Effects of multiple rework on the reliability of lead-free Ball Grid Array assemblies

In this work, the effect of multiple rework cycles (up to 5x) on reliability of lead-free assemblies has been investigated. The test vehicle was designed to capture the reliability impact of rework processes on interconnects of BGA devices, the solder joints of its adjacent components and BGA devices of clamshelled devices. The effects on high aspect ratio plated through hole (PTH) vias and microvias were also considered. A variety of BGA packages, such as FCBGA, PBGA, CABGA, etc. were selected to represent different package technologies and stress levels. The effects of multiple reworks on accelerated thermal cycling (ATC) of lead-free solder joints are extensively explored in this study The experimental and analytical findings showed that multiple reworks significantly degraded the ATC performance of the reworked assemblies by up to 50% in terms of characteristic life. It was also found that multiple reworks affect the PTH vias significantly. The combination of increased lead-free rework temperature excursions and high-end PCB aspect ratios may incur higher stresses in the PTH copper barrel. Early failures were observed post multiple reworks and the subsequent ATC test due to PTH barrel cracking. However, the effects of multiple reworks are less severe for devices with microvias compared to that with PTH vias. Multiple reworks also caused severe degradation of long term performance of devices with clamshell designs. With proper control in rework process, multiple reworks have minimum effects on the adjacent BGA solder joints. In addition, failure analysis was performed in order to study the impact of multiple rework on the microstructure of SnAgCu solder joints. It was found that the IMC layer grows thicker as the number of reworks increases; however, the IMC growth rate and morphology are dependent on the surface finish of the package substrate. The failure mode and failure mechanism after ATC were also compared for different number of rework cycles. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.