Hi Tetsuo,
I have no arguments with most of what you wrote, but I need to set the record
straight on 2 of your points, so people do not draw the wrong conclusions.
[1] You write "the Sn-Ag-Cu system (and the Sn-Cu) system is stronger in all
respects than the Sn-Pb alloys."
That maybe, and is likely, the case--however strength is not a good indicator
of fatigue resistance.
[2] You state "the experience with commercial products that have been in the
field for nearly two years seems to confirm that. This is particularly the
case in regard to thermal fatigue, possibly the main cause of failure in
electronic circuitry, where the lead-free alloys have a much longer life."
There have not been good data, following IPC-SM-785 testing guidelines,
published, that really characterize the creep-fatigue life of of lead-free
solders; without such data nobady can make any statements as to the
inferiority, equaity, or superiority of any of these alloys. To annectotally
refer to '2 years of commercial products in the field' is totally
meaningless; 2 years of cheap [read throw-away] consumer products does not
prove anything, one way or the other. It may be more impressive under the
hood of an automobile, but even that is meaningless because of a total lack
of definition of the actual loading conditions on the solder joints.
So as far as I am concerned, the jury is still out as to the reliability of
lead-free solders.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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