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Date: | Wed, 27 Jul 2005 08:11:54 -0500 |
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At a previous job, we did SMT followed by chip-and-wire on a regular basis
for many years, through several product life cycle iterations. The best
solution to keeping the wirebond pads clean was a peel-off temporary solder
mask. Tech Form, which is now a division of Kester, has a whole series of
temporary masks. Selection of specific product depends on the cleaning
processes you use after SMT. Some can be stencil printed down to just a few
mils thick, so as not to mess up stencil printing your solder paste. Oven
cures in a half hour or less, peels off with tweezers right before die
bonding.
As for gold intermetallics with 40 microinches of gold, we had a
metallurgist work with us and established that as long as time above
liquidus was sufficient, the IMCs were distributed (dissolved?) enough into
the joint so as not to be a risk.
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