Hi Werner,
Very interesting information. Was all silver dissolved in the solder joint during soldering or may the IMC in the interface between solder and the metal surface have consisted of AgSn IMC?
Per-Erik Tegehall
IVF
Sweden
-----Ursprungligt meddelande-----
Från: Werner Engelmaier [mailto:[log in to unmask]]
Skickat: den 21 februari 2003 20:15
Till: [log in to unmask]
Ämne: Re: [TN] Question For Werner (Ag More Detrimental than Au?)
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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