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SolderTips: Printed Circuit Board Bow and Twist Showing Up

Question: We have recently started to see board flexing in one of our circuit board assemblies. This assembly has a 40mm PoP BGA component on it with an integrated heat sink.

We are using a three zone reflow oven and the profile has worked without issue for at least six months. Now the boards are bowing, or curving upwards towards the middle of the board during the process. What should we be looking for even though we know the process has not changed? Or at least we think it has not changed.

Answer: Interesting question, as although, it may not be readily visible the process or the materials have changed to introduce this issue.

The first thing you need to do is to review the profile once again. Look at the conveyor speeds and the width of the conveyor as well. Next I would check to see if the same temperature is across the width and length of the board, as well as the same temperature on the bottom side of the assembly.

Next, from an equipment perspective, check the equipment venting to make sure the air flow has not changed, which would impact oven temperature. Also check the fixture, if used, to make sure the board is secure within the fixture during the reflow process.

From the component perspective you might what to review the weight of the component and the heat sink. Has the manufacturing materials for the heat sink changed, making it heavier?

Lastly, inspect the laminate materials for the boards to make sure that the same material is in use as defined and still has the same Tg.