That is a fascinating response, Werner!
I have never heard anyone contradict the "nickel barrier prevents copper
diffusing into gold" rule. Are there other common industry "myths" you
are willing to discuss?
Thanks for being here... Jack
-----Original Message-----
From: Engelmaier [SMTP:[log in to unmask]]
Sent: Wednesday, April 22, 1998 8:02 PM
Subject: Re: Ni plating and Immersion Au
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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