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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