Abstract
In this paper we report on an investigation into appropriate reflow profiles for BGA joints using Sn-Ag-Bi-In solder, which has a lower melting point than Sn-Ag-Cu solder. We examine the relationship between the interfacial microstructure and mechanical characteristics of the solder joints compared with Sn-Zn-Bi solder. When soldering Sn-3Ag-0.5Cu BGA balls on Cu/Ni/Au pads, Sn-8Zn-3Bi has high joint strength at 503 K or higher and Sn-3.5Ag-0.5Bi-8In has strength at a lower temperature of 493 K. This implies that Sn-Ag-Bi-In type solder is more appropriate for soldering at lower temperatures. A stratified Ni-Sn layer formed on the joint interface when the Sn-3Ag-0.5Cu BGA ball was soldered on a Cu/Ni/Au pad using Sn-3.5Ag-0.5Bi-8In at 483-493 K. At 503k or higher, blocks of (Cu, Ni) 6Sn5phases formed unevenly on the joint interface and lower strength resulted. These results suggest the appropriate reflow thermal profile of the Sn-3.5Ag- 0.5Bi-8In solder joints with the Sn-3Ag-0.5Cu BGA ball on the Cu/Ni/Au pads should be in the range of 483-493K. Even if the reflow peak temperature is over 493 K, slowing down the cooling speed by 2 K/s can prevent a decrease in joint strength.
