Peter,

In our experience bridging occurs because the BGA warps ("pancakes") during
reflow. Wrap is induced by CTE mis-matches between the materials in the
package. We investigated this problem in depth by conducting a series of
designed experiments. Here is what we found:

1.  Use a baffled nozzle
2.  Minimize air flow rates
3.  Maintain temperature on top of BGA package at < 220C
4.  Minimize time above reflow beyond supplier recommendation

We pay very close attention to the above when generating profiles to rework
plastic BGAs that are > 35mm. The bigger the BGA and the finer the pitch,
the more prone they become to bridging at the corners. Some parts have such
a narrow process window that we down-graded them to non-preferred status.

I'll be glad to discuss in more detail. Contact me offline.

Tom Buschor, Harris Corp.
Melbourne, Florida

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