Novel solder pads for self-aligned flip-chip assembly

Self-alignment via solder-surface tension in flip-chip bonding opens the door to low-cost, high-throughput assembly of components with sub-micron accuracy. This is especially impactful to integrated photonics as used for high speed optical communication and sensors [1,2]. Assembly yield hinges on the details of solder-induced forces and on the geometry of the melted solder surface. Low curvature of melted solder is best to balance solder forces for optimal re-alignment yield but leads to shallow contact angles and solder de-wetting on traditional solder pads. We introduce and demonstrate the concept of recessed solder pads with shallow angled edges. Such geometry enables arbitrarily-low curvature of the molten solder surface and even flat or slightly concave shapes. The solder stays anchored at the angled edges of recessed pads and can be made to flow in long and narrow conduits. Both aspects are key to widening the fabrication and process window for the solder-induced chip-alignment technology.