Hi TechNet! I can fill in some holes on the Ni/Pd/Au clarify some of the
questions:

1.   Intermetallics

Some work done by the NCMS consortia and here at Rockwell Collins has shown that
there is a number of intermetallic phases possible but the Pd3Sn intermetallic
phase is brittle and causes solder joint embrittlement similar to the Au4Sn
intermetallic phase. The way to avoid the formation of the Pd3Sn was to keep the
Pd thickness in the 9-12 microinch range. Per Bader's work ("Dissolution of Au,
Ag, Pd, Pt, Cu, and Ni in Molten Tin Lead Solder", December 1969 Welding
Research Supplement) Pd is slow to diffuse into molten solder so the soldering
process time becomes a critical parameter to allow the Pd to diffuse into the
solder and not form the Pd3Sn phase.

2.   Rework cycles

We haven't seen any problems provided the initial solder process was done
properly and the rework process has proper process controls. It seems to be
comparable to tin/lead rework at least up to 3 reworks (if you need more than
that something is wrong!:).

3.   Gold embrittlement

The gold layer on the Pd I am familiar with is an immersion gold which is only
3-5 microinches thick - not enough to cause embrittlement problems. This gold
layer does two things: (1) it improves wirebonding values by 2-3 grams pull
force in comparison to just a Pd/Ni finish; (2) it protects the Pd surface. Pd
is a very cool element and is used in many process as a initiator or "getter. Pd
is used to seed laminate thru holes prior to copper plating and you can find Pd
in your catalytic converter on your car. By putting gold over the Pd surface you
eliminate any Pd surface interactions with the environment. Seems like overkill
to put one noble metal on another noble metal but the properties of Pd are the
reason. Collins has a large wirebond study nearly finished up which includes the
Ni/Pd/Au and we'll be publishing some neat stuff sometime in late 2000.

4.   Fine pitch or BGA components

The Ni/Pd/Au finish is just as flat as immersion tin or immersion silver or
immersion gold so it works for both fine pitch and BGAs.

5.   Pressfit

Sorry but I haven't played in this area yet. Hey Paul K. - haven't you done some
stuff with Pd and pressfit?

Well, hope that helps.

Dave Hillman
Rockwell Collins
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Darrel Therriault <[log in to unmask]> on 12/06/99 11:59:13 AM

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Subject:  [TN] Ni/Pd/Au



TechNet,

A PWB supplier is recommending a Ni/Pd/Au finish for .062" and .093" FR-4 PWB
with mixed PTH/SMT components, as well as press fit connectors.

Any comments/guidance, pro or con,  regarding :

1.   Intermetalics
2.   Rework cycles
3.   Gold embriddlement
4.   Fine pitch or BGA components
5.   Pressfit

All info appreciated.

Regards.......DT

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