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Bonding a LED Substrate to a Heat Slug: More than a Matter of Low Thermal Resistance
Keywords: heat spreader interconnection, low thermal resistance, reliability
High-Power LEDs enable light sources with high brightness. In Power LED devices, a large amount of light is generated in a small area. LEDs are becoming more and more efficient, but they still produce heat. Although they get less sensitive to high operating temperatures, this is still a real challenge. Therefore, an efficient heat path that conducts the heat away from the LED is essential for a proper and reliable operation of a Power LED device. High-Power LEDs products from Philips Lumileds consist of a ceramic substrate with LEDs bonded to a heat slug. For an optimal performance, the interconnection of the ceramic to the heat slug has to fulfill several requirements. Of prime importance is a small thermal resistance, but also reliability, costs, bond strength at operating temperature, and process issues (max T, max P) have to be considered. An existing High-Power LED product from Philips Lighting Lumileds was used as a carrier. These types of packages are used in demanding applications. An example is front lighting of the Audi R8. Three interconnect technologies for ceramic to heat slug bonding were investigated: SAC-soldering, adhesive bonding, and AuSn-soldering. Test samples with comparable dimensions and material properties as current packages on the market were used. Performance and properties of the samples were determined both at zero hours and at different moments during reliability tests, such as thermal shock and exposure to damp heat. Shear strength determinations (both at room temperature and at 130C), transient thermal measurements, X-Ray, C-SAM, and X-sectioning were used as characterization methods. In this paper, results of the investigations will be presented and discussed. The conclusion can already be drawn: Bonding a LED substrate to heat slug is more than a matter of low thermal resistance!
Kees C. Slob, Senior Scientist
Philips Applied Technologies
Eindhoven, Noord Brabant 5656 AE,
The Netherlands

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