Evaluation of Long-term Stability of Bismuth-Tin Solder Joint Properties in Dependence on Reflow Conditions

This work aimed to evaluate the change of properties of joints based on bismuth-tin eutectic alloy soldered under various reflow conditions during long-term accelerated aging tests. Four temperature profiles were used for the reflow soldering of testing boards. The profiles differed in the reflow phase and the pre-heat phase when the soldering flux is activated. Subsequently, the first half of the testing boards was aged for 1000 hours in a climatic chamber at 85 °C and relative humidity 85 %. The second half was closed up in a temperature-shock chamber for 1000 cycles with the following settings: a hot temperature of 125 °C, a cold temperature of -40 °C, and a cycle time of 30 minutes. The solder joint properties – electrical resistance, shear strength, and thickness of intermetallic layers – were measured four times during the aging to observe trends. It turned out from the measurement results that the most stable during temperature-humidity aging were the joints soldered with higher temperature in the pre-heat phase and lower temperature in the reflow phase. On the contrary, the joints soldered with a lower temperature in the pre-heat phase and a higher temperature in the reflow phase were more resistant against the thermal shocks. The shear strength was even higher than before the start of the test. Overall, both pre-heat and reflow settings play an important role in solder joint reliability.