Effects of the Ni electrodeposit on microstructure evolution and electrical resistance of the P-type Bi2Te3 solder joint

Abstract

Abstract A systematic investigation was carried out to evaluate the effect of Ni electrodeposit on the microstructure evolution and electrical resistance of the P-type Bi2Te3-based thermoelectric (TE) joint. In this study, two systems (SAC305/(Bi,Sb)2Te3 and SAC305/Ni/(Bi,Sb)2Te3) were set up. The intermetallic compounds (IMCs) were mainly detected by using scanning electron microscope (SEM), X-ray diffractometer (XRD) and transmission electron microscope (TEM). A model of overlapping joint was set up for electrical resistance testing. Results indicated that the SnTe phase was the major and thermodynamically stable product in the SAC305/(Bi,Sb)2Te3 system, and its porous structure determined the presence of micro-tunnels, which facilitated the diffusion process of Sn and Te atoms, therefore the SnTe layer continuously grew in the whole reflowing duration. In this system, the time exponent was analyzed to be 1.2741, indicating that the growth of the IMC layer followed the linear law, and was reaction-controlled. In the SAC305/Ni/(Bi,Sb)2Te3 system, it was remarkable that the Ni barrier inhibited the rapid growth of the IMC layer, and the dominated product was detected to be (Ni,Cu)3Sn4 whose dense distribution could effectively suppress the diffusion of Sn atoms. The time exponent in this system was 0.5010, indicating that the growth of the Ni–Cu–Sn ternary IMC followed the parabolic laws and was mainly volume diffusion-controlled. After reflowing treatment, the electrical resistance remained at 2.203 mΩ for NiTL (the TE leg with the Ni electrodeposit) system while that of TL (the TE leg without the Ni electrodeposit) system rapidly increased to a higher value about 4.879 mΩ. Owing to the lower growth rate of the IMC layer in NiTL system, it could reasonably conclude that the Ni electrodeposit was of great importance in depressing the electrical resistance.