Sunday, March 15, 2009

Developing Dry-film Images


Unlike copper etching, developing properly laminated and imaged dry-film photopolymers is easily accomplished by swishing your board about in a conventional photographic processing tray. This derives from the fact that the organic and radiation chemists who developed (and continue to develop) these films have done an excellent job of creating an entire class of easy-to-use and environmentally safe materials that are almost as tough as nails if processed according to published guidelines. The benefit of this will become apparent below.

Processing Chemicals

Regardless of the developing equipment you use or the film vendor you buy from, most of the aqueous processable dry-films use the same basic developing and stripping solutions. The films are developed with a 1% (wt.) solution of sodium carbonate (common soda ash) operating at 100ºF (38ºC) ± 5ºF and stripped with a 3 to 5% solution of sodium hydroxide (caustic soda or lye) operating at 130ºF (54ºC) ± 5ºF. You can usually buy premixed concentrates of both from the film manufacturer (or distributor) but you will end up paying as much as 100 times what it will cost you to mix your own. In any case, it is always a good idea to start with fresh solutions until you have a good feel for the life expectancy of the developing and stripping chemistries based on your throughput.

The two easiest dry-film developing systems to implement in a small shop setting are tray developing and bubble-assisted developing. Tray developing is the easiest to set up but may severely limit the density of the circuit patterns that you are able to develop, as well as restrict the maximum hole size that you can reliably "tent" with photoresist. At Think & Tinker, we have tray developed 6" (15.2 cm) by 13" (33 cm) circuit patterns with typical features including 0.010" (0.25 mm) wide traces separated by 0.015" (38 mm) spaces. With care, we were able to tent 0.125" (3.18 mm) dia. through-holes (in 0.155" pads) well enough to survive low-pressure spray etching. Stripping is a straightforward matter of immersing the board in a heated solution of caustic soda and agitating the stripper until the photopolymer lifts off of the copper.

One of the prime advantages of aqueous process films is that the developing and stripping solutions are easy to neutralize (with hydrochloric acid), filter (to remove suspended particulates) and dispose of. In most cases, disposal does not become an issue until 10 to 15 square feet of 1 mil (0.001", 0.025 mm) dry-film has been processed for every gallon of solution.

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