Package-on-Package (PoP) is a popular technology for fabricating chipsets of accelerated processing units. However, the coefficient of thermal expansion mismatch between Si chips and polymer substrates induces thermal warpage during the reflow process. As such, the reflow temperature and reliability of solder joints are critical aspects of PoP. Although Sn[sbnd]58Bi is a good candidate for low-temperature processes, its brittleness causes other reliability issues. In this study, an in-situ observation was performed on composite solders (CSs) made of an Sn-3.0Ag-0.5Cu (SAC305) solder and an Sn[sbnd]58Bi solder that were mixed during reflow at temperatures of 170 °C (CS-170), 180 °C (CS-180), and 190 °C (CS-190). The volumes of the mixed-solder regions were in the order of CS-190 > CS-180 > CS-170. A calculated Sn58Bi-SAC305 isoplethal section of the Sn-Ag-Cu-Bi quaternary system was employed to elucidate the melting behavior of CS-190. CS-190 with Sn cyclic twin boundary and Bi phase preferred orientation of  was demonstrated by electron backscatter diffraction. Moreover, Cu/CS-190/Cu joints with a shear strength of 46 MPa were formed. CS-190 can reduce the brittleness of Sn[sbnd]Bi solder owing to the reduced solid-solution hardening with the decrease in the Bi content. These factors can improve the reliability of PoP technology.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Mechanics of Materials
- Mechanical Engineering