Mike:

The problem of incomplete wetting by the solder has been with us as long as
the HASL process (actually was with us even before HASL, back when the solder
was plated, and then fused, and then "dewetted").  In essence, the problem is
usually caused by some contaminant, even enough Copper oxide can do it,
keeping the molten solder from contacting the Copper metal.

Using fluxes that do not coat well, or are not active enough, or allowing the
panels to sit before HASL, all can cause the problem.   One of the lesser
known, but certainly pretty common causes is incomplete/poor developing of
the soldermask.

And, of course, the whole problem is only exaggerated by the smaller
geometries in the boards today.

Your vendor has to get their process in order, and they should be able to
supply uniformly wetted surfaces.

Rudy Sedlak
RD Chemical Company

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