Fluxless TCB with Atmospheric Pressure Plasma Surface Treatment Enabling Ultra-Fine Pitch Solder Micro-Bumps for 3D Integration

As the semiconductor industry advances towards 3D heterogeneous integration and chiplet stacking, achieving fine-pitch, high-density bonding is increasingly challenging. For current High Bandwidth Memory (HBM), Thermocompression Bonding (TCB) with Non -Conductive Film (NCF) is widely adopted. Flux-assisted TCB offering enhanced solder joint quality is still required for higher performance HBM and high-end computing (HPC) due to reliability requirements. However, flux joining at fine pitches introduces signif icant challenges, including flux residue control, post -bonding cleaning, and underfill surface activation, all of which can pose long-term reliability issues . Consequently, fluxless TCB for microbump interconnects is highly desirable, addressing the critical aspect of achieving oxide -free bonding surfaces. Since both solder bumps and receiving pads oxidize upon ambient exposure, an effective in-situ oxide removal method is essential. In t his study we demonstrate residue-free atmospheric pressure plasma surface activation method for fluxless TCB . Utilizing an Ar with 1-3% H₂ mixture as the source , this gas plasma effectively removes oxide layers from joining surfaces. We successfully developed a novel TCB bonding process, achieving complete wetting and robust bonding of fine -pitch solder joins in die -to-die format. Reliability assessments, including deep thermal cycles ( -50°C to 125°C) for 1000 cycles, showed no bonding defects or underfill voids attributable to the plasma cleaning. We conclude that such an integrated oxidation removal method is crucial for advancing heterogeneous integration and enabling reliable fine-pitch bonding in future 3D stacking applications.