Since the shape of molten solder is determined by its energy minimization, self-aligning effect occurs during reflow process in flip-chip soldering. By using self-aligning effect positional error between chip and substrate can be compensated effectively without additional process or device. In this work, analysis by Surface Evolver and experiment are performed about self-aligning effect in case of circular and non-circular pad.
As the result of analysis of circular pad, self-aligning effect in vertical direction shows correspondence but in horizontal direction shows discrepancy between Surface Evolver and existing modeling. The assumption of the circular cross-section of the molten solder in existing modeling appears to cause the discrepancy. As horizontal self-aligning effect is much smaller than those in vertical direction, larger aligning error can be resulted in the horizontal direction. In case of elliptical and rectangular pad that has same area of circular pad, self-aligning effect in minor direction is better than those in major axis.
The experiment is conducted by using 762㎛ diameter eutectic solder ball and ceramic substrate with circular and rectangular pad. Horizontal displacement is measured by CCD laser displacement meters. The result of experiment and analysis shows correspondence. Therefore, positional error can be decreased effectively by using non-circular pad.
