Hi John,
Phil Hinton is quite right about the solderability protection afforded by a
palladium/gold finish. But please do NOT think of the thin Pd/Au finish as a
'solderable surface', it is not. The Pd/Au finish will protect the
solderability of the surface beneath it, provided it was solderable prior the
application of the Pd/Au layers; it promotes 'wetting' (in the sense of
spreading, not the formation of a metallurgical bond) of the underlying
surface by the liquid solder, because both Pd and Au are dissolve into tin at
much higher rates than do Ni; the wetting (in the more commonly used meaning
of spreading plus the formation of intermetallic compounds to form a
metallurgical bond) to the underlying surface (copper or nickel) is certainly
 enhanced by the faster 'spreading of the liquid solder; the Pd and Au layers
will totally disappear by dissolving into the tin to from PdSn and AuSn IMCs,
which will disperse more or less throughout the solder joint depending on
peak reflow temperature and the time above Liquidus temperature. 

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]>   *
* with <subject: subscribe/unsubscribe> and no text in the body.          *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************