Hi LK & others,
You asked for a rebuttal—here it goes.
Just throwing [[[[[[THERMAL STRESS]]]]]] around is neither elegant nor
simple—simplistic, maybe. And referring to evidence without stating what the
evidence is, what it represents, and under what circumstances it was obtained
proves nothing and only serves to obfuscate.
One always needs to precisely define what one is talking about and the
original statement did not confine itself to HASL, but included all soldering
processes the way I read it anyway. My statement addressed multiple reflow
passes.
(1) I certainly agree that if one HASL pass shows poor wetting, multiple
passes will not improve things as far as solderability is concerned.
(2) The quality and shelf life of a HASL'ed surface will depend on the
thickness of the solder layer above the IMC layers and the storage conditions.
(3) If a second HASL pass is necessary, because too much solder remains and
forms too much of a meniscus for component placement, a second pass will
result in a somewhat thicker IMC layer. But that has no impact on quality and
reliability as long as the solder layer above the IMC is thick enough (~2-3
micro-meter plus) to avoid that the layer above the IMC is not mostly lead-
rich phase (higher Liquidus temperature) or to prevent the IMC growing through
the solder layer (no solderability).
(4) Reflow soldering passes, even multiple ones, will not degrade either the
quality or long-term reliability of solder joints if adequate wetting has
taken place, because the slightly thicker Cu6Sn5 IMC layer has no impact on
long-term reliability of the solder joints (these IMCs are much stronger than
solder); I have seen many solder joint failures because of inadequate wetting,
but never one because of excessive IMC layer growth in a finished well-wetted
solder joint. The reflow temperatures are typically low enough (certainly
lower than the HASL temperature) not to affect the PCB.
(5) I agree that manual soldering operations—single or multiple ones—can
result in a significant degradation of pad and inner laminar bond strength; in
fact, it is the manual soldering operations, where frequently soldering irons
with no temperature limits are used, where most damage due to PTH barrel
cracking, post separation, pad lifting, etc., occurs.
None of the examples of degradation/deficient quality that were cited, are
caused by multiple solder process passes, but result from other problems
(oxidation, contamination, cranking up the solder temperature, slowing down
the conveyor, boards and processes defective, etc.).
Most of the horror stories that I have seen over the years result from
inadequate designs and bad incoming materials/components; processing errors,
while they can occur to fix some problem, are a relatively small part of the
picture. The problems can be inadequate solderability, lack of information of
what materials are being soldered (BeCu, Alloy 42, Ni, Pd, Sn/Pb plating,
solder coating, etc.), inadequate protection of components/PCBs during
shipping, etc.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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