Hi George,
It is not a belief, but a fact. 
The industry found out—and since forgot except for a few old-timers like 
me—in 1982, what silver can do to solder joints when TI moved some of its 
PLCC production to Singapore. There somebody decided to Ag-plate the leads. 
At the first unfortunate recipient of this product, IBM-Austin, the PLCCs 
started to literally fall off the PCBs when slight bending, such during 
fixturing for functional testing, put even very small loads onto the solder 
joints. As a consequence of this scary news (nobody knew about the Ag yet and 
compliant leads were just invented and without history), 17 competing 
companies joined forces and formed the IEEE Compliant Lead Task Force with 
Jack Balde its chairman.  
Silver, it turns out creates an even weaker intermetallic compound (IMC) with 
tin than does gold. We never did determine whether there is a critical Ag 
concentration, similar to the 4% Au number that is bandied about [the 4% 
assumes an uniform dstribution, but typically there is a much larger 
concentration near the interface]. The IMC concentration necessary for this 
effect should be about the same as for Au.  The culprit, as with Au, were 
AgSn IMCs. All IMCs are brittle, most of them are strong except AuxSny and 
AgxSny (mostly Ag3Sn), which seem to be weak to boot. Further, some of these 
(Au, Ag) IMCs come in geometries (platelets) that also result in weak 
intergranular bonds.  When these structures come to the solder joint surface, 
they cause the dull "crazed" appearance.
Because of legal problems (the 2 companies with the lowest PLCC fatigue lives 
threatened to sue everybody involved in the IEEE CLTF), the results, except 
for some preliminary data and progress reports at IEPS and NEPCON 
conferences, were never published.
Nevertheless, the IEEE CLTF accomplished a lot. We found out that for 
accelerated testing one needed to continuously monitor; hence the development 
of the event detector. We correlated lead compliancy with fatigue life (see 
IPC-SM-785 and IPC-D-279). We learned not to Ag-plate SM components.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com 

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