Microstructure and Property of Au stud/Lead-free solder composite joint

In recent years, the miniaturization trend of electronic products requires larger I/O numbers, finer pitch pads, and higher cost. Thus the interconnection of dies and lead-frames thermally and electrically is becoming a challenge for the development of ultra-thin chip and ultrafine pitch packaging. The utilization of flux in traditional FC limits the reduction of bond-pad pith and the forbidden of Pb and the only use of large size lead-free solder can cause more reliability problems. Flip chip (FC) requires under-bump metallurgy (UBM) formation on the pad of the chip, which requires high cost. Stud bump bonding (SBB) FC, as a low-cost flip-chip packaging, are rapidly becoming a common package for high performance applications. This paper no longer paint flux before reflow, but directly apply solder paste, and then complete the bonding of Au stud with the solder paste. The reactions of the interfaces of Au/Cu, Au/Sn0.7Cu, and Sn0.7Cu/Cu are figured out. From the surface of Au stud bump to the solder, the AuSn, AuSn2 and AuSn4 formed. There was no Cu6Sn5 in the interface between Cu pad of substrate and Sn solder, but a 3μm thick layer of (Cu, Au)6Sn5 existed. In addition, the size of the FC joint and the pitch of fish could be controlled by steel stencil, so that it is possible for the reduction of pad pitch in the guarantee electronic product reliability. The composite joint appeared comparable shear strength with that of pure Sn0.7Cu joint, which were 60.1MPa and 65.2MPa respectively.