Hi All,
Jean-Paul is absolutely correct in what he said.
Too many people talk loosely about reliability—one recent column even seemed 
to advocate serendipity. While for many products serendipity works—both for 
SnPb- and LF-solderes, because they do not see much in terms of operational 
temperature excursions, serendipity sure did not work in the case of the GM 
windshield wiper control boards which caused NHTSA to require over 4 million 
high-end GM SUVs and trucks to be recalled because of these solder joint failures. 
But as Jean-Paul points out, solder joint failures are either the result of 
inadequate quality or inadequate design-for-reliability (DfR). In the first 
case the failure mode is brittle interfacial failure due to overstress, in the 
second case it is creep-fatigue in the solder due to inadequate DfR.
Recognizing the different failure modes and root causes is important for 1) 
finding a solution to the problem, and 2) assessing fault in case of 
litigation. For quality-caused failures the fault can be in the design choices, the 
assembly process, the components, the PCBs, or a combination of these; in the case 
of inadequate DfR it is clearly the fault of the design organisation.
Setting quality issues aside, adequate DfR assures that creep-fatigue 
failures do not occur prematurely. Proper DfR will assure reliability for either 
SnPb- and LF-solders.
Whether proper DfR is more or less difficult for a given LF-solder relative 
to SnPb is not clear at this point. Accelerated test (ACT) results indicate a 
somewhat mixed picture; however, one cannot draw relative conclusions from 
side-by-side accelerated tests, because of the significantly different creep rates 
for the solders—they are much more important in ACT than for product 
reliability. 

Werner

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