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BGA mounting sequence

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BGA mounting sequence | 20 March, 2011

We are preparing to run a 2-sided board with BGAs on one side and passives on the other side. We have two schools of thought on which side we should run first.

One school says run the passive side first, then run the BGA side and the surface tension will hold the passive on while the BGA side is reflowing.

The other school states that if we need to rework the BGAs it will be difficult with the passive side populated, so run the BGA side first, then run the passives side either with lower temp paste or with a thermal dampener to keep the bottom side under reflow temp.

Would anyone have an idea of which would work better? Thanks for any suggestions.

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BGA mounting sequence | 21 March, 2011

I would run the passive side first just because the BGA is probably moisture sensitive and I wouldn't want to have to keep track of the time between first and second reflow.

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BGA mounting sequence | 21 March, 2011

Passives first reflow, and BGAs second side reflow. Uses only one heating cycle for your BGAs, and the passives will stay in place just fine given correct profile parameters.

There are a few exceptions, but this is generally how it is done.

My $.02


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