Harmonic and Random Vibration Durability of SAC305 and Sn37Pb Solder Alloys

In this paper, durability tests were conducted on both SAC305 and Sn37Pb solder interconnects using both harmonic and random vibration. The test specimens consist of daisy-chained printed wiring boards (PWBs) with several different surface-mount component styles. Modal testing was first conducted on a test PWB to determine the natural frequencies and mode shapes. The PWB was then subjected to narrow-band excitation at its first natural frequency. Electrical continuity of the daisy-chain nets was monitored to measure the time-to-failure (and hence cycles-to-failure) of the interconnects. The response history of the PWB was recorded with strain gages located near the components of interest. Finite element analysis (FEA) was conducted for each component type, to estimate the transfer function between the flexural strain of the PWB and the strain in the critical solder joint. The predicted strain transfer function was then combined with the measured PWB strain response history to estimate the strain history in the critical solder joints. The solder strain history was used, in conjunction with the failure history, to estimate lower bounds for the fatigue durability (S-N curves) of the solder interconnects. In the first part of this paper, the results show that the SAC305 interconnects are marginally less durable than Sn37Pb interconnects for the harmonic excitation range used in this paper. The durability model constants are found to be very sensitive to the solder stress-strain curve assumed in the FEA. Since the stress-strain properties reported in the literature for these solder alloys vary significantly, the solder stress-strain curves were parametrically varied in the FEA, to assess the resulting effect on the estimated S-N curves. In the second part of this paper, random-vibration tests were conducted to assess durability under step-stress, broad-band excitation. Conventional cycle counting techniques were used to quantify the random excitation histories in terms of range distribution functions. Using the same time-domain vibration fatigue analysis used earlier for narrow-band excitation, the durability trend for the corresponding SAC305 and Sn37Pb solder interconnects under broad-band excitation was found to be similar to that found earlier under harmonic vibration excitation. Comparison between the durability prediction and test results provides a good understanding of the effect of stress-strain behavior on the fatigue constants of these solder materials. The best set of material properties was then used to verify the durability of leadless chip resistor interconnects under quasi-static mechanical cycling.