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Accelerated Life Test of High BrightnessLight Emitting Diodes

陳詠升

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Outline

• INTRODUCTION

• EXPERIMENTAL SETUP

• RESULTS AND DISCUSSION

• CONCLUSION

• REFERENCES

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INTRODUCTION

• In the last fewyears, great efforts have been spent in the improvements of materials and structure in terms of process and reliability. Novel technologies for low defect densities in active region allow a better efficiency, and an efficacy of 110 lm/W at 350 mA has been demonstrated .

• The high junction temperature reached during operation limits the lifetime of devices, and better performing heat sinks are needed in order to operate at high current level.

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Temperature increase for devices operated at 400 mA without heat sink at ambient temperature of about 35 ◦C. The temperature was collected from voltage transient and thermal coefficient.

EXPERIMENTAL SETUP

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Light output decay for different aging temperatures (180 ◦C–230 ◦C) during first 50 h. The time constant of quasi-exponential kinetic decreased with higher temperatures.

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Arrhenius plot of the MTTF70% during temperature aging for LEDs of sets A and B. The line represents the linear fitting of data from set A.

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Light output of LEDs from set A during thermal and electrical aging.X-axis was plotted in log scale in order to identify the exponential decay of 220 ◦C aged devices. Devices have been characterized in the same conditions for different stress test. The kinetics were well correlated for the first 10 h of stress.

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CIE 1964 chromatic coordinates during thermal aging were reportedon the chromatic diagram: LEDs from set A exhibited stronger blue shift.

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Forward voltage measured at 400 mA for LEDs of set A submitted todifferent stress conditions.

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Optical images of surface and cross section of LEDs submitted to (a) 100 h at 200 ◦C and (b) 100 h at 400 mA.

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Electroluminescence images of untreated and aged device of set A biased at (a) low current and (b) high current levels; the (c) schematic diagram was also reported.

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(Above) Cumulative and (below) differential structure function of device of set A submitted to electrical aging at 400 mA.

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CONCLUSION

• The optical decay could be ascribed to a lowering of blue chip emission and a degradation of yellow conversion efficiency.

• The modifications of thermal properties of the materials (die attach, epoxy) could induce the thermal resistance increase detected for aged devices.

• The use of silicone materials rather than epoxy plastic for package will improve the lumen maintenance of such devices.

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REFERENCES

• S. Ishizaki, H. Kimura, and M. Sugimoto, “Lifetime estimation of high power white LEDs,” J. Light Vis. Environ., vol. 31, no. 1, pp. 11–18,Apr. 2007.

• P. N. Grillot, M. R. Krames, H. Zhao, and S. H. Teoh, “Sixty thousand hour light output reliability of AlGaInP light emitting diodes,” IEEE Trans. Device Mater. Rel., vol. 6, no. 4, pp. 564–574, Dec. 2006.