Hi Werner!
     
     The danger of not wetting to the base metal surface can also happen 
     with tin/lead finishes so I don't view the emergence of the 
     alternative finishes as a bad thing - we just need to be smarter than 
     we have in the past and not repeat those same mistakes again. Wetting 
     balance testing is one tool that would give some indication of the 
     wetting phenomena at the base metal surface. Also the SERA test method 
     can give indications if a base metal/surface interaction is occurring 
     at a surface. The new alternative finishes will also push the 
     solderability specifications to the next level of measurement 
     assessment (or at least I hope they do!). If anyone has some ideas 
     about new solderabiltiy test methods the ANSIJ-STD-002/003 committee 
     would love to hear about them. Werner - did the IEEE Compliant Lead 
     Task Group ever publish their investigation findings and where ( I 
     want to get a copy!)?
     
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask] 


______________________________ Reply Separator _________________________________
Subject: Re:Re:Re:Re: silver vs. HASL/OSPs
Author:  [log in to unmask] at ccmgw1
Date:    12/16/96 8:12 AM


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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