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This research studies the motion of void bubbles within molten solder bumps. Surface tension variation on the outer surface of the bump drives a flow that is transmitted to the interior by viscosity. Bubbles inside the bump grow by collisions with other bubbles. A computer model of this process has been formulated. In cases with the melting process from top to bottom, a ring of bubbles collected at the top outer rim of the solder bump. When the melting direction was reversed, the typical result was a single large bubble near the center with a few smaller recirculating bubbles.
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Panton et al. (2003) studied this question.
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