Hi Paul & John,
The original question had to do with soldering only, when one looks at what is
important for edge connectors other considerations become more important. I
agree with most of John's response of 4/23. Au is not a good coating over Cu
to preserve solderability and the primary reason is Au porosity and the
oxydation of the underlying metal. That explains the time-dependent reduction
in solderability as well as the increasing resistivity. But Au does not create
a good "intermetallic bond between the solder, Au, and Ni"; this bond is
brittle and weak and is the reason for gold embrittlement. In fact, proper
reflow soldering with Au present should keep the solder liquid long enough
that the AuSn intermetallics are as evenly distributed throughout the solder
joint as possible to avoid the concentrations leading to gold embrittlement.
Our problem is that we are trying to have one surface preparation do multiple
functions—that typically means that one or all of these functions are
performed less than optimally as compared to a single-function surface. But,
we do live in the real world, where what is best is frequently too expensive.

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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