Material characterization of a novel lead-free solder material — SACQ

The wafer level ball grid array (WLBGA), a silicon die-size package, offers the small form factor and high-performance packaging solution. Good board-level reliability under drop impact is achieved on account if its light weight. There is, however, a limitation on the reliability of the solder joint at board level, subject to thermal cyclic loading. This places a limit on the silicon die-size window in which WLBGA packaging can be applied. Unlike other plastic BGAs, a WLBGA is a silicon chip directly mounted onboard without a plastic interposer. This causes a larger coefficient of thermal expansion (CTE) mismatch between silicon and organic printed circuit board (PCB), which leads to higher solder joint stress. This scenario gets even worse with increasing die size where corner solder joints are further away from neutral points. This study considers methods for extending the board-level reliability window in the face of increasing die-size requirements. One way to improve the reliability of the solder joint is by applying underfill epoxy material to protect the joints. Another possibility is by adopting lower CTE board material for the PCB. These methods are, however, not cost-effective. A more cost-effective solution is introduced by developing a new solder alloy with better creep and mechanical properties to enhance the solder joint integrity. A novel solder alloy material was successfully developed and proven to enhance the solder joint reliability, when subject to thermal loading, without degrading the board-level drop-test reliability. This study presents the characterization of the new solder alloy. Its visco-plastic constitutive behavior was tested by arranging tensile tests under constant strain-rate loading at various temperatures. The constitutive model is constructed by fitting the experimental data collected through a 9-parameter Anand model. The validity of the material model was verified by comparing numerical analyses with the experimental results.