In Situ Observation of the Ga- and Cu-Based Substrate Reaction by Synchrotron Microradiography

Gallium (Ga) and Ga-based alloys have attracted much attention in the microelectronics industry due to their low melting points at room temperature and below. Compared to traditional Sn-based solder alloys, which require processing at 240 °C and above, near-room temperature liquid sintering with Ga-based alloys reduces energy costs and prevents damage to heat-sensitive electrical components during the soldering process. Mechanically sound joints depend on the formation of higher melting point intermetallic compounds (IMCs) such as CuGa 2, the main IMC that forms between Ga- and Cu-based substrates, which has certain desirable properties compared to the IMCs formed from the reaction between traditional Sn-based alloys and Cu substrates. However, the reaction between Ga and commonly used Cu substrates is slow, and the IMCs do not form within practical time frames, hindering the application of Ga-based solder alloys. To accelerate the reaction between Ga-based alloys and substrates during soldering, it is essential to know the mechanisms behind the formation of the IMCs. This study investigates the reaction between liquid Ga and Cu-6 wt %Ni substrates during heating and cooling using in situ synchrotron imaging. It is shown that the formation behavior of CuGa 2 differs during the heating and cooling processes. The results of ex situ experiments and density functional theory (DFT) calculation are compared with the in situ experiments to provide insights into the reaction between Ga- and Cu-based substrates.