Void formation in flip-chip solder bumps. I

Void formation in solder bumps occurs due to the formation and coagulation of gas bubbles from the combustion of flux during the solder reflow process. The formation of such voids can result in the failure of solder joints under the application of load. This study aims at determining the factors which affect the formation and growth of these voids. It consists of evaluating numerous cross-sections of solder bumps, as well as the numerical modeling of the flow and bubble dynamics within the solder bump during the solder reflow process. The cross sections reveal several cases in which a large void is formed within the solder bump. Generally, in the absence of such a void, a large number of smaller bubbles accumulate at the top of the joint, and often result in crack propagation through them. A simplified, two-dimensional numerical model to simulate the motion and coalescence of bubbles in a solder bump has been developed. A recirculative flow within the melt region is assumed. This recirculation is caused by the temperature differential between the exterior and interior boundaries of the melt region. A heat-transfer analysis predicts the movement of the melt front during reflow. It is hoped that this model, along with a study of joint cross sections, will lend some insight into the factors that affect the formation and distribution of voids within solder bumps.