Unfortunately, I am not aware of any non-destructive way of telling whether
you have or not have good wetting to the base metal surface when that surface
had been covered by a metal easily soluble in tin. That is one of the dangers
with metallic surface coatings like Au and Ag; the other danger, of course,
is insufficiently long time above liquidus for the uniform dispersal of the
Au- or Ag- intermetallic compounds with tin to avoid too high a concentration
of the brittle and weak intermetallics. 
The effect of Ag-Sn intermetallics was amply demonstrated in 1983 when
Ag-plated PLCC-J leads from TI fell off circuit boards made by IBM-Austin as
the result of the brittle and weak Ag3Sn intermetallic compounds. This event
caused the formation of the IEEE Compliant Lead Task Group with 23 competing
companies participating to investigate this occurrence and to gain
understanding in the reliability achieved with compliant leads. 
I also, as a matter of principle, do not like the many overplates and other
coatings that seem come up with increasing frequency; every interface is a
potential source of trouble. I go with the motto from the Motorola quality
concept, to eliminate all unnecessary potential trouble sources--or more
simply put, I like the K.I.S.S. principle. 

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-mail: [log in to unmask]

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