Rod,

I have had good success with electrolytic nickel, palladium and gold.  The
electroless and immersion Pd and Au combinations have produced some good and
some bad for me, and I would not recommmed them without testing a specific
system to my requirements.  The electrolytic palladium and gold should be over
electrolytic nickel and kept on the thin side, both about 8 microinches [0.2
microns].  Thicker palladiium coating [30 microinches] can cause strange
things to happen to the solder joint after thermal aging, the metallurgical
cause has not agreed to by all.  The nickel shoud be at least 80 microinches
[2 microns], if you are going to use it for higher temperature applications.
I have not compared a underplate of 20 microinches of nickel against 200
microinches of nickel underplate for 80 deg. C operations, but it should be
okay.  However, 200 microinches of electrolytic nickel makes the barrel a lot
stronger and it will withstand much greater thermal cycling and solder dip
testing as compared to electroless nickel or solder coated copper in the
holes.

The Ni/Pd/Au has good shelf life (6 months plus when normally protected) and
the solder stays stuck to it even when cycled up to 200 Deg C (the solder has
to be Sn 96 or one of the higher melting temperature solders, of course).
Sn60, 62 or 63 solders all solder well with the normal soldering processes.
The basic solderability seem to be about the same as the electroless nickel/
immersion gold system when it has not been thermally stress or steam aged.
The thermal conditions experienced when bonding heat sinks or stiffeners to
the board (200 deg C for about an hour) does not affect the solderability.

I saw a note on technet about palladium plating causing polymerization of
organic material in the flux and cleaning materials, perhaps due to its
ability to act as a nacent hydrogen donor, but I have not experienced anything
that I could relate to this.  Perhaps the gold overplate prevents the
reaction.

Unless you can accept pattern plate where the Ni/Pd/Au acts as an etch resist
or you can plate the conductors after etching by having an electrical contact
to all exposed conductors, it is not a drop-in for ENi/IAu, immersion silver
or OSP wherein isolated conductors may be plated or coated.

Phil Hinton

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