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
On Thu, 1 Oct 1998, Werner Engelmaier wrote:
> Hi Neil,
> Are you sure you have de-wetting? You may never have had any wetting in the
> first place or you may have too much brittle and weak Au-Sn and/or Ag-Sn
> intermetallic compounds (IMCs) near the interface, which with upwards of 2
> microns of Ag or Au, and particularly with 2 microns each of Ag AND Au, is
> likely. You can tell the difference by analyzing the lead surface. If there is
> little or no Sn present, you never had wetting, if you have Sn and Ag/Au
> present it is the IMCs.
> The Ag does not reduce solderability, it is similar to Au in this respect.
> With this much Au/Ag underplating with nickel will NOT help. The Ni-underplate
> only helps if the brass is not solderable by itself.
>
> 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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