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October 1998

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 2 Oct 1998 13:22:57 EDT
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In a message dated 10/2/98 11:44:31, you wrote:

>Mr Engelmaier,
>
>Are you sure about some of the statements made, with regards to Ag and gold
>being similarly solderable, and that the nickel is a means of gaining a
>solderable surface?
>
>Nickel underplate is simply a diffusion barrier for copper, upon which the
>gold is plated to prevent the nickel from tarnishing. A copper surface (not
>sure about brass) is very solderable, hence the development of OSPs, and
>the like, however, gold cannot be used as a solderability preservative on
>top of the copper because the copper will diffuse through the porous gold,
>hence, nickel is first plated, followed by a typical, 8 microinches gold
>layer. Too thick a gold deposit will lead to solderability issues with the
>Au-Sn intermetallic, hence the ratio of solder thickness to gold thickness
>must be controlled closely, to achieve a good bond. Thermosonic wire
>bonding has different parameters.
>
>LK
Hi 'LK,'
I think we pretty much said the same thing, but came from different
perspectives or emphases.
Certainly, Au or Ag should not be plated on Cu; there is not any need for it
and OSP or Sn/Pb are much better. The nickel is not just a diffusion barrier,
but also the surface to which the actual solder connection is made. That means
the Ni has to be kept solderable, hence the Au-overplate, but it also means
that it has a good bond to whatever is underlying the Ni or you could get
adhesive failure of the Ni from the underlying metal. Alternatively, the Ni
could be thick enough to form a quasi-exoskeleton that is selfsupporting.
And the problem with thick gold overplates is not so much solderability, but
too much Au-Sn IMCs, which are not only brittle, as are all IMCs, but also
weak (most IMCs are rather strong).

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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