Void formation in solder bumps occurs due to the formation andcoagulation of gas bubbles from the combustion of flux during the solderreflow process. The formation of such voids can result in the failure ofsolder joints under the application of load. This study aims atdetermining the factors which affect the formation and growth of thesevoids. It consists of evaluating numerous cross-sections of solderbumps, as well as the numerical modeling of the flow and bubble dynamicswithin the solder bump during the solder reflow process. The crosssections reveal several cases in which a large void is formed within thesolder bump. Generally, in the absence of such a void, a large number ofsmaller bubbles accumulate at the top of the joint, and often result incrack propagation through them. A simplified, two-dimensional numericalmodel to simulate the motion and coalescence of bubbles in a solder bumphas been developed. A recirculative flow within the melt region isassumed. This recirculation is caused by the temperature differentialbetween the exterior and interior boundaries of the melt region. Aheat-transfer analysis predicts the movement of the melt front duringreflow. It is hoped that this model, along with a study of joint crosssections, will lend some insight into the factors that affect theformation and distribution of voids within solder bumps
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