Oh, yes. This often happens when the solder mask isn't properly cured,
particularly with HASLed boards. The flux gets into the mask and doesn't
come out, at least quickly, again until the mask is exposed to
temperatures above its Tg. Anyway, I've had customers talk to me about an
ECM problem, thinking it's a flux problem when it was really a mask
problem. Of course, sometimes customers really have flux problems, but not
always.
I believe Terry Munson has written about this issue in his advice column
in one of the trade magazines.
Talk to your PCB fabricator. Sometimes a UV "bump" during mask cure help
alleviate this issue. Thorough washing with HOT DI water after HASL helps
also.
And yes, HASL fluxes trapped in boards can lead to CAF problems.
Karen Tellefsen - Electrical Testing
[log in to unmask]
908-791-3069
"Kane, Joseph E
(US SSA)"
<joseph.kane@BAES To
YSTEMS.COM> [log in to unmask]
Sent by: TechNet cc
<[log in to unmask]>
Subject
[TN] PB Surface Contamination
05/07/2009 09:14
AM
Please respond to
TechNet E-Mail
Forum
<[log in to unmask]>
; Please respond
to
"Kane, Joseph E
(US SSA)"
<joseph.kane@BAES
YSTEMS.COM>
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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