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I recently came into possession of two large (approx. 3x4 ft) copper-clad fiberglass boards from a junk sale. These boards are consistent with typical printed circuit board material (thickness, flexibility, internal appearance, etc.), save for a blue coating on both sides. Here are two small pieces I cut off for testing:

blue boards

Here I've sanded off some of the coating to reveal the copper underneath (the cuts are from me testing my PCB mill):

cut and sanded

The copper is conductive (unsurprisingly), and the coating is not.

Any ideas on what the coating might be? The boards are probably around 5-10 years old, and were covered in a thin plastic sheeting when I obtained them.

Specifically, I'd be interested in knowing (a) how to remove the coating without damaging the copper (solvents? acids? photoexposure?) and (b) if I could use the coating in some way to help make printed circuit boards (as a resist material? ablate with a laser cutter?).

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up vote 5 down vote accepted

The boards appear to be photoresist boards, i.e. bare PCBs factory coated with photoresist material, then protected by a thin sheet of plastic. Here is an example of such PCBs.

Over time, even if such boards are stored in a relatively dark place, the photoresist gets exposed and becomes unusable. It just needs to be scrubbed off for the bare PCB to be used like any other copper PCB.

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Hm. What do I scrub it off with? Sandpaper hasn't worked particularly well. The boards have been stored in god-knows-where so if it's photoresist, the resist has long gone bad. – nneonneo Jul 29 '14 at 2:16
Depending on the specific type of resist either acetone or dilute caustic soda in lukewarm water should work. Soak a piece of the board overnight in each and then rub off with paper towels. Please wear nitrile gloves and protective eyewear. Wash the boards afterwards with a lot of warm running water. – Anindo Ghosh Jul 29 '14 at 2:24
Scotch Brite scouring pads is what I use. Photo-resist actually hardens with exposure to light and/or age. The warm solvent soaking process, and mechanical scrubbing, will probably be your best bet. Sand paper will get gummed up rather quickly..especially for the sizes you are talking about. You can get the scrubbing pads from your local grocery store. If they get gummed up, you can rinse them clean and continue. Here is a link to a list of stripping solvents: photoresists.eu/photoresist_1932.html – Enemy Of the State Machine Jul 29 '14 at 4:49
I have had success with cleaning photoresist by using ethanol. – jnovacho Jul 29 '14 at 7:25
@AnindoGhosh: Sodium Hydroxide worked well! Thanks! – nneonneo Jul 30 '14 at 18:23

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