Mr. Werner.
I must agree with Paul. During my short 20 years in this incredibly wild and wonderful
industry, I have been involved with evaluations of numerous studies using SEM, SAM, EDX,
electrographic, and yes, even the olllddd nitric Bell Jar Test. The reason- potential Copper
migration to the surface of a connector or wire bonding pad. From these studies, I have always
concluded, assumed (ass-u-me?), and been under the impression that NI was used as an undercoated
layer of material for gold for a couple of reasons, One of these reasons was as a "buffer
material' to inhibit the oxidation and propagation of copper oxides to the surface. Since gold
is a very porous metal, it is stated that approx. .001" of Au is necessary to overcome this
issue, is used alone. If Ni is used underneath (.0001"), then the Au thickness can be much
thinner. Obviously, most of our studies have been performed for reasons of contact integrity,
but some of the testing by some of the Large OEMs and suppliers of Elec. NI/Imm. AU have come up
with the same need for a buffer metal (nickel). In solderability applications, the Au is
actually used as a deterrent for Nickel oxide formation and at the .000003" to .000005" range,
typically "diffuses" into the solder joint with minimal embrittlement concerns, and creating an
intermetallic bond between the solder, Au, and NI. . I am not a metallurgist (God knows, I have
enough trouble spelling the word!), but have been involved during the R&D phase of alternate
coatings to solder with some interesting results. I am not going to say that elec. Ni./Imm. Au.
is the only or the best alternative, but I believe that it is one alternative that has had some
success. One of the trends I noticed during the earlier phases of alternative coatings was that
the East Coast was more influenced by Elec. Ni/Imm. Au from the European side and the West Coast
was more influenced by OSP's from the Far East side. Now we are seeing a myriad of alternatives
and within the next couple of years should have a "shake out" to the best ones for soldering,
ultrasonic wire bonding, and thermalsonic wire bonding. Just my 2 cents worth (alright, a
quarter). JOHN WAITE
Paul Gould wrote:
> Hi Werner,
>
> I would like to clarify this point with you:-
> >I do not know 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.
>
> If this is not true, then I too have been guilty of giving out
> misinformation. The solderability of a gold on copper surface
> deteriorates for some reason and I thought this was due to copper
> migrating into the gold to the surface. This also causes contact
> resistance to rise on edge connectors even with significantly thicker
> gold deposits on copper. The plating of Nickel on the copper reduces
> this and it would seem that the Nickel does act as a barrier. If this is
> not the reason then what is the mechanism, and why do we need to plate
> nickel on edge connectors?
>
> Best Regards
> Paul Gould
> [log in to unmask]
> Isle of Wight,UK
>
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