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April 1998

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Subject:
From:
Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 22 Apr 1998 23:01:44 EDT
Content-Type:
text/plain
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text/plain (43 lines)
Hi Scott,
You have a legitimate question, but the reponses you got....well.
The intermetallic compounds (IMCs) of concern for solder joints are all tin
based; thus we have AuSn, AuSn2, AuSn4, Ag3Sn, Cu3Sn, Cu6Sn5, Ni3Sn2, Ni3Sn4,
Ni3Sn7.
All of these IMCs are brittle, the important ones CuxSny and NixSny are
strong, wheras AuxSny and AgxSny IMCs are weak, leading to what has been
termed 'gold embrittlement' when concentrations get too high. The dissolution
rates into Sn are for Au:Ag:Cu:Ni about1000:200:30:1.
Originally, Ni was used as a barrier layer for Ag-metallizations to prevent
the formation of Ag3Sn IMC. On PCBs the Ni layer provide the possibility to
provide a single surface treatment that is both solderable (albeit
significantly less so than Cu) and wirebondable. I do not no where the notion
of "Cu diffuses into the Au" or that"Ni acts as a barrier layer to prevent the
formation of CuAu intermetallics" comes from, but that is not the case.
The Au on the Ni has the primary function of preventing the oxydation of the
Ni (much the same as the solder coverage of Cu pads); it also helps the
spreading —not the wetting—of the liquid solder over the Ni surface. To
properly wet to Ni you need higher reflow temperatures (about 15C) than for
Cu.
If you do not need wirebonding, you are definitely better of with solder
coated Cu.

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]

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