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This work reports on the moisture inducing delamination in light-emitting diode (LED) packages and its effects on thermal characteristics. The LED samples were subjected to moisture preconditioning followed by heat block testing. Transient thermal measurements were performed to investigate the thermal behavior of the delaminated LEDs. Increase of thermal resistance with the degree of delamination was observed from the transient measurement. The thermo-mechanical calculated from coupled-field FEA simulation agree well with the micro graphical evidence. The calculated hygro-mechanical stress increased with the preconditioning time. It was found that the thermo-mechanical stress plays more important role than the hygro-mechanical stress for the development of delamination in the LED packages. Moisture pre conditioning for 3 hrs and 6 hrs under 8 5/spl times/C/8 5 RH conditions was found to make little contribution to the delamination between the chip and lead frame.

Thermal transient measurements of high power GaN-based LEDs with multi-chip designs are presented and discussed. Once transient cooling curve was obtained, the structure function theory was applied to determine the thermal resistance of packages. The measured total thermal resistances from junction to ambient were 19.87 K/W, 10.78 K/W, 6.77 K/W for the one-chip, two-chip and four-chip package, respectively. The contribution of each component to the total thermal resistance of the package can be calculated from the cumulative structure function and differential structure function. The total thermal resistance of multi-chip package is found to decrease with the number of chips due to parallel heat dissipation. Very useful thermal design rule for high power multi-chip LED package is analogized from the experiments. It was found that the effect of the number of chips in a package on the thermal resistance depends on the ratio of partial thermal resistance of chip and that of slug. Thermal resistance for full color multi chip LEDs, where each chip is driven independently, was measured as well and the implication was discussed.

In this paper we present thermal analysis of three kinds of ceramic package designs for high power LEDs and thermal characterization of high power LED array system. The analysis was made by transient thermal measurement and thermal simulation using the finite volume method (FVM). For the package design, thermal behaviors, as are described in thermal resistance, of the three packaging designs were compared and evaluated as functions of bulk thermal resistance, spreading resistance, and surface roughness. The deviation between the simulated results and measured data were attributed to the different surface roughness in the interfaces between the packaging components. For the system design, the emphasis is placed upon the investigation of junction temperature rise of LED array for a limited range of boundary conditions which include design effect of heat pipe, convection condition, and ambient temperature. It was found out that the measured junction temperatures and thermal resistance of LED array are increased with the input power and ambient temperature and decreased with the air velocity. An analytical thermal model analogous with an equivalent parallel circuit system was proposed and was verified by comparison with experimental data.

Thermal transient measurements of high power GaN-based LEDs with multi-chip designs are presented and discussed in the paper. Once transient cooling curve was obtained, the structure function theory was applied to determine the thermal resistance of packages. The total thermal resistance from junction to ambient considering optical power is 19.87 K/W, 10.78 K/W, 6.77 K/W for the one-chip, two-chip and four-chip packages, respectively. The contribution of each component to the total thermal resistance of the package can be determined from the cumulative structure function and differential structure function. The total thermal resistance of multi-chip packages is found to decrease with the number of chips due to parallel heat dissipation. However, the effect of the number of chips on thermal resistance of package strongly depends on the ratio of partial thermal resistance of chip and that of slug. Therefore, an important thermal design rule for packaging of high power multi-chip LEDs has been analogized.