Abnormal Shear Performance of Microscale Ball Grid Array Structure Cu/Sn–3.0Ag–0.5Cu/Cu Solder Joints with Increasing Current Density

The shear performance and fracture behavior of microscale ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with increasing electric current density (from 1.0 × 103 to 6.0 × 103 A/cm2) at various test temperatures (25 °C, 55 °C, 85 °C, 115 °C, 145 °C, and 175 °C) were investigated systematically. Shear strength increases initially, then decreases with increasing current density at a test temperature of no more than 85 °C; the enhancement effect of current stressing on shear strength decreases and finally diminishes with increasing test temperatures. These changes are mainly due to the counteraction of the athermal effect of current stressing and Joule heating. After decoupling and quantifying the contribution of the athermal effect to the shear strength of solder joints, the results show that the influence of the athermal effect presents a transition from an enhancement state to a deterioration state with increasing current density, and the critical current density for the transition decreases with increasing test temperatures. Joule heating is always in a deterioration state on the shear strength of solder joints, which gradually becomes the dominant factor with increasing test temperatures and current density. In addition, the fracture location changes from the solder matrix to the interface between the solder matrix and the intermetallic compound (IMC) layer (the solder/IMC layer interface) with increasing current density, showing a ductile-to-brittle transition. The interfacial fracture is triggered by current crowding at the groove of the IMC layer and driven by mismatch strain at the solder/IMC layer interface, and the critical current density for the occurrence of interfacial fracture decreases with increasing test temperatures.

The In-Situ Observation of Grain Rotation and Microstructure Evolution Induced by Electromigration in Sn-3.0Ag-0.5Cu Solder Joints

The in-situ observation of Sn-3.0Ag-0.5Cu solder joints under electromigration was conducted to investigate the microstructure and grain orientation evolution. It was observed that there was a grain rotation phenomenon during current stressing by in-situ electron backscattered diffraction (EBSD). The rotation angle was calculated, which indicated that the grain reorientation led to the decrease of the resistance of solder joints. On the other hand, the orientation of β-Sn played a critical role in determining the migration of Cu atoms in solder joints under current stressing migration. When the angle between the electron flow direction and the c-axis of Sn (defined as α) was close to 0°, massive Cu6Sn5 intermetallic compounds were observed in the solder bulk; however, when α was close to 90°, the migration of the intermetallic compound (IMC) was blocked but many Sn hillocks grew in the anode. Moreover, the low angle boundaries were the fast diffusion channel of Cu atoms while the high grain boundaries in the range of 55°–65° were not favorable to the fast diffusion of Cu atoms.