Hi Colin,
First, CTE's are not rates—they are time-independent.
Second, CTE's are the same on heating and cooling, with the exception of polymers which can show a hysteresis behavior.
Third, indeed, the cooling rates can too fast—remember, that you have quite a different structure after soldering than before.
Werner
-----Original Message-----
From: colin mcvean <[log in to unmask]>
To: [log in to unmask]
Sent: Thu, Aug 13, 2009 11:04 pm
Subject: [TN] Reflow profile
Folks,
From my previous question regarding pattern crack we are seeing on a wire-bonded
flex-rigid PCB, I have now found more evidence that the presented FPCB is ok,
but after reflow for SMT assembly, this is where the cracking is first found
(rather than after wire-bond process). The reflow profile being used peaks at
245oC, but rapidly cools from peak to 135oC in about 30s. This seems to me to be
too rapid a cool rate, and with the pad metallics being 0.2uM gold over 10uM
Nickel over 30uM copper pad. Are CTE rates on cooling the same as CTE rates for
heating? I think I may have CTE mismatch on the coatings, which when cooled too
rapidly, are causing the crack. Any metallurgists out there willing to comment?
Thanks
Colin
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