Bogert,
White staining on soldermask is usually caused by moisture absorption
during post-processing of the PWB, for example HASL or Ni/ Au finishing.
You may be seeing it one one side only for several reasons, some of which
are to do with the way the soldermask was applied & subsequently processed
by your supplier. During soldermask processing the main steps that can
affect white staining would be tack dry, exposure level and cure (post
bake). As you only see white staining on one side of the panels you could
ask your board supplier how the panels are processed and see whether the
component side had less tack dry or cure time as I would assume both side
will have similar exposure levels.
However one process that is side specific is the wave solder process as
poor rinsing of the flux can also cause white stains. Also if a hot panel
hits cold rinse water, the thermal shock can contribute to this effect.
Depending on the severity of the white staining, re-baking the panels
may have some effect - what conditions did you use?
My feelings would be that if the white stain is due to moisture
absorption there will always be a site for future breakdown/ dendritic
growth. For high performance/ MIL spec applications I would get your panels
redone and get your OEM to check rinsing conditions on the solder line.
For the board supplier, you could ask him to confirm with his soldermask
supplier the optimum processing conditions for the mask that he is using.
Just my 0.02 Euros worth...
David Albin
Coates Circuit Products (UK)
-----Original Message-----
From: Bogert [mailto:[log in to unmask]]
Sent: 30 March 2002 22:51
To: [log in to unmask]
Subject: [TN] White residue on LPI Solder Mask coated (and incompletely
cured) PWAs, Is it a technical concern?
3/30/002
Folks, we have an OEM who purchased FR-4 printed wiring boards from a
MIL-P-55110 qualified board manufacturer. The OEM wave soldered the PWAs
using HF1189A water washable, organic flux and cleaned the assemblies using
DI water in an in-line cleaning process. Subsequent Omegameter 600 ionic
cleanliness testing passed. However, almost all of the solder side of the
PWAs exhibited white residue. Basically it looks like a water mark. While
you can remove the residue using abrasion, such as by a pencil erasure, it
comes back, even after several subsequent baking operatioins and additional
in-line cleaning. The OEM sent out samples for outside chemical evaluation
and found minute traces of carbon. The OEM traced the problem to incomplete
curing of the LPI solder mask. The white residue only appears on the solder
side of the PWA. There is no evidence of the residue on the component side
of the assembly.
The OEM proposes that a sample assembly be sent out for SIR testing, and
that if the test passes, they will request us to accept the condition as
is. My concern is that over time in a humid environment, we may experience
leakage current paths that cound cause operasting failures. The OEM has 20
PWAs that are part of the lot. However, only 6 experience this
condition. It appears that I should reject all PWAs that show evidence of
the white residue. I am concerned with the presence of carbon, even in
trace amounts. The OEM noted that almost everything will show traces of
carbon, so not to worry.
Bottom line is, is there any technical concern if the white residue is
present? Any input on this topic wo\uld be appreciated.
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