Creep and crack propagation in flip chip SnPb37 solder joints

Abstract

The lifetime determining event in flip chip packages is the fracture of solder joints. Crack initiation and crack growth in micro solder joints, however, are supposed to differ very much from that of bulky samples of steel, nickel, copper etc. that are usually used in fracture mechanics tests. Hence, the commonly known fracture laws do not hold for FC joints while alternative laws have not been established yet because of a lack of experimental data. The paper presents the results of reversible shear tests on flip chip solder joints under isothermal conditions. Two micro shear testers have been designed and built for this task. One tester is optimized to achieve high precision. In contrast to similar setups, this tester is actively compensated for its finite stiffness. Therefore, it is able to record force displacement hysteresis with a resolution of better than 1 mN and 20 nm force and displacement measurements, respectively. The second tester works very similar but fits in a UHV chamber. In this way, it enables in-situ SEM observations during the test. The results of this study show that the deformation behavior of flip chip solder joints to be more alike to that of bulk samples with a comparable micro structure than it is commonly believed based on published data. The parameters of the determined creep equation indicating what deformation mechanism dominates at what strain rate. The results of the creep tests are compared with that of crack growth experiments. The influence of different deformation mechanisms on the crack growth rate is discussed.