Influence of Nickel and Bismuth Addition on the Mechanical Shear Strength of SAC+ Ni, Bi Solders under Isothermal Aging and Multiple Reflows

The transition from tin-lead to lead-free solders, prompted by increasing environmental and health concerns, led to the consideration of the SAC (Tin-Sn, Silver-Ag, and Copper-Cu) solder alloys for various interconnects in electronic packaging. SAC alloys are a popular choice due to their low environmental impact, reliable mechanical properties, excellent solderability, compliance with various regulations like the Restriction of Hazardous Substances (RoHS), and relatively low melting points suitable for electronic manufacturing. However, when used in high-temperature environments, SAC alloys have been experiencing significant degradation and the solder joint's reliability decreases with time because of microstructural changes. Adding alloying elements to SAC, at different percentage weights, bring unique properties to the solder joint reliability. Nickel and Bismuth when added to SAC improve mechanical strength and aging resistance. The objective of this study is to evaluate the solder joint reliability for the SAC+Ni+Bi (SAC-3%Bi, 0.05%Ni) solder alloy by performing the solder ball shear test on the test vehicle. Ball Shear strength of solder joints that went under isothermal aging and multiple reflows were evaluated for the test vehicle whose size is 12 x 9 mm. It was observed that the shear strength of SAC+Ni+Bi outperformed SAC305 by 30% under isothermal aging conditions and 25% under multiple reflows. In addition to this, IMC growth rate was slow for SAC+Ni+Bi in comparison to SAC305. This study finally concludes that SAC+Ni+Bi outperformed SAC305 for both isothermal aging and multiple reflow test conditions. Hence, SAC+Ni+Bi solder alloys could be employed for high reliability applications.