In a message dated 02/17/0 14:54:31, [log in to unmask] writes:
>Besides what it written in STD-001B, it might be interesting to add some
>requirements about lead phase size and homogeneity, intermetallics thickness,
>etc. to get more reliable assemblies.

Hi Christophe,
STD-001B does not address reliability; for reliability information you need
to go to IPC-D-279, Design Guidelines for Reliable Surface Mount Technology
Printed Board Assemblies.
Since the grain structure of solder is unstable and coarsens with time and
temperature, it does not make much sense to somehow specify solder joint
microstructure. True, if you fatigue cycle fine grained solder in an
accelerated test you get about twice the life as compared to coarse grained
solder, but for actual product this is immaterial since within a year's time
the initial fine grained solder is virtually indistinguishable from initially
coarse grained solder.
Further, I have yet to see solder joint failures as the result of
intermetallic layer being too thick; on the other hand, if you solder to Ni
or Alloy 42, you will have a difficult time determining the IMC layer
thickness.
For the reliability of solder joints, the most important quality aspect is
adequate wetting; on copper the obvious presence of IMC layers is proof of
that, on Ni--good luck. However, in most cases of solder joint failure it is
the result of  inadequate solder joint quality, but caused by improper
designs allowing large cyclic thermal expansion mismatches to fatigue the SJs.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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