Radiation doses effect on micromechanical properties, microstructure evolution and thermal stability of Sn-Ag-Cu solder

Radiation particles with enough energy to break the chemical bonds of the materials, such as gamma radiation, may affect the performance of the solder joints. In this research, the physical and mechanical properties, microstructure evolution and thermal stability of lead-free solder joints radiated with gamma radiation were investigated. Prior to characterization, the solder pastes of tin-silver copper (SnAg3Cu0.5; SAC305) was manually deposited on the printed circuit board using a stencil printing method and reflow soldering process. Subsequently, the solder was exposed to low doses (5-50 Gy) of gamma radiation. As gamma radiation dose increased, the thickness of the intermetallic compound (IMC) layer of the solder increased suggesting that gamma radiation had coarsened the grain size of SAC305 solder. The contact angle remained to have good wettability. XRD results show the presence of the β-Sn, Cu6Sn5 and Ag3Sn phases. The nanoindentation test revealed the occurrence of pop-in events due to the atomic distortion and dislocation density of the SAC305 samples. The hardness and reduced modulus values increased up to 15 Gy owing to the disturbance in the atomic arrangement of the solder material. However, the values decreased as the radiation dose increased (50 Gy) which could be linked to the presence of voids and irregular IMC. The eutectic phase area showed a parallel trend to the hardness value obtained due to the coarsening of β-Sn and IMC particles in the solder. Microstructure observation of the solder revealed that Cu6Sn5 and Ag3Sn compounds dominated the intermetallic layer in the Sn matrix. Thermal analysis showed that the radiation does not affect the performance and reliability of the solder. These findings are expected to be used as a basis in understanding the effect of radiation on the physical, mechanical properties, microstructure evolution and thermal stability resulting to the reliability of solder joints.
Keywords: micromechanical, microstructural, solder, thermal stability, radiation