Hi Peter,
You repeat what I regard as an unsubstantiated old-wives-tale: "Time and
temperature increase the thickness of the brittle intermetallic layer that is
the solder joint, rendering the solder joints increasingly susceptible to
fracture."
In all my years in this business, I have never seen a SJ failure caused by
'too thick' an IMC layer. Of course, IMC layer thickness will increase with
time and temperature, but (1) IMCs--with the exceptions of AuSn and AgSn
IMCs--are much stronger than solder even though they are brittle; I have
NEVER seen a fracture in the IMC. (2) It takes long exposures of about 150
hours at 155C--I have seen the results from up to 1,000 hours at 155C
[TUMunich & TI]--to get to thicknesses I would get concerned about, and not
so muchabout the IMC layer but the structure of what is left of the SJ.
I certainly would agree that eposure to elevated T's of PCAs should be as
imited as possible, but nnot because of concerns for the SJs.

Werner Engelmaier

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