Vibration Durability Assessment of Sn3.0Ag0.5Cu and Sn37Pb Solders Under Harmonic Excitation

Abstract

In this paper, the vibration durability of both SAC305 & eutectic SnPb interconnects is investigated with narrow-band harmonic vibration tests and finite element simulation. The tests are conducted at the first natural frequency of the test board using constant amplitude excitation. Compared to broad-band vibration durability test, the advantage of harmonic test is less complexity, hence, less uncertainty in the desired fatigue constant. A time-domain approach, reported in the literature 0, was adapted in this study for the fatigue analysis. The test board consists of daisy-chained components, to facilitate real-time failure monitoring. The tests are conducted at different loading amplitude to obtain the durability data points located at both low cycle fatigue and high cycle fatigue region. The SAC305 solder was found to have lower fatigue durability than the SnPb solder, under narrow-band harmonic excitation, which is consistent with results from broad-band vibration test 0, as well as with results of mechanical cycling studies and repetitive mechanical shock studies conducted earlier 0. The test board was first characterized before the vibration durability test. A modal test was conducted to determine the mode shape and natural frequency, which were used to decide the excitation frequency of durability test. Flexure strain amplitudes at critical locations on the PWB were first characterized with strain gages at each level used in the test excitation. Detailed local finite element analysis was then conducted to estimate the strain transfer function between the PWB flexure strain and the strain in the critical solder joints for selected components (BGA256, LCR1210 and LCR2512). Then the strain history in the critical solder joint was estimated from the measured PWB strain, using the strain transfer function. Finally the solder strain and the measured time-to-failure data were used to estimate durability model constants for a generalized strain-life fatigue model 0 for both SAC305 and eutectic SnPb solder. Destructive failure analysis (cross-sectioning, polishing and microscopy) was used to confirm that the failure was by solder fatigue.