Development of simultaneous transferring and bonding (SITRAB) process for µLED arrays using Anisotropic Solder Paste (ASP) and Laser-Assisted Bonding (LAB) Technology

We have transferred and bonded a $\mu \text{LED}$ array to the patterned substrate using SITRAB (Simultaneous transferring and bonding) and ASP (Anisotropic Solder Paste). ASP contained epoxy as a base matrix, solder powder, and a small fraction of polymer balls and was applied to the substrate using screen printing. We have arrayed $\mathrm{20}\times \mathrm{20}$ of $\mu \text{LEDs}$ on the interposer, which is PDMS laminated glass wafer. The interposer was picked-up by quartz bonder-head and aligned to the substrate. After aligning the interposer to the substrate, the bonder-head moved to the substrate and applying pressure. While applying the pressure, the homogenized laser with a wavelength of 980nm was irradiated through the quartz bonder-head for a few seconds. In this process, transferring and bonding the $\mu \text{LED}$ array are performed simultaneously, so we named this process SITRAB. After the SITRAB process, the $\mu \text{LED}$ array bonded substrate was cured at 120°C for 2 hours. Because the ASP contains no vaporizable substance, there is no fume during the SITRAB process, and the cleaning is unnecessary after the post-cure process. Using ASP material and the SITRAB process, a $\mathrm{20}\times \mathrm{20}\ \mu \text{LED}$ -array was successfully transferred and bonded to the substrate. We observed the light output from the $\mu \text{LED}$ array bonded to the electrodes and measured a cross-sectional SEM image to analyze the bonding joint.