Low-temperature attaching of LED chips using SnBiIn solder nanoparticles

Low melting point solders, particularly those with melting points below 100 °C, are gaining increasing attention for applications in 3D IC hierarchical packaging, flexible devices, and biological devices. Previously, we have done some research on the development of novel solder alloys. In this work, we will focus on the study of the application of low melting point solders in die attachment of LED chips. The SnBiIn solder particles of varying sizes were prepared using a heat-assisted ultrasonic method, as reported in our previous research. To develop an industrially applicable die attachment technology, we began by analyzing the oxidation mechanism of SnBiIn solder nanoparticles. Our analysis revealed that oxidation is primarily caused by the outward diffusion of tin and indium. We also showed that a formic acid atmosphere can effectively replace flux at low temperatures to facilitate the sintering of the solder particles. Successful bonding with Au/Ni pad chips was achieved at 80 °C in an acidic atmosphere for 5 minutes. With prolonged time or increased temperature, the intermetallic compounds (IMCs) at the interface transitioned from Au-(Sn, In) IMCs to Cu-(Sn, In) IMCs, due to the thin Au layer and Cu gradient diffusion. This study successfully demonstrates the LED die attachment methodology for Au/Ni pad chip bonding using SnBiIn micro-nano particles, providing strong experimental evidence for the application of low-melting-point solder in device packaging.