Hi Ryan & Earl,
Ryan got it right--thicker IMCs are not better, it is just not the big 'bad'
deal some people make it out to be. Any evidence of an IMC layer is proof of
a metalurgical bond to the base metal; so thicker does not proof anything
further about proper 'wetting.' For some base mtals, in particular
electrolytic Ni and Alloy 42, it is difficult to get any good indication of
IMCs.
The thicker the IMC layer(s), the more Sn has been used up from the adjacent
solder region and the more Pb-rich region is present. However, there is no
evidence that that has a measurable bad effect [as rework cycle experiments
have shown]--but clearly it is not better. One of the reasons why thin HASL
layers have a limited shelf life, is that the outer surface contains a layer
of non-solderable Pb-oxide and the layer underneath is too Pb-rich--and
therefore has a higher Liquidus temperature--and does not reach a
sufficiently low viscosity during reflow to break up and sweep away the
Pb-oxide layer [this mechanism was pointed out by Colin Lea].
While I agree that "Times were much simpler when ?wetting,? ?smooth,? and
?shiny? were terms embraced and accepted by everyone in industry," they were
definitely not better--also we did not push the envelope as much as now.
Also remember, there are experts and then there are 'experts.' Lots of
practical experience does not guarantee scientific understanding and vice
versa.

Werner Engelmaier

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