There is no good choice.
Board Finishes: Industrial/Battelle Class 3 Environment [Reliability Knowledge Gaps: For use of Pb-free solders in High Reliability Applications, J Smetana, iNEMI Availability of SnPb-Compatible BGAs Workshop, March 1, 2007, Alcatel-Lucent]: * ENIG, OSP, and immersion silver ALL are subject to corrosion in Battelle Class 3 environments (creep corrosion and/or fiber assisted electrochemical migration, blistering/peeling of conductive corrosion products, etc.) * Only SnPb HASL and immersion tin survive this environment ** SnPb HASL is not Pb-free ** Immersion tin has poor solderability performance in Pb-free assembly (and whisker concerns) * SnCu HASL - has not been tested for this issue but� Greatly undesirable (we don't want this!) ** Additional severe thermal shock to PWB ** Same issues with non-flat pads as SnPb HASL ** Extremely limited availability ** Tin whisker risk - worse than immersion tin ** Cu dissolution ** Process control at fabricator ** Etc. NO PATH FORWARD at this Time!
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