Hi TechNet! This discussion on Pd has been wonderfully fun from a
metallurgical viewpoint! One segment of information that has not come to
light but really links the various responses together is the thickness of
the Pd plating. Early test programs and investigations used relatively
thick coating thicknesses (greater than 20 microinches) which promoted the
formation of Pd/Sn intermetallic phases. The Pd/Sn intermetallic phases are
brittle and resulted in solder joint crack failures (as documented in early
TechNet posts). However if the Pd thickness is specified in the 9-12
microinch range the formation of the brittle intermetallics can be avoided
and solder joint reliability is not compromised. The use of Pd is very
similar to Au  - there is a specific region of the phase diagrams which we
need to reside in to avoid solder joint reliability problems. The use of Pd
as a surface finish will find its place in electronics assembly because it
is both wirebondable/solderable - but due to cost and process
considerations it will not be the next 'universal' finish. Its just another
surface finish option in our design/assembly 'tool' boxes.

Dave Hillman
Rockwell Collins
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Subject:  Re: [TN] Palladium query


Hi Terry,

TI publish a useful "Palladium Finish User's Manual" which may give you
more information on palladium lead finish on components. According to
this, the finish is as follows (viewed with Cu leadframe at bottom of
list/outer layer at top):
        Palladium min 3u" (0.075micron)
        Nickel Plate 40 - 60u" (1.0 - 1.5micron)
        Palladium/Nickel Strike max 5u" (0.125micron)
        Nickel Strike max 5u" (0.125micron)
        Cu. Base Leadframe

The key role of the palladium is to protect the underlying layer of
nickel from oxidation during storage/assembly. The palladium dissolves
into the solder joint, and these joints can look significantly
different to those made to a solder dip finish leadframe.

The exact role of the underlying Palladium/Nickel Strike and Nickel
Strike layers I am not sure. Perhaps other TechNet Minds can advise?

Hope this helps,

Clive ffitch
Matra BAe UK
Stevenage, England

-----Original Message-----

Fellow TechNetters,

Can someone please enlighten me as to the purpose of a Palladium
Barrier on
components / PCBs.

Best regards

Terry Exell
BAE SYSTEMS, Plymouth, UK

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