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| Date: | Fri, 12 Jul 96 14:51:13 EDT |
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This is to give some input to the attached note about a nickel-
immersion gold finish.
We currently plate Ni-Au surface finishes for solderability and
wire bonding. For solderability only immersion gold is required, but
for wire bonding, a thicker gold is recommended: this one is deposited
by an electroless process.
So, to globally answer the 4 questions asked:
Immersion gold is an exchange reaction between the substrate (Ni)
and the gold ion in solution, by which gold gets deposited on the
surface. This is a self limiting reaction since when no more Ni is
exposed to the solution, the reaction stops. The maximum thickness
you'll get from such a process is of the order of 1/10 micron and
should be adequate for solderability.
Electroless gold is an autocatalytic process, which means that the
deposition will proceed as long as the part is in solution. This way,
a thicker deposit can be plated for wire bonding applications.
It is generally recognized in the literature that a soldered joint
should not contain more than around 2-3% Au in order to avoid
embrittlement.
I hope this answers some of your questions.
Are you interested in this surface finish as a replacement for an
existing one, or would it be required for a new application?
Richard Langlois
IBM Bromont, Canada
email: [log in to unmask]
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From: [log in to unmask] (APeder01)
Subject: FAB: Electroless Nickel - Immersion Gold
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My company is in the process of evaluating the use of Electroless
Nickel - Immersion Gold as a finish, for both SMT and mixed technology
PWBs. Is this a good plating system to evaluate?? Is there any
"conventional wisdom" that we're missing?
Secondly, some questions about the gold deposition.
1. Is "electroless gold" the same as "immersion gold?"
2. How thick can we expect the gold to deposit?
3. Is the gold deposition process a self limiting one?
4. How much gold is required to adequately keep the nickel solderable?
Any other comments regarding this topic is appreciated.
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