Mechanical characterization of Sn-Bi solders having low melting point temperature subjected to thermal cycling is important for understanding the mechanical response and reliability of lead-free microelectronic assemblies. While SAC (Sn-Ag-Cu) is often the standard soldering alloy of choice in industry, Sn-Bi solder with lower melting point temperature is becoming more widely used in industry due to the requirement of lower reflow temperature, and to reduce warpage of the components and printed circuit board (PCB).

The 42%Sn-58%Bi low melting point temperature solder (LTS) paste is often used to mount ball grid array (BGA) packages with SAC solder balls on the board. When such assemblies are reflowed, mixed Sn-Bi/SAC solder joints with prominent interfaces between the SAC and the Sn-Bi regions are formed. Mechanical characterization of such mixed solder joints subjected to different thermal cycling conditions is crucial for better prediction of the reliability of electronic packages. In this study, quarter symmetry 3D models of a plastic ball grid array (PBGA) package with eutectic 42%Sn-58%Bi (58Bi-42Sn) solder joints, mixed SnBi (58Bi-42Sn) and SAC305 solder joints, and homogeneous SAC305 solder joints were made. The constitutive Anand parameter coefficients for both SAC305, and 58Bi-42Sn solders were used to model their non-linear mechanical behaviors. The quarter symmetry models of the PBGA package with these three different solder joint configurations were subjected to a thermal cycling condition, ranging between −40 to 125 °C, with a 90 minute cycle of 15 minutes ramps and 30 minutes dwells. During thermal cycling, the viscoplastic solder alloys are subjected to plastic deformations which eventually lead to their failure as crack initiates and propagates due to thermal fatigue loading. Hence, for all three solder joint configurations (eutectic Sn-Bi, mixed Sn-Bi/SAC, and homogeneous SAC305), the nonlinear plastic work accumulation in the solder joints was used to identify the critical solder joint where the maximum plastic damage occurs. Once the critical joint was identified, the average nonlinear plastic work accumulation (per unit volume) per cycle in the critical solder joint was then used to predict the thermal fatigue life (TFL) of each solder joint configuration, for 3 different PCB surface finish conditions (ENIG, ENEPIG, and ImAg).

From the TFL approximations, it was evident that the mixed Sn-Bi/SAC joint had the highest thermal fatigue life, followed by the SAC305, and the eutectic SnBi (58Bi-42Sn) joints. Even though the failure during thermal cycling occurs at the top SMD end of the mixed SAC/SnBi joint like the other eutectic and homogeneous joint configurations, yet significant amount of plastic work accumulation at the SAC/SnBi interface was observed as well. Thus, the study provides an estimation of the thermal fatigue life of mixed LTS and SAC solder joint configurations, as well as significant insights of the failure mechanisms of these solder joints under thermal cycling exposures with different PCB surface finishes.

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