One aspect in the ever-challenging thermal management is the design and deployment of effective thermal interface matierlas (TIMs). Gallium-based low melting temperature alloys (LTAs) have been proposed as candidates for next generation TIMs due to their high thermal conductivity (~30 W/m K) and conformability. However, the poor wettability, embrittling, and corroding effect of Ga on metals, including Al and Cu, have limited their use by the electronics industry. Studies on the relationship between the evolution of thermal properties and interfacial reactions between Ga-based TIMs and metal substrates are thus vital for creating a path forward.

 

This work investigates four aspects of Ga-based low melting temperature alloys in their role as TIMs: the interaction between Ga and metal substrates, the change in the thermodynamic behavior of the liquid metal alloy, the evolution of the thermal performance, and mitigation strategies against Ga corrosion. The eutectic Ga-In alloy (EGaIn) was chosen as the base liquid metal alloy for its thermodynamic simplicity and a Ni-plated Cu substrate was used as a representative metal substrate. We created Cu-LTA-Cu joints by sandwiching EGaIn between two Ni-plated Cu substrates using an oxide-mediated 2-stage wetting method. Such joints were submitted to either an isothermal accelerated aging at 125°C or a reflow process to imitate the assembly process. At different time intervals during aging, the thermal conductivity and thermal interface resistance of such joints were measured with an infrared (IR) temperature mapping setup, an enhanced version of the ASTM D5470 standard reference bar method. An increase in thermal conductivity and a decrease in thermal contact resistance were observed after reflow and accelerated aging. Subsequent characterization of the LTA-Cu interface using SEM and EDS reveals that rapid interfacial reactions between EGaIn and both Ni and Cu substrates take place at elevated temperatures. SEM images of the cross-sections of substrates show spalling of the Ni plating as a result of the corrosion by Ga. The enhanced adhesion between EGaIn and the substrate as a result of IMC formation together with the consumption and spalling of the Ni-plating contribute to the increased thermal conductivity and decreased thermal contact resistance observed after accelerated aging. In analyzing the change in thermodynamic behavior of EGaIn during aging, we observed a change in the solidification and melting temperature of the alloy as the aging time increases. In situ XRD reveals this to be the result of the stabilization of the metastable β-Ga phase.