Patterned die-to-die thin film bonding for 3D chip stacks with integrated microfluidic cooling

A novel semi-fluxless die-to-die bonding solution is presented, developed and successfully tested. Consisting of patterned thin film lead-free solder, this method was developed to meet the constraints given by the integration of state-of-the-art cooling structures into a 3D chip stack, such as minimizing the gap between the dies, sealing the active solder pads from a conductive coolant fluid and sealing the edges of the chip to prevent leakage. Non-standard solder shapes were designed for some of these purposes, and reflow experiments led to the understanding of the effect of solder aspect ratio on UBM wetting behavior. Finally, the quality of the thin bond was assessed through Focused Ion Beam cross-sections as well as daisy-chain electrical measurements.