I'd be willing to place a small bet that despite all due diligence you have 
already unwittingly built some product using components with a tin/lead 
termination finish. Many suppliers flipped over to a lead-free finish before the 
industry woke up to RoHS and often didn't care to give us much notice of the 
fact. For most of us this didn't matter, but I'm glad those that should care 
about this do!

Temperature should not be an issue as the parts without Sn/Pb coating will 
now be designed to survive Pb-free temperatures. The only area where you 
might be very concerned would be BGAs.   

The NPL in the UK did a good study of the reliability of mixed alloy solder 
joints, but concentrated mostly on varying levels of Pb contamination in Pb-
free alloys, so this won't help you and whisker detection per se was not part 
of this project either.

Nigel

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