Printed Indium Inks for High-Density Chip Interconnects

High Energy Physics detectors with large channel counts and fine granularity require higher-density interconnects (HDI). However, current assembly technology (flip-chip, etc.) uses solder-alloy bumps to interconnect devices that are stiff and cause warping of devices, possibly causing distortion in the detector. Current state of the art processing and patterning can achieve high resolution interconnects with submicron resolution on 2D surfaces but not around corners, edges and curved surfaces. Applied Nanotech proposes to use indium inks/pastes that can be printed with sufficiently small pitch and feature sizes (<50 um) to be used as HDI. Indium has the advantage of being malleable and compliant even at cryogenic temperatures. Printing of indium has advantages for printing circuits on 3D objects or curved surfaces, around edges and corners without having to add bulky connections. Applied Nanotech developed, evaluated, and demonstrated indium inks/pates as high-density interconnects (HDI), in partnership with our subcontractors. Various compositions were formulated with pure indium/indium oxide and fluxes (mixtures of different additives). Aerosol Jet (AJ) printer was used to print features of different indium inks. The smallest indium trace was about 40 um thick with a pitch of 200 um. Indium bumps were deposited on a glass slide. Smallest printed bump was about 60 um in diameter with a pitch of about 100 um. Indium interconnects were successfully printed by an AJ printer between indium traces in daisy chains. An Indium solder paste was successfully reflowed on a FR4 board in an industrial oven at about 190°C. In phase II, we propose to develop indium-based formulations for aerosol jet printing and other methods to create interconnects/traces and micro solder-bumps (with a pitch of <50 microns), connect arrays of thinned integrated circuits to corresponding patterned substrate (with a pitch of <50 microns), demonstrate 3D integration of multi-layered printed interconnects, demonstrate interconnects that will not introduce mechanical stresses at cryogenic temperatures, and develop indium solder paste. The technology presented will provide indium-based ink/paste materials that would allow for the high-resolution, high-density printing over 3D surfaces, and higher-performance electronics for detectors and other devices. Indium-based solders/interconnects are excellent for thermal management in electronic devices, for soldering two materials that have different coefficient of thermal expansion rates, excellent for bonding non-metallic surfaces like glass and quartz.