Collapse of a liquid solder bump under load1

In this work the collapse of a solder bump as a result of the weight load of a chip or a package is analysed. A new expression is derived for the opposing bump force that carries the load. Its validity is checked on known solutions. Calculated spring constant and hydrostatic pressure are compared between the new force based approach and the more conventional approach based on increase of free surface. It is shown that for a spheroid bump shape, the results are qualitatively similar but not quantitatively the same. It turns out that the spheroid bump shape does not fully satisfy the Young Laplace (YLP) equation. The new force expression is used to develop an alternative bump shape which satisfies the YLP equation better. It is expected that the new force expression and the new alternative collapsed contour will contribute to better reliability assessment of solder bump interconnects.