Your problem cannot be handled with a 'fatigue resistant' solder; you must
essentially reduce the thermal expansion mismatch to near zero, because in
the 175 to 200oC range no solder is 'fatigue-resistant' which would imply
resisting creep. However, I am puzzled by to -55oC; I cannot imagine an
actual application down to -55oC--even the USAF no longer talks about -55oC
since no electronics actually experiences this temperature. 

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-Mail: [log in to unmask]

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