Effect of assembly sequence on shear behavior of solder joints in BGA under board-level structure

Abstract

In this paper, the shear behavior of solder joints in BGA package structure was studied by test. The four kinds of solder joints used in the research are Sn-3Ag-0.5Cu, Sn-0.3Ag-0.7Cu, Sn-0.3Ag-0.7Cu-0.07La and Sn-0.3Ag-0.7Cu-0.07La-0.05Ce. The influence law of the assembly sequence on the mechanical properties of the solder joints under board-level structure were analyzed. At the same time, the fracture form and fracture mechanism of the two kinds structures were analyzed with the aid of SEM(Scanning Electron Microscope) and EDS(Energy Dispersive Spectrometer). Results show: Firstly, when the assembly sequence under board-level are the same, the shear strength of the high silver solder joints(SAC305) is much larger than that of the low silver solder joints(SAC0307), the shear strength of low silver solder with rare earth elements is greatly improved and can basically reached the level of high silver solder joints shear strength. Especially, the shear strength of low silver solder joints with mixed rare earth elements(SAC0307-0.07La-0.05Ce) is more closer to to that of the high silver solder joints. The shear strength of the four components solder joints in the Cu/SAC/Ni structure is lower than that of Ni/SAC/Cu, and the low silver solder joint is more sensitive to the assembly sequence of reflow soldering. Secondly, the broken displacement of solder joints under Ni/SAC/Cu board-level structure is greater than the solder joints under Cu/SAC/Ni board-level structure, which shows the great plastic performance of the solder joints under Ni/SAC/Cu board-level structure. Finally, the solder joints of Cu/SAC/Ni board-level structure are broken down in the Cu/SAC side under the action of shear loading, but the solder joints of Ni/SAC/Cu board-level structure are basically broken at the Ni/SAC side. It shows that the assembly sequence of reflow soldering has certain influence on the BGA under board-level structure.