Solder Mobility for High-Yield Self-Aligned Flip-Chip Assembly

Self-aligned flip-chip assembly with sub-micron accuracy is of particular importance to low-cost manufacturing of single-mode opto-electronic components. The concept of alignment via surface tension force of melted solder has been proposed over two decades ago and appears simple. Yet, its effective working into manufacturing requires solving a few fundamental issues. In prior work, we introduced the concept of solder reservoirs which provide a solder volume self-balancing mechanism to notably enhance self-alignment yield. In this paper, we show that the effectiveness of reservoirs is impeded when the solder wetting of pads or the solder mobility between pads and reservoirs is limited. We therefore studied a wide variety of metal stacks and identified candidates for substantial wetting and solder mobility improvement. We ranked the metal stacks for solder mobility using traditional wetting angles as well as speed of wetting along narrow tracks. First test parts, manufactured with the improved metal stacks, show the expected benefit in increased yield for chip alignment.