1) What process or material problem could have caused the conductive
filaments to form, and how to prevent it from happening again?  My best
guess is contamination residue from the etch-back process.

**Les, I think Susan Mansilla hit the nail on the head.  Almost all the SEM
shots of dendrites that I have seen had the same tin, lead, copper that you
see.  I agree that the cause is most likely insufficient tin/lead etch
resist removal.  The only caveat might be - are you sure the dendrite grew
"under" the mask, as opposed to "through" the mask?  I have seen some cases
were poorly developed solder resist had dendrites grow through the bulk of
the solder mask, rather than under it.

2) If the PWB passes shorts testing and end-item functional test, is there
any concern with continued use in a humid environment given that the
contamination is trapped under a layer of solder mask as well as a layer of
Type UR conformal coating.

**Any concern?  Maybe.  What is the design life of the product and what are
the consequences of failure?  You have seen the failure mechanism on some
product, so it CAN occur.  How often will the unit be powered in the field?
How constant will the humidity be while power is on?  Urethane coatings are
pretty good at moisture inhibition and solder masks are a combination of
acrylics and urethanes. Moisture diffusion will be slower.  Growing
dendrites under a polymeric layer takes a much greater forcing function to
occur.  How much risk depends on how bad the contamination is.

Any info available on how long solder mask and conformal coating can retard
moisture entry?  I know that neither material will prevent eventual
moisture entry.  Is there any data that shows how long these materials will
retard moisture from causing dendrite growth problems?

**None of the data that I have done has quantified the link.  Dr. Craig
Hillman, University of Maryland, CALCE Center, has done some work in this
area and presently has such a project ongoing, if memory serves.

Doug Pauls

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