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High Density Attachment of HB-LED Die to Heat Sink Surfaces Using Aerosol Jet Printing
Keywords: Aerosol Jet, heat, direct attach
High LED junction temperature leads to reduced LED output and color shift, and are a factor in most LED failure mechanisms. Every interface and layer of packaging between the junction and the heat sink increases the device operating temperature necessitating more aggressive heat sink design. Packaging also increases the footprint of the device, limiting packing density. The ideal design would have the bare LED die attached directly to the heat sink with a micron-scale bond of thermally conductive adhesive. This paper discusses the use of an Aerosol Jet® system to create micron-scale bonds of adhesives for both top and bottom-side (flip-chip style) connections. The system is also used to write topside connections, replacing wire bonding. In order to limit the temperatures of LED junctions with power densities of 250 W/cm2 to below 100 °C, it is necessary to limit the thermal resistance of a direct die-heat sink bond for a 0.4 W, 0.16 mm2 die to approximately 100 C/W. Assuming 50% coverage, this implies a thickness of approximately 1.5 um for an unfilled adhesive (K=0.2) or 40 um for a filled adhesive (K=5). For a 0.16 mm2 die this requires an adhesive volume of 65 pl and 1600 pl respectively, far below the 17,000 pl in the 8 mil hemisphere from a syringe dispense. Aerosol Jet systems are able to produce patterned deposits with total volumes in the 100 pl range. This paper will describe the results of work with top and bottom-side attachment of 450 nm InGaN LED die to thermally conductive substrates.
Kurt K. Christenson, Senior Scientist
Optomec Inc.
St. Paul, MN

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