The Effect of Homologous Temperature on the Thermal Cycling Performance and Failure Mode of a Bismuth-Tin Low Temperature Solder Alloy

ABSTRACT Low temperature solder (LTS) alloys based on the bismuth-tin (Bi-Sn) binary system are being developed and commercialized to address specific electronic assembly applications. Eutectic Bi-Sn solder (58 wt.% Bi and 42 wt.% Sn) has a melting point of 138 °C compared to the near-eutectic, de facto standard, SAC305 solder (96.5 wt.% Sn, 3.0 wt. % Ag, 0.5 wt.% Cu) with its onset of melting at 217 °C. The low melting point of Bi-Sn solder joints presents a potential reliability limitation in applications requiring thermal fatigue resistance. Thermal cycling is the method used to evaluate fatigue resistance. For Bi-Sn solder joints, the upper temperature extreme of the thermal cycling profile typically is limited to 100 °C due to the low melting temperature of the alloys. However, because device operating temperatures may now exceed 100 °C, it is necessary to adjust the thermal cycling profile to account for increased acceleration to failure above 100 °C. To address the effect of the upper temperature extreme, this paper presents thermal cycling results for Bi-Sn ball grid array (BGA) solder joints using three distinct test conditions: 0/100 °C, 15/115 °C, and 25/125 °C. Thermal cycling data for SAC305 is used as the performance baseline. The results show that the failures are accelerated faster in the Bi-Sn solder than SAC305, and the difference is significantly greater with the 25/125 °C test condition. The findings are discussed in terms of the homologous temperature effect, microstructural evolution, and failure mode as the upper temperature extreme is increased.