What is Photo Chemical Machining?
The PCM Process by Professor David Allen, Cranfield University,UK. D.Allen@cranfield.ac.uk.
Terminology and products
Photo Chemical Machining (PCM) is also known as Photo Etching , Photo Chemical Milling, Photofabrication and Chemical Blanking. It plays a valuable role world-wide in the production of precision parts and decorative items, mainly sheets and foils.
Such products include:
- Color television shadow masks
- Integrated circuit lead frames
- Surface mount paste screens
- Heat ladders, plates and sinks
- Optical attenuators, choppers and encoder disks
- Grills, grids, sieves and meshes
- Washers, shims and gaskets
- Decorative ornaments
- Signs, plaques and nameplates
The PCM process:
artwork generation and phototool production
Various methods exist for the production of phototools. The most commonly-used method is to produce a phototool of the correct size by using a laser photoplotter to selectively expose a photographic film according to computer-aided design data. Once this master image has been made it may be ‘stepped and repeated’ and contact printed to form a multiple image, registered, double sided phototool for use in production.
Before coating the metal with photoresist, the metal is thoroughly cleaned to remove all dirt, rust, greases and oils so that good adhesion to the photoresist is obtained.
Photoresists are UV light-sensitive polymers that may be applied to the metal as a liquid by dip coating, flowing, roller-coating or electrophoresis. Dry film photoresists are applied by hot roller lamination.
The coating of the metal sheet with photoresist sensitises it. When this sensitised metal is put into the double-sided phototool and is exposed to UV light on both sides, an image is formed in the photoresist. This is then developed in a liquid formulation to form an adherent, durable image on both sides of the metal.
Virtually all materials can be etched, although some are etched more readily than others. The vast majority of PCM companies etch a very wide range of metals employing a relatively innocuous etchant, an aqueous solution of ferric chloride. It is important to ensure that the by products of the etching process are soluble in the etchant solution so that etching can proceed quickly. To maintain a constant etch rate, lower environmental impact and improve economics, some companies regenerate their ferric chloride etchant in-house.
Stripping and inspection
The final process of PCM is to strip off the resist, ensure that the metal is clean and that the dimensional specifications requested have been satisfied.