Effect of isothermal aging on the microstructure, shear behavior and hardness of the Sn58Bi/Sn3.0Ag0.5Cu/Cu solder joints

Abstract

In order to decrease the hardness and brittleness of eutectic Sn58Bi (SnBi) solder joint, Sn3.0Ag0.5Cu (SAC) bulk was added into SnBi/Cu by a two-step soldering process to obtain solder joint with superposition structure. The microstructure, shear behavior and hardness of Sn58Bi/Sn3.0Ag0.5Cu/Cu (SnBi/SAC/Cu) during isothermal aging were investigated in comparison with SnBi/Cu and SAC/Cu solder joints. Experimental results indicated that the addition of SAC bulk affected the formation and growth of the β-Sn dendritic in the SnBi bulk in SnBi/SAC/Cu. The relative area and grain size of β-Sn in the SnBi bulk were enlarged due to the increasing concentration of Sn. The atomic diffusion during aging led to the volume reduction of SAC bulk in the SnBi/SAC/Cu joint. The addition of SAC bulk suppressed the brittleness of SnBi/Cu and transformed its failure mode from brittle failure to brittle-ductile failure after reflow. Such effect was weakened and the shear force and failure mode of SnBi/SAC/Cu joint were similar to those of SnBi/Cu, as the solder joints were aged for 600 h at 80 °C. Furthermore, the addition of SAC bulk significantly decreased the hardness of SnBi/Cu before and after aging. Keywords: Solder, Isothermal aging, Microstructure, Shear behavior, Hardness