Hi Bill,
You sort of got it right.
The CTEs act instantaneously on the materials which they characterize; if 
they are no constraint.
If they are constraint, as by being attached to a material with a different 
CTE, then the CTE-mismatch of the materials results in a compromise position 
with both materials in stress, one in tension the other in compression, and 
elastic strains [in gross CTE-mismatches you can get plastic deformation, as for 
instance in 'resin recession' in PCB vias].
Now these materials wil just sit there forever.
If, however, these materials are not directly attached, but have a creeping 
interposer [read solder],than the elastic strains in the 2 materials [and in 
the solder to a lesser extent] will unload as plastic strains into the solder as 
the result of the solder creeping.
When the 2 materials have reached the positions that would have had without 
the solder, the stress has reached zero [the stress relaxation is complete], 
the creep process is complete, and the plastic deformation in the solder is at 
its maximum.
Because the creep behavior as well as the creep-fatigue characteristics of 
SnPb solders are well understood and models mathematically describing these 
processes exist, IPC-9701 can be a 'Performance Standard' for SnPb solders.
Since we do not have the same level of understanding for SAC solders, 
IPC-9701 will remain a "Recommendation" for SAC solder until such time as the level 
of understanding for Pb-free solders has rached a sufficient level. 
The ramp rate is not very important, since for most real thermal cycles, the 
creep process is fast enough to prevent a significant build-up of stresses 
during T-changes. 10 minute dwells are not sufficient for complete creeping even 
for SnPb solders until you get to temperature in excess of about 110C or so; 
they certainly are infufficient to get "to the coast" with SAC solders.

And we sure are not smart enough at this time to "re-rate" the "test 
strength" at this point in time.
No, higher modulus of elasticity does not mean "less elastic," it means 
higher stresses for the same displacements. Metals, other than steel, do not have a 
truly elastic behavior; that is why 'yield strength' is a defined quantity, 
whereas for steel one has an actual well-defined 'yield point'

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com



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