Printed Circuit Design & Fab - July 2008 - (Page 27)

ture functions” which provide detailed internal information for power-LED packages, revealing die-attach failures and other structural integrity problems. Conclusions LED technology offers enormous potential to save energy and enhance lighting quality and reliability. Thermal management is critical in LED design in order to meet performance, service life and cost requirements. Systems designers have a wide range of options to consider in meeting thermal challenges. The latest generation of CAD-embedded thermal and fluid simulation software enables design engineers to diagnose thermal problems, evaluate alternative designs, and iterate rapidly to an optimal solution. The final design can be qualified with measurements at the prototyping stage to ensure manufactured tolerances (e.g., interface thicknesses) meet the thermal design requirements and to identify any manufacturing problems. The knowledge gained can be used to improve future design simulations. pcd&f FiGurE 4. Temperatures inside the lamp shown with a 50% slice. ! YOU’RE FiGurE 5. visualization of the flow trajectories (through convection). Convection brings cold air inside the lamp while hot air escapes through the slots. ! Physical Testing Physical testing is too costly and time consuming an approach to use to investigate speculative design changes, but is highly effective for both validating the final design and in investigating manufacturing issues. Physical testing can confirm the material property values used in the simulation check bond line thickness, and identify problems such as voids in the die attach. The leading approach takes advantage of the fact that the temperature is proportional to the forward voltage drop of a specific device. After determining the forward voltage drop at a specific measurement current, a large current is applied to heat the LED. Then this current is turned off while another much smaller test current is applied to make the measurement. The test current used to characterize the device and the forward voltage have to be identical. The forward voltage is measured very quickly before the junction has the chance to cool down. The ability to monitor the temperature change with respect to time can provide detailed information on how heat flows through each layer in the path from the junction outwards. This allows direct measurement of the key thermal resistances in the heat flow path, such as the die attach resistance. Sophisticated measurement hardware that can measure the temperature change of the device within a few microseconds of being powered off is required, due to the LED’s fast thermal response. This type of thermal transient measurements can be used to generate highly accurate “strucJULY 2008 rEFErEncES 1. rebello, Jagdish, iSuppli – “The Solid State lighting Industry: At a Critical Crossroad,” September, 2006. 2. u.S. Department of energy, energy efficency and renewable energy, Bulletin Pnnl-SA-51901, “Thermal Management of White leDs,” February 2007. 3. Philips White Paper, “understanding Power leD lifetime Analysis.” dr. John Parry is the research manager at at Flomerics, and can be reached at john.parry@flomerics.co.uk. printEd circuit dESign & fAB 27 http://www.photoplot.com

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