Hi Steve
It may be a photo construct, but everything looks dulled with a patina of
something to me..
The white residues in the repair area look like flux puddling which has not
been completely removed. The area around the nut/bolt looks different,
ingress of something into damaged laminate area, but looking at a picture is
not the same as eyeballing direct!
The heating idea is a pretty good diagnosis, short of a pukka lab
examination. Flux resins melt from say 80C, so if you have partially removed
them they will melt and revert back to clear, thereby becoming invisible.
So the question would be why didn't they clean off properly?
So this may be a cleaning issue or possibly
- The resist is slightly porous from under curing or some reason. Try
pre-baking a board for a couple of hours before dabbing some flux on and
cleaning and then seeing if you generate white residues. Do the same with an
unbaked board and an un-fluxed board at the same time as a control.
- Another possibility: incompatibility. The rework flux is in someway
incompatible with the original flux and the cleaner is "developing" that
incompatibility like a photo. If you are using an IPA based liquid flux for
rework and this is applied to a normal resin base flux residue, the original
residue doesn't see as you do, another flux. It responds to the IPA and
partially dissolves in it. Partially removed flux is generally harder to
remove the second time around. This is very much the case with very low
solids no resin/rosin flux types which in this context are really just dirty
IPA. Alternatively the cleaner is set/selected for the assembly flux not the
rework material.
Hope this helps
Regards
Mike Fenner
Indium Corporation
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, January 20, 2009 1:43 PM
To: [log in to unmask]
Subject: [TN] Strange Mask Appearance
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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