Werner,
If gold does not diffuse into the copper, or vice versa, on gold
plated directly over copper without a barrier coat, then I personally have
seen magic. Many years ago, not knowing better, I worked for a very small
company that made and used exclusively solder coated boards for printed
wiring assemblies. An end user asked for gold edge connectors. We
fabricated the boards, and asked an external house to plate the gold. And
that is what they did: Gold only over copper. The boards looked good on
their return, but over the weekend, the gold disappeared, leaving the copper
with a slightly bronze look that continued to fade over time. Where did the
gold go?
Greg Anderson
Advanced Manufacturing Engineer
GE Fanuc Automation, N.A.
----------
From: Engelmaier[SMTP:[log in to unmask]]
Sent: Wednesday, April 22, 1998 11:01 PM
To: [log in to unmask]
Subject: Re: [TN] 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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