This experimental research studies the formation of void bubbles within molten solder bumps in flip-chip connections. A theory based on thermocapillary flow reveals that the direction of heating influences void formation. Twelve chip-bump substrate assemblies were investigated using different heating profiles. A database on sizes and locations of voids in solder bumps was constructed. The observation on cases with melting direction from bottom to top supports the numerical study based on thermocapillary theory. The results show that a single big void is near the bump center with a few small voids near the edge when the melting direction is from bottom to top during solder reflow. When the melting direction was reversed, many small voids appear near the bottom edge with big voids in the middle of the bump. This experimental finding does not completely agree with the interpretation on the formation of voids by thermocapillary theory. However, the results do show that heat flux direction plays significant role in the formation and distribution of void bubbles in molten solder.

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