In the development of compact and thin IC modules of heat resistant type (over 150℃) for automobile applications, the mechanism that deteriorates the SnPb solder bump joints had to be clarified. The samples were put to acceleration test under the temperatures of 150℃, 165℃ and 180℃ respectively, and the cross sections of the solder bump joints were examined at fixed intervals of time during the test. The result shows that the deterioration of the joints is caused by voids and cracks made by mutual diffusion between Cu and Sn at the joint. Furthermore, we found that the width of the alloy formed by the mutual diffusion can be expressed by the solid state diffusion equation. This made prediction of the life of solder bump joints possible. As the result, the life of about 25,700 hours at 150℃ is obtained.
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