with the increasing requirements on guaranteeing the color uniformity and improving the luminous efficacy and manufacturing efficiency, a wafer level chip scale packaging (WLCSP) technology has been developed by thermally impressing a thin multiple phosphor film on a LED wafer, then being segmented into individual LED chips. In this paper, a high power white LED Chip-on-Board (COB) module with high color rendering index (CRI, Ra > 93) and tunable correlated color temperatures (CCTs) is prepared by the flip chip technology. In this COB module, the tunable color temperatures are achieved by using two types of white LED CSPs with different target CCTs of 3000 K and 5000 K. The thermal and photochromatic properties and the photochromatic stability of the COB module are studied through both experiments and simulations. The results show that: 1) The measured spectral power distribution (SPD) intensity of the prepared COB module is smaller than the arithmetic sum of SPD intensities of its each series connected CSPs, which may be attribute to the light absorption happened among CSPs; 2) The junction temperature and driven current have the different effects on the photochromatic properties of COB module (i.e. luminous flux, CCT and CRI); 3) The nonlinearity of luminous flux as a function of driven current and junction temperature should be considered in its modeling.

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By solving the problem of very long test time on reliability qualification for Light-emitting Diode (LED) products, the accelerated degradation test with a thermal overstress at a proper range is regarded as a promising and effective approach. For a comprehensive survey of the application of step-stress accelerated degradation test (SSADT) in LEDs, the thermal, photometric, and colorimetric properties of two types of LED chip scale packages (CSPs), i.e., 4000 °K and 5000 °K samples each of which was driven by two different levels of currents (i.e., 120 mA and 350 mA, respectively), were investigated under an increasing temperature from 55 °C to 150 °C and a systemic study of driving current effect on the SSADT results were also reported in this paper. During SSADT, junction temperatures of the test samples have a positive relationship with their driving currents. However, the temperature-voltage curve, which represents the thermal resistance property of the test samples, does not show significant variance as long as the driving current is no more than the sample's rated current. But when the test sample is tested under an overdrive current, its temperature-voltage curve is observed as obviously shifted to the left when compared to that before SSADT. Similar overdrive current affected the degradation scenario is also found in the attenuation of Spectral Power Distributions (SPDs) of the test samples. As used in the reliability qualification, SSADT provides explicit scenes on color shift and correlated color temperature (CCT) depreciation of the test samples, but not on lumen maintenance depreciation. It is also proved that the varying rates of the color shift and CCT depreciation failures can be effectively accelerated with an increase of the driving current, for instance, from 120 mA to 350 mA. For these reasons, SSADT is considered as a suitable accelerated test method for qualifying these two failure modes of LED CSPs.@en

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI > 80, CCT ~ 3000 K and 5000 K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.@en