Ken,
Your shorting problem may be due to inadequate curing of the component
adhesive, an outgassing product from the adhesive cure or an unclean surface
under or around the adhesive.

Not all adhesives, even those marketed for "electronics use", are suitable
for use (in my opinion).  Our guidelines are that an adhesive should have
less than 100 ppm of chloride, as measured by ion chromatography.  Some
adhesives are designed more for metals joining and have much higher levels of
chloride.  Chloride, mixed with humid conditions, creates leakage currents.

Your observation that the problem goes away after a thermal process suggests
that the problem is ionic contaminant related.  Electrochemical failures need
three things to occur - electricity, moisture, and an ionic contaminant.
 Take away any one of the three and no problems.  In your thermal process,
you drove away the moisture and the problem went away.  I bet it would come
back after exposure to humid conditions.

Since you can find no physical evidence of a dendrite (yes, they can grow
through adhesives, but takes more voltage) it tells me it is a contaminant on
the surface or absorbed into the adhesive.  What are you using as a flux in
the wave solder process and how is the flux applied?  Does this happen
consistently or was this a one time problem?

If you used old adhesive (past its shelf life), then it may not have cured
fully and may have absorbed flux in wave soldering.  The same effect would
occur if you had good adhesive but insufficiently cured it.

Hope this was helpful.

Doug Pauls
Technical Director
CSL

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