Bonding of micro LEDs using wet reflow process of indium bumps based on SU-8 solder mask

• SU-8 enables single-step lithography, greatly simplifying fabrication and reducing cost. • The method yields high-quality In bumps with improved roughness, uniformity, and strength. • A low-temperature glycerol-assisted wet reflow facilitates oxide mitigation and improves bonding reliability. This study proposes an innovative wet reflow process utilizing SU-8 photoresist as the solder mask for fabricating In bump arrays in Micro LED packaging. Conventional solder masks such as SiO 2 or metal layers involve complex processes, elevated temperatures, and limited compatibility with flexible substrates. In contrast, SU-8 enables mask patterning via single-step UV lithography, greatly simplifying fabrication and effectively reducing manufacturing complexity and cost. The optimized process achieved a 480 × 640 In bump array with excellent morphology: surface roughness (R a ) reduced from 0.65 μm to 0.126 μm, height non-uniformity improved from 4.8 % to 0.28 %, and shear strength increased nearly tenfold to 1.106 N. Using glycerol as an eco-friendly wet reflow medium facilitated oxide mitigation and enhanced bump uniformity and bonding reliability. The results demonstrate that this low-temperature, efficient, and scalable approach offers clear advantages for high-yield Micro LED integration, particularly in flexible and high-resolution display applications.