Submicron Au-Particle Bumps for High-Density Flip-Chip Mounting

To perform flip-chip bonding, bumps must be formed on electrodes on the substrate of a chip. Depending on the intended application, various methods, such as soldering, plating, and stud bumping, are used to form bumps. Furthermore, submicron metal particles are used as bonding materials because they easily undergo surface diffusion owing to their high surface energies, thereby resulting in low-temperature sintering. Submicron particles used for mounting are typically mixed with solvents to form a paste. However, these pastes tend to travel up chip walls and cause short circuits between electrodes, making them unsuitable for flip-chip bonding with pitches below several hundred micrometers. Therefore, we developed a method to form fine Au-particle bumps by combining submicron Au-particle paste with a resist process. The resulting bumps had a porous structure that allowed compression in the direction of pressing without lateral expansion, thereby accommodating the substrate steps and enabling high-density mounting. In this study, we explored the formation of Au-particle bump shapes that are suitable for various applications. By altering the aperture diameter, pitch, and thickness of the resist patterns, we formed bumps with a minimum diameter of 5 μm and height of 10 μm. However, when the aspect ratio was too high, the bumps collapsed owing to insufficient resist exposure, and when the bump diameter was too large, shape defects such as depressions were observed. These results enabled the selection of appropriate bump diameters and heights according to the size, roughness, and warpage of the electrodes on the substrate. As electronic devices continue to be miniaturized and integrated, high-density bonding on nonplanar surfaces is required. We aim to utilize the unique advantages of Au-particle bump bonding in applications that require high density and reliability, which has not been possible thus far.