Analysis of Mechanical Behavior of Hybrid SAC-LTS Joints Under Temperature Cycling With a Modified Garofalo Creep Model Based on Bi Concentration

Low-temperature solders (LTSs) are gaining popularity in surface mount technology (SMT) for their lower melting points, thereby reducing reflow temperatures, and minimizing warpage, cost, and energy consumption. Hybrid SAC-LTS joints, formed by mounting ball grid array (BGA) packages with Sn-Ag-Cu (SAC) solder balls onto printed circuit boards (PCBs) using LTS paste, exhibit a varying distribution of bismuth (Bi) from the PCB Cu-pad to the component Cu-pad. Bi diffusion restricts the coarsening of intermetallic compound (IMC) particles and layers during thermal cycling, potentially increasing reliability during thermal cycling. However, excessive Bi concentrations at the PCB side can lead to Bi-embrittlement, making it a prevalent failure location. This study focuses on fitting creep test data for SAC+X%Bi alloys and modifying a Garofalo creep model to account for Bi diffusion in hybrid joints. In addition, a finite element model of a BGA assembly with hybrid joints was developed. The Bi distribution in the joints was modeled using multiple layers, where the Bi-concentration gradient in each layer was chosen to replicate observations in actual joints that were mapped by the Energy-Dispersive X-ray Spectroscopy (EDS) technique. The modified Garofalo creep model was implemented in the finite element analysis (FEA) of the hybrid joints, and the nonlinear creep work accumulation in the solder joints was computed. The FEA results were correlated with board-level thermal cycling (BL-TC) failure analysis results using the Syed model for thermal fatigue life (TFL) calculation. This analysis proposed a dedicated constant in the Syed model for hybrid joint failures at the PCB end, which can be utilized in future FEA simulations of hybrid joints with modified loading conditions for informed decision-making.