Mr. Werner,
The fact of the matter, with regards to the primary purpose of nickel, in
the gold plating process of boards and tips, is that the nickel layer
prevents the migration of copper to the surface and ultimately the
formation of surface oxides, stress lines and low peel strenghths.
In the case of a soft gold deposit, electrolytically applied, over the
entire circuitry, a solderable surface is not being sought, but rather
enhanced surface or skin conductivity. Tips are plated with a hard deposit
because of the physical demands of plugging and unplugging, again a
solderable surface is not a requirement. The gold deposits range from 25 to
100 microinches. There is still a need for nickel to prevent the migration
of copper through the much porous gold, and affect the conductivity and
physical strengths of the deposit, hence leading to failure.
However, I believe that we have deviated from the main issue, and original
question; why are we doing electroless nickel-immersion gold, when the
copper/Tin intermetallic, is stronger than the nickel/tin intermetallic.
The main reasons are as follows:
Copper left bare will oxidize readily and lead to solderability issues at
the assembly level, hence, a solderable coating must be provided that does
not oxidize.
HASL has been the workhorse over the decades, and continues to enjoy
preferential treatment; however, issues with HASL and lead content have
lead to the development of alternative finishes. However, nothing solders
better than solder on solder.
One particular finish is electroless nickel-immersion gold, the soldering
occurs on the nickel, the nickel/tin intermetallic is fairly solderable.
The immersion gold is applied as a simple antioxidant coating, typically
2-6 microinches. If however, the immersion gold coating is applied as an
antioxidant coating over the bare copper, then the copper will migrate
through the gold layer and lead to solderability problems.
I hope the above addresses the initial concerns of the Technetter, who
asked the question, Scott.
Regards,
Lenny Kurup
EMX Enterprises Ltd.
905-475-8000
> From: Engelmaier[SMTP:[log in to unmask]]
> Sent: Wednesday, April 22, 1998 11:01 PM
> To: [log in to unmask]
> Subject: Re: [TN] Ni plating and Immersion Au
>
> Hi Scott,
> You have a legitimate question, but the reponses you got....well.
> The intermetallic compounds (IMCs) of concern for solder joints are
> all tin
> based; thus we have AuSn, AuSn2, AuSn4, Ag3Sn, Cu3Sn, Cu6Sn5,
> Ni3Sn2, Ni3Sn4,
> Ni3Sn7.
> All of these IMCs are brittle, the important ones CuxSny and NixSny
> are
> strong, wheras AuxSny and AgxSny IMCs are weak, leading to what has
> been
> termed 'gold embrittlement' when concentrations get too high. The
> dissolution
> rates into Sn are for Au:Ag:Cu:Ni about1000:200:30:1.
> Originally, Ni was used as a barrier layer for Ag-metallizations to
> prevent
> the formation of Ag3Sn IMC. On PCBs the Ni layer provide the
> possibility to
> provide a single surface treatment that is both solderable (albeit
> significantly less so than Cu) and wirebondable. I do not no 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.
> The Au on the Ni has the primary function of preventing the
> oxydation of the
> Ni (much the same as the solder coverage of Cu pads); it also helps
> the
> spreading -not the wetting-of the liquid solder over the Ni surface.
> To
> properly wet to Ni you need higher reflow temperatures (about 15C)
> than for
> Cu.
> If you do not need wirebonding, you are definitely better of with
> solder
> coated Cu.
>
> 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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