Some guesses at the contamination problem:

The edges of the board are protected prior to wave solder because
a large ground plane passes through this region.  The ground plane
is copper, plated with 60/40 tin/lead.  This ground plane is covered
with SR1000 solder mask.  When the ground plane is heated, the
tin/lead reflows and damages the solder mask.

One theory is that poor flux cleaning prior to adding the solder mask
is leaving residue.  This residue then penetrates the solder mask
when the board is heated.  Tape and stiffeners act to trap the
residue against the board.  Does anyone know if this is viable?
Boards are supposed to be cleaned with water prior to the addition
of the SR1000.

The only reason I question the tape theory is that we have been
using the same high temp Kapton style tape for this operation for the
past two years.  We've only just started experiencing contamination
problems.  Both old and new lots of tape have yielded the same
results...

Unfortunately, I don't have any older bare boards to test against the
new ones.

We are using MG 422 silicone based conformal coat.






Ian Slater, P.Eng.
Manufacturing Engineer
GE Harris Energy Control Systems
phone: (403) 214-4753
Fax:   (403) 287-3107

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