Capillary-Assisted Automated Silver Direct Bond of Chip-on-Chip Through a Novel Sponge-Dipping Approach

Abstract

Silver direct bonding is an organic-free joining technique for chip-on-chip power electronic assemblies that eliminates the need for sinter paste, but also removes the temporary chip-tacking function that the paste normally provides. This article presents a simple and cost-effective sponge-dipping approach, in which a water-soaked sponge module is integrated into a conventional pick-and-place process. The resulting capillary forces retain the chips in position during multichip stacking without requiring a dedicated dispensing unit or hydrophobic surface patterning. Experiments on five-chip stack assemblies with three chip footprints ( $4\times 4$ , $8\times 8$ , and $10\times 10$ mm ${}^{2}$ ) show that capillary assistance increases the yield to 100% for the two larger footprints, eliminates complete placement failures across all chip sizes, and reduces misalignment. Scanning acoustic microscopy (SAM) and cross-sectional scanning electron microscope (SEM)/energy-dispersive X-ray (EDX) analyses reveal no significant mineral contamination at the Ag–Ag bonding interface, indicating that the applied deionized (DI) water does not measurably affect bond quality under the tested conditions.