Effects of Extended Dwell Time on Thermal Fatigue Life of Ceramic Chip Resistors

Abstract

The understanding of the effects of temperature cycling parameters, such as dwell and ramp times, mean cyclic temperature, and temperature range, on the fatigue life of solder interconnects is critical for qualification and reliability testing. After the solder achieves complete stress relaxation, a further increase in dwell time does not decrease the fatigue life of solder interconnects. Studies have shown that an increase in dwell time beyond a certain limit (10–20 minutes) has no effect on the fatigue life of eutectic tin-lead solder when cycled at peak cycle temperatures at or above 100°C. The duration of stress relaxation is longer in SnAgCu and SnCu solders than in eutectic tin-lead solder, resulting in higher creep damage accumulation and thereby reducing the fatigue life of solder interconnects. Experimental data for modeling of the effects of extended dwell time (beyond 60 minutes) on the temperature cycling reliability of tin-silver-copper solders is limited. In this study, forty 2512 ceramic chip resistors soldered onto an FR4 board using SAC105, SAC305, SN100C, and eutectic SnPb solders were subjected to temperature cycling tests with dwell time durations of 10 and 120 minutes, respectively. Resistors soldered on standard and narrow pads were compared to study the effects of pad size on thermal fatigue reliability. In eutectic SnPb-soldered narrow pad resistors, the increase in dwell time to 120 minutes did not change the cycles to failure. However, SnPb-soldered standard pad resistors showed a decrease in fatigue life with the increase in dwell time. For SAC105- and SAC305-soldered narrow and standard pad resistors, the 120-min dwell decreased the thermal fatigue life, compared to the 10-min dwell. The thermal fatigue life of SN100C-soldered narrow pad resistors increased when the dwell time was increased to 120 minutes, while that of the standard pad decreased. In the case of narrow pad resistors, extended dwell may be have annealed the SN100C solder, making it more robust to solder fatigue.