Nanocopper as a soldering alternative: Solder-free assembly

Abstract

A nanocopper-based interconnect material was developed as a robust, high-performance alternative to solder. This new solder-free nanocopper material overcomes an inherent limitation of traditional solders wherein the operating temperature is limited by the processing temperature. For the first time, an interconnect material is capable of operating at temperatures not only equal to but even far above its original processing temperature. Being pure copper, the material can form contacts with 5-10× the thermal and electrical conductivity of typical solder systems. The material rheology can be tuned for drop-in replacement of solder on standard PCB assembly lines and other industrial paste dispensing equipment. Such nanoparticle based interconnects can exhibit improved creep resistance and enhanced reliability in low- and high-temperature operating environments. The nanocopper material is obtained by reducing a copper salt with sodium borohydride in the presence of an amine surfactant mixture that controls particle size and protects the nanoparticles from oxidation. The manufacturing process has proven readily scalable with a 1 kg pilot plant currently in operation and a path to a continuous low-cost manufacturing process. For the present work, a readily dispensable nanocopper paste was formulated to bond commercial LEDs to a thermal heat sink. To evaluate the quality of the formed bulk copper interconnects, a large number of test samples was fabricated to measure mechanical strength. Shear strengths exceeding 70 MPa have been achieved.