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Improved Heat Dissipation for high power LED systems via a Nanocopper-based metal SMT
Keywords: LED die bonding, thermal interface, nanocopper adhesive
AuSn solder is often used for SMT of high power LED dies due to its high temperature capabilities and decent thermal performance. However, the material has reached its thermal limits for ever increasing power levels of new LED die products and laser diodes that have moved into the kW range. Therefore, there is a need for a replacement material with higher thermal performance, lower cost and improved process conditions. A nanocopper-based interconnect material was developed as a robust, high-performance alternative. 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 5-10x 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 with a suitable rheology to bond commercial LEDs to a thermal heat sink using commercial dispensing equipment. 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 80 MPa have been achieved using 4-5 min reflow profiles with peak temperatures in the 260-280 C range. Once fused, the nanocopper reverts to bulk copper raising its melting point to 1084 C. Since normal rework is therefore impossible, a suitable rework has been developed and patented.
Dr. Alfred A. Zinn, LM Fellow
Lockheed Martin Space Systems Company
Palo Alto, CA
USA


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