>     Recently we have seen dewetting (~20 to 30%) that appears only on some
>of the fine pitch pads.  Larger pads with various components on the same
>boards are ok.  This has been seen on HASL boards from two different
>suppliers.  There have been no reports of problems with our regular daily
>production lots.
>     Any opinions would be welcomed.
>

Create a Theory
Poor rinsing in the soldermask developer allowing soldermask to remain, and
get deposited/dried on the small pads.  I suspect that solution is trapped
in the "deep" slot between the soldermask and the pad and then is blown out
onto the pads by the dryer.

Test the Theory:
Is the smallest gap between the soldermask and the pads on the fine pitch
parts?  Does this occur only when there is a little misregistration of the
mask, creating very small gaps between mask and pad?

Sell the Theory:
The theory explains why only the small pads, and not the large ones (the
small gap area in only near the fine pitch), the random nature (20-30% due
to dryer splatter), and one some boards (changes in registration, rinse
cleanliness, orientation of the gaps with respect to spray nozzle and dryer
air knife.)

Disclaim the Theory
The above theory explains all the reported observables, any correlation
with reality is entirely coincidential.


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                           George Franck
  PWB Product Assurance                     Phone (703) 560-5000 x2648
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