JW wrote in
:

Hello,

We have a problem with one of our circuit board vendors who applied
solder mask to the gold fingers on a lot of 100 boards. Naturally, the
100 boards were built up and the problem wasn't discovered until test.
Does anyone know of a way to remove it without losing the gold on the
fingers? Solvent or ??

Thanks.


I tried acetone and strong caustic soda solution, the stuff doesn't budge.

Assuming chemical is useless, and physical is likely to wreck the bits you
want, try optical.

An IR pulsed laser should be a good match for absorption by dark green, and
the gold will reflect it, as will the copper, and the partial trasparency
of the substrate should dissipate safely what isn't absorbed by the resist
layer.

In order of rising price (and general butchness) try hair removal lasers,
tattoo removal lasers, and stone cleaning lasers. Take care not to angle
the thing such that it can reflect off gold or copper right back up the
waveguide, as that will likely damage or even instantly destroy the laser
cavity, especially if it's a direct-from-diode sort of pulsed laser.