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Hi Tom - there hasn't been much published on this topic in the last 5-10
years as most folks wave solder processes are reasonably controlled, the
SMT capacitor manufacturers have better/more consistent fabrication
processes and the preheat capacity of modern wave solder equipment is much
better than in the past. A "general" industry consensus was reached that
SMT capacitors temperature had to be within 100C-125C of the wave solder
temperature. A combination of adequate preheat parameters, use of the chip
wave, the physical distance between the chip wave and the laminar wave,
and the wave solder pot temperature all came into play to avoid thermal
shocking the components. We conducted an number of investigations looking
at the process/parameters and found we could get a heating rate of
5-6C/second and not have any thermal shock issues. Lead-free solder
processing is going to have everyone probably giving the thermal shock
issue a second look as there have been some suggestions that we may need
controlled cooling of our assembles as they exit solder process equipment.
If controlled cooling becomes necessary, then the thermal shock concern
that Werner detailed could be an ugly balancing act to achieve. There were
some good papers on the whole SMT capacitor/wave solder thermal shock
topic published in the early 1990s.
Dave Hillman
Rockwell Collins
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"Gervascio, Thomas L" <[log in to unmask]>
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[TN] Wave soldering SMT capacitors
A friend of mine asked a good question, why is that most SMT capacitor
specs call out for a maximum heating rate at SMT of 2-4 C/sec but the same
specs allow a temperature delta at wave soldering of 100-125 C? In case of
wave, the time between preheat and soldering causes a heating rate in
excess of the SMT heating rates. But it is OK to subject the part to the
wave solder but not SMT using those heating and cooling rates?
I never could recall a logical explanation. Called several capacitor
manufacturer's and they seemed to dance around the question.
Any information would be helpful
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