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Reply To: | (Leadfree Electronics Assembly Forum) |
Date: | Mon, 1 May 2006 13:37:43 EDT |
Content-Type: | text/plain |
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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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