Experimental study of void formation in solder joints of flip-chip assemblies
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Voids in solder bumps have been a critical reliability issue as electronic components continue to shrink in size. This dissertation addresses the formation of voids in solder joints of flip-chip assemblies. Specifically, experimental techniques have been developed to study how the heat flux direction affects the occurrence of void formation. Three types of solder pastes including high-lead solder (90Pb/8Sn/2Ag), eutectic solder (63Sn/37Pb), and lead-free solder (95.75Sn/3.5Ag/0.75Cu) have been studied in the present experiments. For high-lead solder, the tests were performed on flip-chip connections to ceramic substrates. As predicted by studies from thermocapillary driven flow theory, the melting direction was found to have significant effect on the formation of voids and their final distribution. Solder bumps that were heated and cooled from top have the smallest void volume and the least probability for finding a defective bump. This represents the best reflow methodology to minimize voids for high-lead solder. Reversing the flow direction during cooling, the bumps show highest percentage of defective bumps and the highest void volume. This demonstrates that for high-lead solder, the effect of cooling process on the formation of voids is very significant. These results are of major significance to manufacturing process. Experiments on lead-free and eutectic solder were carried out on flip-chip connections to FR4 substrates. Both solders show much smaller void volume than high-lead solder. Compared with eutectic solder, lead-free solder has a slightly higher void volume and a higher percentage of defective bumps. For lead-free and eutectic solders, the two cases with heating solder samples from top to bottom show less defective bumps and smaller void volumes. Experimental results were compared with the numerical simulation based on thermocapillary theory. The case of heating from bottom and cooling from top agrees well with the computational results. The agreement of final void distribution inside the solder joints with reference to thermocapillary theory is encouraging. These findings will lead to better void management in solder joints of flip-chip connections.