Rate-dependent properties of Sn–Ag–Cu based lead-free solder joints for WLCSP

Abstract

The increasing demand for portable electronics has led to the shrinking in size of electronic components and solder joint dimensions. The industry also made a transition towards the adoption of lead-free solder alloys, commonly based around the Sn–Ag–Cu alloys. As knowledge of the processes and operational reliability of these lead-free solder joints (used especially in advanced packages) is limited, it has become a major concern to characterise the mechanical performance of these interconnects amid the greater push for greener electronics by the European Union. In this study, bulk solder tensile tests were performed to characterise the mechanical properties of SAC 105 (Sn–1%wt Ag–0.5%wt Cu) and SAC 405 (Sn–4%wt Ag–0.5%wt Cu) at strain rates ranging from 0.0088 s −1 to 57.0 s −1. Solder joint array shear and tensile tests were also conducted on wafer-level chip scale package (WLCSP) specimens of different solder alloy materials under two test rates of 0.5 mm/s (2.27 s −1) and 5 mm/s (22.73 s −1). These WLCSP packages have an array of 12 × 12 solder bumps (300 μm in diameter); and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM) as their silicon-based interface structure. The bulk solder tensile tests show that Sn–Ag–Cu alloys exhibit higher mechanical strength (yield stress and ultimate tensile strength) with increasing strain rate. A rate-dependent model of yield stress and ultimate tensile strength (UTS) was developed based on the test results. Good mechanical performance of package pull-tests at high strain rates is often correlated to a higher percentage of bulk solder failures than interface failures in solder joints. The solder joint array tests show that for higher test rates and Ag content, there are less bulk solder failures and more interface failures. Correspondingly, the average solder joint strength, peak load and ductility also decrease under higher test rate and Ag content. The solder joint results relate closely to the higher rate sensitivity of SAC 405 in gaining material strength which might prove detrimental to solder joint interfaces that are less rate sensitive. In addition, specimens under shear yielded more bulk solder failures, higher average solder joint strength and ductility than specimens under tension.