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)
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.