Photoresist & Soldermask Lamination
For the hobbyist, as well as the small board shop, copperclad can be reliably laminated using either a conventional "hot-roll" (or "heat-shoe") laminator or a lower cost "pouch" or "pocket" laminator (for very small volumes). Regardless of the method you choose, many of the same considerations apply.
Before using your laminator, there are a few precautions that should be observed.
- To prevent the possibility of electrical shock, do not operate near sinks, bathtubs, etc.
- Always check the electrical requirements printed on the bottom of your laminator before plugging into a wall outlet.
- If an extension cord or "outlet expander" is used, check its power rating to insure that it is compatible with the requirements of the laminator.
- Do not use the unit if the power cord or case is damaged in any way.
- Do not allow the power cord to touch hot surfaces.
- Turn the power off when the unit is not in use, unattended or not intended to be used.
- Do not rest anything (including your hand) on top of the unit when it is in operation as it can get quite hot.
- Many people are sensitive to the fumes produced by the thermal lamination of dry-film photopolymers. Only use these products in a well ventilated room or, if available, under a fume hood. Some laminator manufacturers now offer self-contained fume handling systems that effectively eliminate the emmission of these mildly toxic vapors into the environment.
- Always wear eye protection when operating the laminator.
- Never wear loose clothing or jewelery that might be drawn between the lamination rollers.
- Always wear gloves when cleaning and handling copperclad prior to dry-film lamination
- Dry-film lamination must be performed under incandescent or UV-proofed fluorescent lights.
- Store laminated boards in a light-tight enclosure if they will not be processed immediately.
- Your work area must be as free from airborne particulate contamination as possible.
Dry-film photoresist and soldermask
Many dry film products are supplied as a laminate consisting of two to three layers. In the case of common photopolymers (e.g. DuPont Riston 4615 photoresist), there are three layers; a peel sheet (or release-liner), the resist film, and a ultraviolet (UV) transparent cover sheet.
- Peel sheet: A polyolefin release liner that covers the thermally active adhesive on the bottom of the photopolymer. This layer is typically automatically removed (stripped) by the laminator immediately prior to lamination.
- Resist film: A UV curable, aqueous-developable, super-viscous-liquid film.
- Cover sheet: The outer covering of the multilayered film is usually made of heat resistant polyester (PET, a.k.a. Mylar). Since it functions as an oxygen block (see below) as well as a protective layer, it should remain in place on the outer layer of the resist until the board is ready to be developed. The cover sheet should be left in place at least 15 minutes after exposure and should only be removed imediately prior to developing.
Note on selecting dry film soldermask:
One of the purposes of all soldermask materials (liquid and dry-film) is to totally encapsulate underlying circuit elements, protecting them from the corrosive aspects of the intended operating environment. To accomplish this with a dry-film soldermask, you must chose a film that is thick enough to flow over and around the pads and traces of your circuit during lamination. A good rule of thumb to use, is to select a film that is at least twice as thick as the copper being covered. In more practical terms, if you are covering 1ounce (35 micron) copperclad, you should use a film that is at least twice as thick as the 0.0013" thick copper layer. For this application, a 3 mil (0.003") film would be preferred. Failure to follow this simple rule might result in thin, capillary gaps along the edges of pads and traces. These gaps are just about the perfect size to wick, and retain the corrosive fluxes used during wave and/or hand soldering, which could lead to the eventual failure of the board.