Hi Werner,
Thanks for the interesting story. So, if silver forms brittle IMC with tin at
about 3%, why do people use (and pay for) Sn62/Pb36/Ag2, Sn96/Ag4 and even
Sn95/Ag5? Are all these silver-bearing alloys inherently brittle?
Is there any hard evidence that the failure mode in this particular TI case back
in 1982 was related to Sn/Ag IMC and not to something else -- say for instance
-- electroplating defect?
Eric Yakobson
Alpha PC Fab
[log in to unmask] on 05/05/99 03:01:20 PM
To: [log in to unmask]
cc:
Subject: Re: [TN] Silver coating on connectors
In a message dated 5/5/99 12:25:22, [log in to unmask] writes:
>Hi Werner,
>Sorry, I've never heard of "silver embrittlement", which is worse than gold
>embrittlement before. Perhaps you could enlighten us? Also, I'm not quite
sure
>I understand how the nickel barrier layer would help to eliminate it.
>Thanks.
>Eric Yakobson
>Alpha PC Fab
Hi Eric,
The industry found out
๙and since forgot except for a few old-timers like
me
๙in 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.
Silver, it turns out creates an even weaker intermetallic compound (IMC) with
tin than does gold. The necessary IMC concentration necessary for this effect
is about the same as for Au.
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, 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.
If you have a nickel barrier over Ag-containing component metallization, the
Ni will prevent AG from coming in contact with Sn. Of course, the Ni needs to
be protectd by a Au layer to prevent oxydation.
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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