Mornin' All!
 
We recently built several boards and discovered that we had to remove
two DIP IC's on the boards, re-program the parts, and then put them back
into the board.
 
We used our Air-Vac PCBRM-12 to do the job which made the task a simple
one. But after coming out of our inline DI wash, I noticed some
discoloration at the area where we did the rework:
 
http://stevezeva.homestead.com/files/Mask_Discoloration.jpg
 
At first I thought it was white residue and tried to use a brush with a
little saponifier and scrub the area. No luck. Tried alcohol, no luck
either.
 
After looking closely at it, I could see that it wasn't really residue,
but that the mask was discolored. If you look at the picture at the area
around the screw and nut, you'll see some of same type of discoloration
which was the outline of the liquid water soluble mask we applied prior
to wave solder.
 
I remember something that somebody posted on the Technet a long time ago
about using a heat gun on areas like that, and it might clear up. I have
a Hakko board pre-heater, so I put one of the assemblies on the
pre-heater and brought the board up to about 125 C. then let it cool
down. Removed the board and flipped it over, and like magic everything
cleared up!
 
I'm glad that it worked, but what was it that might have caused it? Mask
that wasn't fully cured? If that was the case, why wouldn't wave solder
finish the cure? Or baking? We wash and bake all our bare PCB's prior to
building as SOP.
 
 
Steve

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