Effect of gamma irradiation on microstructural evolution and mechanical properties of Sn3Ag0.5Cu solder joints

Abstract

Sn3Ag0.5Cu (SAC305) Ball Grid Array (BGA) solder joints were exposed to 60Co source gamma irradiation at a dose rate of 1 Gy(Si)/s from 0 to 200 h in air. The effects of gamma irradiation on the microstructure evolution and mechanical properties of the solder joints were investigated. β-Sn interacted with O2 in the air and was oxidized to SnO2 on the solder joint surface after gamma irradiation. The oxidation initiated at the corners of the solder joint/pad interface, extended to the entire solder joint/pad interface, and finally occurred at the solder joint matrix. Microcracks usually grow around SnO2/β-Sn interfaces. The interaction of gamma rays with β-Sn and SnO2 is the primary reason for solder joint oxidation. Gamma irradiation generated atomic vacancies, breaking bonds, and dangling bonds in β-Sn and SnO2 crystals through the Compton Effect, which promoted the physisorption and chemisorption of O2 on their surfaces, as well as the oxidation of β-Sn and growth of SnO2. The interaction between the gamma irradiation and solder joints can be divided into two stages. SnO2 was nucleated at lattice defects induced by the interaction between gamma rays and β-Sn crystals in the first stage. The growth of the SnO2 nuclei resulting from a large number of crystal defects in SnO2 and the potential difference between SnO2 and the β-Sn surface induced by the Compton Effect occurred in the second stage. According to the differential scanning calorimetry (DSC) analysis, the melting temperature of the SAC305 solder alloy decreased with increasing irradiation time. According to the nanoindentation test results, with an increase in irradiation time, the microhardness of the solder joints first increased, then decreased, and finally increased again.