Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

Boards Bowing

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Boards Bowing | 30 September, 2005

We are having a random problem with boards bowing significantly. Its a large through hole board with only one QFP on it. The bowing occurs during the SMT reflow. The only conclusion I can come to is a problem at the board house that made it. I just went through about 100 boards and although they all have minor deflection which I would expect from reflow, one of them is bowed significantly only on one end. If you lay it on a table the middle of the board on one end is about .24" off the table while all four corners are on the table. The board is 10" by 12". The bowing occurs in different areas. The copper is pretty evenly distributed. One was actually bowed up in the middle of the board so bad the through hold parts did not solder in the wave. Anyone have any clue what would cause this?

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Boards Bowing | 3 October, 2005

is this a single brd or multi up array? What is the thickness? where were the brds made, state/country? I had an issues with a brd which was fabed in mexico during the humid months there was mosture trapped in the brds which caused many strang things including warping solderability issues. I backed the brds (with weight on them so they would not warp) and then ran them. Still had soldering issues but warping was gone.

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Boards Bowing | 3 October, 2005

Its a single board, one up. 4 layer, pretty simple, no large copper pours. We get these consigned from our customer so I cant work that end of it but wanted to make sure to cover everything I can. The oven it runs in was just checked out by the factory, its a Research inc. Deltaflow 10LN running slow and the profiles are fine. If it was a moisture problem, I an not sure the short time in reflow would be enough to get rid of the moisture so I will try to find out if the customer (they do the through hole parts and wave) is having problems with blow holes.

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Boards Bowing | 3 October, 2005

If the PTH plating thickness is proper, your customer probably won't see the effects of moisture during soldering processes.

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Boards Bowing | 3 October, 2005

When we see warp and twist, we usually jump the fabricator for: * MLB construction (not homogeneous or unbalanced - too resin rich or poor) * Supplier process capabilities (lamination cycles) * Material selection (usually cheap and dirty), etc.

Warp depends on: * How close the board gets to its Tg during the reflow cycle (the closer you get to the Tg the worse the warp gets) * Amount of copper on each side and the balance between the two * Thickness of the board * Overall length x width * Time under temperature * Rate of cool down * Multiple thermal cycles, such as rework, are never a good thing.

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