Fabrication and Mechanical Properties Improvement of Micro Bumps for High-Resolution Micro-LED Display Application

Recently, many significant strides have been made for the integration of micro-light-emitting diodes (Micro-LEDs) in display. However, the reflow process is usually accomplished under formic acid or hydrogen ambient, which is irritating, flammable, explosive, and very expensive. For reliable micro bumps in the flip-chip package, it is essential that the correlations among mechanical properties, fracture mechanisms, and interfacial reactions of micro bumps formed on a silicon substrate be understood. In this article, a novel under bump metallurgy (UBM) structure based on SiO ₂ openings was described, as a result of which the indium micro bump arrays with a pitch of 40 $\mu \text{m}$ and a diameter of 15 $\mu \text{m}$ were successfully fabricated by the reflow process via glycerol. The effect of reflow time and reflow temperature on the reliability of indium micro bumps was also investigated. Experimental results showed that the average shear strength of indium micro bumps peaked at the reflow conditions of 200 °C for 180 s and decreased with further increasing reflow time and temperature. It was considered that the stable AuIn ₂ intermetallic compounds (IMCs) formed toward the solder joint improved the shear strength after reflowed at 200 °C for 180 s. As the reflow time and temperature increased, the grain size of IMCs increased, which weakened the interfacial connection. In addition, the transfer and bonding reliability of Micro-LED chips through the flip-chip bonding process have also been discussed.