In Japan most companies are moving towards Sn-Bi lead finishes (typically
Sn-2Bi), with the NiPdAu finish (traced to Texas Instruments) commonly in
second place, and some companies accepting coarse grained matte Sn. It has
been reported that the Bi succeeds in eliminating whisker concerns, while it
is generally recognized that matte Sn directly on the Cu-based leadframe
removes most but not all concerns.  In Europe, the popular finish is coarse
grained matte Sn over Ni on leadframe to avoid the Cu-Sn intermetallics
commonly thought to be a source of localized stress thought to be a prime
driver for whisker growth.  Bi as a lead finish is a possible concern while
Pb remains on boards or is present as a contaminant in solder waves.  The
Sn-Pb-Bi eutectic melting temperature is approximately 98 Celsius.  It is
thought that concentration of this ternary liquid phase during cooling of
solders with a reasonably wide pasty range can occur due to the sweeping of
this phase away from solidification fronts, due to low solubility, during
cooling to result in concentration of this material at the interface with
the hottest, or slowest cooling solder joint interface.  The NiPdAu finish
avoids all of these problems, but is the most expensive, and not all
companies can transfer mold to leadframes with an external Au surface
(however thin) and achieve good Pb-Free MSL ratings.

Leo

------------------------------------
Leo M. Higgins III, Ph.D.
ASAT Inc.
Central Region Mgr. / Director of Sales
[log in to unmask]
3755 Capital of Texas Hwy-So
Suite 100
Austin,

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