Interfacial reaction of In-Ag thermal interface materials with Ti/Ni/Ag backside chip metallization and its influence on thermal resistance

Indium-based thermal interface materials (TIMs) are being increasingly adopted in TIM1.5 chip-to-lid bonding for high-power packages due to their high thermal conductivity and mechanical compliance. However, the rapid interdiffusion and formation of intermetallic compounds (IMC) between indium and typical chip metallizations compromise diffusion barriers, affecting long-term reliability. In this study, pure In and In-Ag alloy TIMs (In-2Ag and In-10Ag) were investigated in a representative Si/Ti/Ni/Ag | TIM | Ag/NiV/Cu sandwich structure bonded at 260 °C for 5 min under 3.5 MPa. Bulk TIM heat transport properties were first measured over controlled bond-line thicknesses, and then the package-relevant thermal resistance of the bonded stacks was assessed. The results show that Ag alloying increases thermal resistance and reduces effective thermal conductivity relative to pure In. However, cross-sectional images reveal that the chip-side Ag metallization fully dissolves into the In matrix of the pure In TIM and directly reacts with the Ni barrier, forming Ni28In72 IMCs and exhibiting pronounced Ni consumption. In contrast, In-Ag TIMs form AgIn2-dominated IMC layers that effectively suppress Ni consumption. At the assembly level, the total thermal resistance increases from 0.271 K/W (pure In) to 0.355-0.359 K/W (In-Ag), highlighting a composition-dependent trade-off between thermal transport and interfacial stability.