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Stripping solder / gold plating pcb fingers

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  • Stripping solder / gold plating pcb fingers

    We recently had a need to gold plate some printed circuit board edge fingers -- quickly. Once you get the solder stripped off, the normal Caswell stuff works great, assuming your layout busses the pins together on a part of the board that will be sawn off later. We used nickel, then gold. I'd asked their tech support about what would strip approximately 7 mil thick solder off of approximately 3 mil copper without eating the copper, and they didn't know, so since I now do, I thought I'd share.

    First, there are some commercial Solder Strippers you could find with a google search. I didn't try any of them so I can't comment. But reading the MSDS on some of them, they seem to be composed of mixed nitric acid and hydrogen peroxide. So, I went back to the chemistry set and tried this, using the common 70% nitric acid mixed with about 1/3 to 1/2 by volume 12% hydrogen peroxide (this is "40 volume", which you can get at a beauty supply store, the stuff in the grocery store or pharmacy is only 3% and may not work. The 40 volume stuff hurts skin almost as fast as the nitric acid, it's not tame at all.). It works well applied with a Q tip or the slightly more exotic "Eye-deal" tip -- you do this just like brush plating. The fact that the tip will hold only so much of this concoction helps keep you from going through the copper as well. I haven't tried this in a bath yet as it's plenty quick to do by hand.

    Wear rubber or better gloves and eye protection of course.

    You can dip this in SP degreaser when done to stop any residual action, or just use baking soda solution. Don't soak the board in degreaser for any length of time, as it oxidizes the remaining solder badly. This works about like wiping off powder coat with acetone -- shazamm.

    Nitric acid alone eats either solder or copper fairly rapidly, but it's very nasty to work with (fumes, and will eat through most gloves -- or nearly anything -- in time). Adding the peroxide slows it down somewhat on both, but more so on the copper. Up to half peroxide seems to work OK, and the diluted nitric acid doesn't fume visibly anymore.

    Why would I care? Consider this. I can get solder plated PCBs for about a $40 setup charge, in two day turnaround from AP Circuits ( a good outfit I've been dealing with for decades). A typical order costs about $100. If I want "factory" gold fingers and solder mask, most board houses will charge $400-700 setup, and take around 20 days. Since my business does a lot of prototyping with lots of changes, it was well worth it to bring this in house.