No-Heat Soldering using Supercooled Liquid Metal Microcapsules

This proposal addresses the problem of dead zones and reliability issues of thin high energy particle detectors caused by the interconnect process limitations of current solder bump technology used to attach them to mounting substrates. Highly sensitive and high-density interconnected sensors are necessary for the continued advancement of high energy physics and upgrades to facilities such as those at Fermilab and the European Organization for Nuclear Research. These advancements can continue by using large format chips, with massive amounts of fine pitch interconnects in a high reliability component able to withstand the cryogenic operating conditions of the detecting systems. Large format chips have challenges forming and or keeping the flip chip interconnects joined at the device edge because of coefficient of thermal expansion mismatch causing dynamic warping of the package dimension before, during, and after soldering. This same issue causes mechanical stress upon cooling. Many Department of Energy funded research efforts go towards the searches for dark mater, understanding dark energy, understanding of neutrinos and their role in the universe, discovery of more unusual physics, and understanding the Higgs boson. Advancing the effectiveness and reliability of the miniaturized detectors gives researchers and engineers more and better data and would further the Department of Energy’ goals of advancing the field. The defects that are caused by dynamic warping in soldering are addressed in the proposal by a unique approach to soldering using supercooled liquid metal microcapsules. Supercooled liquid metal microcapsules contain metal solder alloys in a stable flowable liquid phase well below the melting point of the metal. This allows the application of liquid metal solder at temperatures dramatically lower than normal solder reflow temperatures, even down to ambient. This new material allows manufacturers of thin flip chip assemblies to use solder metal alloys, on demand, and at the temperature that flattens out the flip chip at the right geometry to make full metal solid interconnects on the device edges and minimize mechanical stress under cryogenic operating conditions. The phase I project will be a proof-of-concept solder replacement demonstration on a thin test die with a high density of interconnects of a fine pitch. The supercooled liquid metal microcapsules will be developed using SnAg3.0Cu0.5 alloy to have the required supercooling depth, purity, printability, and wettability to generate solder interconnects at 100 °C instead of the normal reflow temperature of 250 °C. The demonstration will be supported by analysis of the interconnect themselves and the number of interconnects made on the device edges. The phase II will take the developed proof-of-concept and its advancement towards soldering an actual readout chip and silicon sensor together and to meet reliability standards and cryogenic operation specifications. The phase III commercialization of the fully realized solder replacement product and application process will involve the large-scale production of the product. The defects and damage caused by coefficient of thermal expansion mismatch are problematic not only in the flip chip bonding process of component-level assembly, but also are seen at the printed circuit board level of assemblies. Electronics manufacturers in consumer electronics, automotive, medical technology, and other sectors are facing commercial market demands to develop products that are thinner, flexible, lighter weight, or have higher-density miniaturization with increased functionality. Supercooled liquid metal microcapsules will allow these industries to meet this demand with new designs and materials that are otherwise unsuitable for high-heat interconnect processes, while at the same time improving operating efficiency by forming interconnects 3X to 15X faster than current methods and reducing associated operating energy costs by up to 70%.