Interfacial reaction and IMC growth kinetics at the Bi2Te3/Ag interface during isothermal aging

Abstract

Bonding materials that can withstand high operating temperatures are essential for increasing the conversion efficiencies of thermoelectric power-generation devices. Sn-based solders commonly used to join thermoelectric materials and electrodes have the disadvantage of limiting the maximum operating temperature to 150 °C because of their low melting point. To overcome this limitation, in the present study, Ag nanoparticle paste with high-temperature stability, low electrical resistivity, high thermal conductivity, and printability was used for bonding a Bi₂Te₃ thermoelectric material and an electroless nickel immersion gold (ENIG)-plated Cu electrode.

We performed isothermal aging at 200 °C from 0 to 1000 h to analyze the microstructural and thickness changes in intermetallic compounds (IMCs). Initially, a heterogeneous distribution of the IMC layers was observed at the bonding interface; over time, the IMC structures became clearly distinct. The IMC thickness increased from ≤3.0 μm (initial) to 45.6 μm in 1000 h, growing by a factor of approximately 15.2. We analyzed the growth kinetics of AgTe and BiTe IMCs. The growth exponent n of the AgTe IMC was found to be 0.3, indicating grain boundary diffusion with grain-growth control, whereas the n value of the BiTe IMC was 0.5, which is consistent with volumetric diffusion-controlled growth. These differences in the growth behavior indicate that different diffusion mechanisms affect the reliability and performance of the bonding interface.