Yes, we used to see this. We had HASL (I spared you the many Deweyisms
that could have followed) as a final finish. SMOBC that had not properly
cured was the major contributor. Other times poor pre-coat surface prep
and some questionable fusing fluids.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Joseph E (US
SSA)
Sent: Thursday, May 07, 2009 6:13 AM
To: [log in to unmask]
Subject: [TN] PB Surface Contamination

A cleaned bare PB is exposed to bake or reflow, and comes out with
measurable ionic contamination.  The board is dirtier at the surface
after 
a heat excursion.

The contamination isn't coming from the oven, handling, or anything
external.
Heat causes something to move from the inside of the board to the
outside.

We haven't done any ion chromatography or other testing yet to
run this down, but has anyone heard of or experienced anything similar?

In our normal process, we clean everything multiple times, but if the 
contaminant builds up under solder mask, or in other areas that are 
difficult to clean, we could have SIR or corrosion problems around fine 
surface features or in high voltage areas.

And if this ionic contamination doesn't just move up to the surface,
but is somehow moving around inside the board, could it collect in 
some places and lead to CAF or other problems?

Joe Kane
BAE Systems
Johnson City, NY 


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