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Printed Circuit Board Assembly & PCB Design Forum

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SMT - PCB combo process

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SMT - PCB combo process | 12 October, 2012

Hi, I want to ask if someone saw the combo process at smt lines for panel array (bot & top per both sides), I just perform some experiments and it works perfectly, and I validate the T hus (Cg/Pa) and no one component were dropped after the 2nd reflow, and i take some cycle times, the two most important improvements where than the changeover was eliminated and the cycle time is decreased in 15% (on my configuration). But my huge concernd is that the designer still don't believe on this improvement, he want to see a DFM (i can not perform it), someone of you can give me your opinion about how I can sell this improvement.


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SMT - PCB combo process | 15 October, 2012

Is the designer's concern that the parts will fall off when hanging upside down during reflow? We've been doing this for decades and our test if we think a part is marginal is to glue a 2nd (and sometimes 3rd) part to the back and send it through reflow. If the part doesn't fall off with double its weight then it'll be fine. When we first started doing this we had a formula that used part weight, wettable area, and surface tension to see if the part would stay on, but these days we can pretty much eyeball it and run an experiment with the ones we're not sure of. All but the heaviest parts will be fine.

You also need to be concerned about moisture sensitivity. You need a system for controlling the time between 1st and 2nd reflow if you have moisture sensitive parts.

Lastly, make sure none of the parts have restrictions on the number of heat cycles. Probably not an issue, but just to be thorough count up the number of heat cycles the board will see (eg. 2 reflows, 1 possible repair, selective wave, etc...).

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