Microstructure evolution and acceleration factor (AF) of micro-solder bumps in through-silicon-via (TSV) in high temperature storage (HTS) conditions

In our study, the microstructure and reliability of micro-solder bumps of through-silicon-via (TSV) packages were assessed in high temperature storage (HTS) condition with several bump structures. The packages were assembled by molded underfill (MUF) processes, and to test the different resin environments, the MUF conditions were modified. Because HTS requires a long period of time in the formal reliability conditions, the estimation of acceleration factors (AF) is critical for the practical evaluation purpose. Elevated temperature (180oC) was used as an acceleration condition against the normal condition (150oC). AF was estimated by measuring the intermetallic (IMC) thickness during HTS. Two main mechanisms for IMC growth were considered in the process of obtaining AF: one is the volume shrinkage during IMC formation and the other is the solder diffusion along the lateral side of under-bump metallurgy (UBM). Based on these, the AF of 180 oC HTS referenced on 150oC was estimated to be in the range of 5-7. To verify the accuracy of AF, two AF values from the actual failure time and IMC growth were compared (between 180oC and 200oC), and there was a good consistency. Assuming the same failure mechanism during HTS for the entire temperature region in this study (125-200oC), we concluded that AF between 150oC and 180oC is also valid. That is, the condition at 180oC for 145hr can replace the condition at 150oC for 1008 hr. Neither showed an actual failure, meaning the conditions are safe.