Indium-based Flip-chip Interconnect for Cryogenic Packaging

Flip-chip die attachment manufacturing has seen a dramatic increase in recent years due to improvements in electrical performance and reduced form-factor. There have been increasing efforts recently in chip-packaging for qubit devices operating at cryogenic temperatures. Flip-chip assembly process can help to reduce existing cumbersome wirings, latency and enable heterogeneous integration [1],[2],[3].Alloys containing indium have frequently been used in processes that need low-temperature operation. Indium has the benefits of being ductile, cryogenically stable, and having strong thermal and electrical conductivity as a solder material. Indium-based solder is preferred due to its superconductivity, soft and malleable characteristic for reliability. In this manuscript, we demonstrate a flip chip process using 99.9Indium (99.9In) solder ball with a diameter size of 100µm.