Root Cause and Solution to Mitigate the Hot Tear Defect Mode in Hybrid SAC-Low Temperature Solder Joints

ABSTRACT Tin-Bismuth (Sn-Bi) based solders are increasingly used in motherboard manufacturing to improve surface mount technology (SMT) process yield by reducing temperature at SMT reflow. This reduces induced warpage, carbon emissions, and electricity consumption. The majority of Ball Grid Array (BGA) components are configured with a Tin-Silver-Copper (SAC) solder ball. Therefore, SMT with a low temperature solder (LTS) paste will form a hybrid SAC-LTS solder joint. The hybrid solder joints may experience a unique set of failure modes due to the different metallurgical properties of SAC vs Sn-Bi. This paper will introduce one of the failure modes called ‘Hot Tear’. The mechanism for how Hot Tear defects occur during the reflow process is detailed. Impact of SMT process parameters (paste volume ratio, varying Bi% weight in solder paste, reflow parameters) and its impact to Hot Tear defect mode are discussed. Finally, process recommendations are provided to mitigate the failure mode.