You are already too cold for bottom side chip capacitors. It is very common
for bottom side to exceed 130-150C (110-140C on top) prior to wave
soldering. If you have top side components failing with a cold profile,
check the top side reflow profile.

One solution I have used for wave soldering sensitive LCDs was to put a
heat sink on top of just the part. This kept it 10-20C colder than the rest
of the panel, provided the sink was removed immediately after exiting the
wave. Heat sink material depends upon whether top side preheat is used.
-------------
Original Text
From: C=US/A=INTERNET/DDA=ID/TechNet(a)IPC.ORG, on 5/14/98 11:37 AM:
We are experiencing failures of .25% film capacitors, presumably from heat
damage during wave solder.  Top side preheat temperatures measured 210F,
within the process range we like.  Tried cooling it down to 163F topside,
got more solder defects, and some caps still going out of tolerance.  We
suspect that parts can't take thermal transfer during transit through the
wave, where topside gets about 100F warmer (of course part bodies see
less).
Manufacturer says no, it's our preheat, still too high at 163F, even though
they claim that the film can withstand 284F in the preheat zone.  Boards
are
at least 10 layers with planes, and soldering cooler than 163 will mean
lots
of defects, especially where the RMA doesn't dry out or activate properly.
Speeding up the conveyor results in lots of top side insufficients.  Hand
soldering works, but is expensive.  Can't live with these parts, can't
flush
them.  Should we consider cooler preheat, or are there other wave solder
tricks we can try?

Joe Kane
Lockheed Martin Control Systems

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