so that no metals are introduced into the etchant, preserving its integrity. All of the etchants are monitored and tested for free acid and the ORP (oxidation reduction potential) is documented twice daily. PEI was one of the first companies to introduce chlorine as an oxidizing agent for our ferric chloride and cupric chloride etching. The chlorine re-oxidizes these agents and keeps them “healthy,” eliminating downtime and keeping production at an optimum level.
The metal sheets are placed in a large conveyorized spray system that sprays them from both sides with the appropriate etchant chemistry, dissolving the bare metal. The speed of the conveyor is carefully calibrated so when the sheets approach the final rinse chamber, they are completely etched into the proper dimensions.
After etching and rinsing, the sheets are ready to have the photoresist stripped off.
At this point, all of the etched metal sheets still have the photoresist remaining on the un-etched portions. They are either tank-stripped, or stripped on PEI’s 30-foot conveyor-ized spray stripper. The resist is removed using a sodium hydroxide-based solution, and once stripped, the sheets are cleaned in city water, then rinsed in deionized water and dried.
After the photoresist has been removed, the parts or sheets go on to be inspected and packed for shipment to the customer or prepared for secondary operations such as plating, forming, lapping, welding, soldering, etc.