Mian Tao

Modeling and Parametric Study of Light Scattering, Absorption and Emission of Phosphor in a White Light-Emitting Diode

In this study, we proposed and demonstrated an effective approach to model and predict spectral power distribution (SPD) for a phosphor-converted light emitting diode (pc-LED). For emission and excitation, broadband diffuse transmittances of 1 mm YAG:Ce phosphor plates with different concentrations were measured by a spectrophotometer. For emission, it was found that transmittance for all wavelengths was almost identical. This result indicates that emission spectrum prediction could be simplified by simulating the radiant power of the peak wavelength only. At the peak wavelength (560 nm), our simulation results, in which optical constants were calculated by the Lorenz-Mie theory, agreed well with our measurements. For excitation, a novel transmittance measurement setup based on an LED goniophotometer was proposed to obtain the absorption coefficient. By adjusting the optical parameter in our ray-tracing model to fit measured transmittances, accurate absorption coefficients were determined. Based on our calculation and measured optical parameters, we simulated the radiant power of the blue light and yellow light of commercial white LED packages. By expanding the total blue and yellow power into linear combinations of wavelengths in both regions, we successfully predicted the SPD of our LED package. Our predicted SPD has good agreement with the measured results.Copyright

Application of Nano Silver sintering technique on the chip bonding for flip-chip and vertical light emitting diodes

The bonding layer in the light emitting diode (LED) device of flip-chip or vertical structure is the most critical part to the device performance since it supports the electrical and thermal interconnection between the chip and its carrier. Therefore, the chip bonding process has become one of the major issues in the practical manufacturing. In this study, we developed a novel bonding technique using silver nano particle sintering. The raw bonding material with silver nano particles is in a paste form similar to the traditional Ag-epoxy. The entire bonding process is almost the same as the Ag-epoxy adhesive bonding. Meanwhile, it can provide excellent thermal and electrical performance better than the Ag-epoxy. Vertical and flip-chip LEDs were both chosen for demonstrating the silver nano particle bonding technique. After the test vehicles were prepared, diverse tests were conducted. The thermal performance was measured by the transient thermal analysis method. The electrical and optical performance were tested by an integrating sphere system. In addition, destructive shear tests were performed in order to characterize the shear strength. From the experimental results, it can be concluded that the silver nano particle sintering technique could be a promising bonding technique based on the assessment of performance, processing, and cost.

Experimental characterization of adhesion strength between the silicone encapsulant and the bottom of a SMD LED leadframe cup

This paper presents an experimental study on the adhesion strength between the silicone encapsulant and the bottom of the leadframe cup of a 3528 SMD LED. In this study, a new test method was proposed using silicone to mount a piece of dummy silicon chip to the bottom of the leadframe cup. Afterwards, a bond tester was employed to apply the loading on the edge of the dummy silicon chip to shear the silicone underneath. In addition to the characterization of adhesion strength, parametric studies were also performed to investigate the effects of shear height and shear speed on the adhesion strength. Detailed information about sample preparation and experimental procedure will be presented in the paper.

Thermal characterization of a printed circuit board with thermal vias for the application of high brightness light-emitting diodes

The power of state-of-art LED has been substantially raised for the continuous demand of high lumen output. Limited by the overall efficiency, at least 70% of the total input power would be transformed into heat. As a result, thermal management gained more and more attention. For the sake of reducing the material cost and also further improving the thermal performance, the FR-4 printed circuit board (PCB) cooperating with thermal via as the alternative of metal core PCB (MC-PCB) is attracting more and more attentions. In this study, the LED devices using thermal via for heat dissipation were fabricated and characterized. By means of the transient thermal characterization, the thermal performances of these devices were estimated. Firstly, the thermal resistance of the LED component was achieved through the dual thermal interface method by surface mounting the components with two types of soldering materials of different thermal conductivities. Then, the junction temperatures and the structure function of every sample were measured and the corresponding thermal resistances of their PCBs were obtained. Through the comparison among all the samples, it can be observed that the FR-4 PCB with fully filled thermal via showed a prominent thermal performance better than the MC-PCB.