Development of bonding process for flexible devices with fine-pitch interconnection using Anisotropic Solder Paste and Laser-Assisted Bonding Technology

We bonded 70μm-thick Si chip and PI substrate with anisotropic solder paste (ASP) using laser-assisted bonding (LAB). ASP contains 5 vol.% of Sn/58Bi type 5, 6 vol.% of nonconductive PMMA balls with diameter of 20μm and thermosetting resin. We have applied ASP on the metal pattern on PI substrate. We have aligned the 70μm-thick Si chip on the PI substrate, and applied pressure to keep the bond line thickness uniform. Then, homogenized laser of which wavelength is 980nm, was irradiated on the Si chip for 5 seconds. When the laser is irradiated to the Si chip, the Si absorbs the laser and its temperature rises. Due to the high thermal conductivity of the Si and electrodes, the heat flux is concentrated on the electrode. Thus selective-solder joint is formed only between the electrodes of the chip and substrate. After the bonding process, we have cured bonded devices at 80°C for 2 hours. Measured contact resistance was ~0.03Ω. There were no vaporized substances during the LAB process and the cleaning process is not necessary because the ASP contains no conventional flux and solvent.