BACKGROUND:
On one assembly, we use semi-automatic air displacement paste dot dispensing
and a semi-automatic placement system. The computer says where to put the
paste and parts, the operator places them by eye in the correct location.
With normal accuracy, we get a few solder balls that squeeze out from under
the chips at the side which are easy to pick off. Recently, we got a batch
that have lots of solder balls trapped under the middle of the parts. It was
a newly trained operator that built these assemblies.

QUESTION:
What is the most likely cause?
What is the best way to rework a chip part with a solder ball under the
middle?

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