Mechanics of solder/IMC interface of lead-free solder interconnects in ball grid array assembly

Abstract

The lead-free Sn-0.4Ag-0.5Cu (SAC405) solder arrays provides an interconnection between the electronic package and printed circuit board (PCB) of the assembly. The ball grid array (BGA) test assembly was exposed to thermo-mechanical loading during solder reflow cooling and subsequent reliability temperature cycles. This could contribute to the solder/IMC interface damage and cracking. Finite element (FE) analysis was utilized to model the BGA assembly under the prescribed temperature loading profile. The unified constitutive (Anand) model was used to describe the temperature- and strain rate-dependent response, and solder/IMC interface damage of the SAC405 solder interconnects. Solder reflow cooling begins from the assumed stress-free reflow temperature of 220 °C to 25 °C at 1.3 °C/s. This was followed by temperature cycling between 125 and -40 °C with 15 minutes dwell time at upper and lower peak temperature levels. The temperature ramp rate of 11 °C/min was used. Results show that the most critically stressed solder joint was the one located underneath the edge of the silicon die. The solder/IMC interface crack initiated at the interface near the package side of the assembly. Catastrophic fracture of the BGA assembly was predicted during the first temperature cycle due to the relatively low input strength and toughness of the brittle-like solder/IMC interface properties. The solder/IMC failure prediction could not be inferred solely from inelastic strain in the bulk solder joint but should also consider the damage of the interface. This study shall benefit the newly designed BGA packages through rapid generation of reliability data and by providing insight into the reliability aspects of the BGA assembly with interface fracture issues.