Hi TechNet - Roger documented some of the good points on the use of Pd in
wirebonding applications and I have some for the soldering end of the
spectrum. Pd can (and should) be considered a solderable finish provided
the base metallurgy is understood. Pd forms an intermetallic phase with Sn
(PdSn4) that can result in a solder joint embrittlement similar to the
reactions of gold and tin (Au4Sn) intermetallic phase. Industry testing has
shown that if the Pd plating thickness is controlled to be in the 6-10
uinch range that the embrittlement issues can be avoided. Pd does require a
few soldering process modifications - a bit slower speeds due to diffusion
issues and some fluxes are not effective at removing the oxides. The NCMS
consortia has published some work on Pd and there is also paper -
"Demonstrating the Relationship Between Wirebondability and Solderability
of Various Metallic Finishes for Use in Printed Circuit Assembly", SMI
Conference Proceedings 1996, authors Hillman, Bratin, Pavlov - that
contains some useful information. There is still considerable
debate/testing/research to be conducted on the various alternative finishes
before the entire family of "alternative finishes" can be considered a
replacement for tin/lead. Take one look at HP's paper on the fracture of
BGA's on immersion gold finished boards published at the IPC Surface
Finishes Conferences - the information is not all there yet!
Dave Hillman
Rockwell Collins
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Roger Massey-G14195 <[log in to unmask]> on 02/01/99
11:26:53 AM
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Subject: Re: [TN] Palladium underplate for Cu-Ni-Au system
Benny,
Th Pd is there to bond onto!!!
Pd is "pretty common" in Au bonding (note the flying commas!) and
is used as the reliability for these is thought to be pretty damn
good, this due to Pd being fully miscible in Au, so there are no
intermetallics to worry about. As the layer is far harder than Au,
capillary life is shorter and you'll need to play with power etc, some
have found that a shorter capillary is needed although Ive never seen
the need myself.
There are some concerns about the system in that if you give it
enough heat and time, all the Pd can be leached into the Au and leave
you with a 2nd foot lift. (this only really happens if the Pd is
incredibly thin instead of just stupidly thin, OK results with 3-5 u
in Pd!) The other thing to watch for is that the Pd-Sn intermetalics
form during soldering if its too thick, so keep it thin.
If you have to bond them, watch the temperature on the plate as Pd
oxidises and goes green at high temps.
Good Luck
Roger Massey
Metallurgist and wire bonder
Motorola AIEG
UK
______________________________ Reply Separator
_________________________________
Subject: [TN] Palladium underplate for Cu-Ni-Au system
Author: "TechNet E-Mail Forum." <[log in to unmask]> at #email
Date: 29/01/99 15:13
Hi technetters,
Does anybody know the purpose of a palladium layer between the Ni and Au
for
wire bonding PCB? Recently, I've got a request to bond gold wire
(thermosonic) on PI flex which is plated with electroless Ni and immersion
Au. On one sample, the gold thickness is reduced and compensated by a
electroless palladium layer above Ni; while the other just is the normal
electroless Ni and immersion Au. The phosphorus contect is around 6-8%.
I would appreciate if anyone can advise me on the palladium function, cost
implication (cheaper by reducing the gold?), and bondability of this kind
of
metalization.
Regards, Benny.
ASM Assembly Automation
Email :[log in to unmask]
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