Tamir,
Run an experiment to see if the problem is related to board moisture
absorption of adhesive over temperature during cure. Take a reasonable
quantity of boards where one would expect to see failures, bake out half of
the boards prior to assembly at 125 degree C for 8 hours and then run both
sets of boards with only chips attached to see if solder shorting occurs.

Section portions of the boards under the components from the board bottom
up and look for bubbles or voids between the two different lots.

If it still occurs then examine the adhesive cure profile to make sure that
the adhesive does not exceed manufacturer's recommended maximum temperature
during cure. I have seen incidence where there is an exothermic reaction
between the resin and hardner during excessive temperature cure that
results in out gassing or bubbling of the adhesive.

Again section portions of the board from the bottom up to look for voids
under the part.

Regards,


John Maxwell
Please contact me off line at [log in to unmask] if there are any further
questions.

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