Zefeng Zhang

Reduction of Die-Bonding Interface Thermal Resistance for High-Power LEDs Through Embedding Packaging Structure

Thermal management is a key issue for high-power light-emitting diodes (LEDs). In this study, a novel packaging structure was proposed to reduce the die-bonding interface thermal resistance and provided a new idea for LED heat-dissipation. The LED chip was embedded into a square groove at the lead-frame substrate. The gap between the edges of the LED chip and square groove was filled with boron nitride/silicone composite. So the heat generated from the chip could be dissipated from the side surfaces to the substrate. The experimental results show that the heat-dissipation ability of LEDs has been significantly improved by the new embedding packaging structure. The die-bonding interface thermal resistance can be reduced by more than 20%. The junction temperature rise can be reduced by 14%. Due to the fully embedding of LED chip in the square groove, the light intensity distribution is slightly shrunk and the light output power is slightly reduced by about 8%.

Thermal dissipation enhancement of LED filament bulb by ionic wind

The aim of this paper is to study the heat dissipation enhancement effect of LED filament bulb by using ionic wind. A LED filament bulb with 6 filaments is used for studying. There are two holes drilled at the end of the bulb and the ionic wind blows into the inner space of the bulb through the holes that will strengthen the heat convection of the filaments. The ionic wind generator includes the needle-net electrode, the flow channel and a high voltage DC power supply. The temperature of the filament is measured by an infrared thermal image instrument. According to the experiment results, the drop of filament surface temperature can reach to about 15°C with the corona discharge volatage higher than 9kV.

Effects of residual stress in the membrane on the performance of surface micromachining silicon nitride pressure sensor

In this paper, influences of the initial stress on the surface micromachining silicon nitride pressure sensor was analyzed and discussed. The residual stress in the diaphragm is a major issue that makes significant difference on the performance of surface micro machined pressure sensors, such as warpage, micro-cracking, delamination, debonding, and nonuniform stress profile of the membrane. The pressure was operated at the room temperature 25°C and barometric pressure 101KPa. As the pressure is multiplied by a safety factor, 120KPa is utilized in this paper. The film is 80μm in diameter and 1μm in thickness, deposited by low pressure chemical vapor deposition (LPCVD). Finite Element Method with ANSYS software was processed to obtain the pressure-displacement curves and the stress profiles of the non-stressed diaphragm and prestressed diaphragm with 100MPa. The theoretical solutions were proposed to verify the validity of the FEA model. Subsequently, the mechanical-electrical models were adopted to investigate the input-output characteristics of the silicon nitride pressure sensor under various pressures. It turned out that the simulation results were greatly in accordance with the theoretical values. The FEA models were proved to be effective. And there was great difference between diaphragm with residual stresses and nonprestressed diaphragm in the voltage output. As a results, the residual stresses in the silicon nitride film have a big difference on the mechanical performance and the output of the surface micromachining silicon nitride pressure sensor.

Study on conformal phosphor coating for phosphor converted white LEDs through ionic wind patterning

The morphology of phosphor layer in white phosphor converted white LEDs is very important since it plays a vital role in optical performance such as correlated color temperature (CCT) and angle color uniformity (ACU). Owing to its outstanding optical performance, conformal phosphor coating process has attracted plenty of attention since it was proposed. In this article, a new method through ionic wind patterning for realizing conformal phosphor coating was proposed. The principle of such a patterning method was based on the interaction between ionic wind and liquid encapsulation. Masks were applied to control the ion flow for obtaining the desired patterning. As a result, compact and uniform phosphor encapsulation layer was successfully fabricated, its geometry was presented and its optical performance enhancement was demonstrated by experiments. Compared with the traditional dispensing coating process, the angular CCT deviation of the proposed structure is reduced from 1630K to 366K, when the averaged CCT is ~5400K.

Enhanced thermal conductivity of QDs-polymer film for light-emitting diodes via electrospinning

Quantum dots (QDs) have been developed as a promising light-converting material for light-emitting diodes (LEDs). However, QDs-polymer film always shows bad thermal performance. In this paper, a new designed structure of QDs-polymer film via electrospinning was proposed to enhance the thermal conductivity of the QDs-polymer film. After the measurement of density, specific heat and thermal diffusivity, the thermal conductivities of the proposed and traditional QDs-polymer films were calculated and compared. Finite-element simulation was conducted to assess the working temperature when both kinds of QDs-polymer films were packaged in the remote QD-LEDs. Results show that the thermal conductivities in thickness direction and in plane have been enhanced by 64.6% and 423.1% at 25 °C. The highest temperature in QDs-polymer film is decreased from 92.9 °C to 88.0 °C at the driving current of 300 mA.

Fabrication of microlens array encapsulation layer based on porous film by breath figure method for chip-on-board light-emitting diodes

High light extraction efficiency is strongly required in many illumination applications. In this study, a new method to fabricate microstructure array based on porous film and silicone gel (OE-6550, Dow Corning, USA) molding process for the blue chip-on-board (COB) packaging light emitting diodes (LEDs) is proposed and demonstrated. In this method, the porous film on the polymer surface is a polystyrene(PS) film. Use the chloroform to dissolve the PS, and after the volatilization of the chloroform solution the PS film has a porous structure with controlled size. Stable and ordered microstructure array is obtained after the solidification of the polymer on the porous film, and thus convex microstructure array is achieved. Experiments are conducted for performance verification. The results show that the radius of the micro-craters in this method were easily controlled in the range of 3μm to 12μm simply by changing the solution height of chloroform evaporating process. The microstructure array is using a molding process and transferred to surface silicone gel layer. The fabricated silicone gel with convex micro lens array is applied as the top encapsulation layer for COB packaging LEDs. Due to the suppression of the total internal reflection (TIR) between the encapsulant and air, the light extraction efficiency (LEE) is enhanced. The optical measurement results show that the LEE of the COB packaging LED with microlens array increased by 15.58% compared with that with flat encapsulation layer.

Fabrication of Freeform Lens Based on Ionic Wind for Chip-on-Board Light-Emitting Diodes

In this study, a novel method for the fabricating silicone lenses is proposed based on ionic wind. In such a method, the normal stress on the air-liquid interface is induced and adjusted by ionic wind, which is applied around liquid lens material. Through adding a specific shape of mask, the lens can be tailored into desired freeform morphology. Experiments are conducted and freeform lens of dual stripe and meniscus shapes are fabricated by configuration of specific mask. Besides, the influence of thickness of silicone gel and the shape of micro-patterns upon optical performance is analyzed by optical simulation, the Monte Carlo ray-tracing method. Results show that the improvement of LEE with the dual stripe lens reached 41.02%.

Thermal performance and reliability management for novel power electronic packaging using integrated base plate

Due to the mismatch of coefficient of thermal expansion (CTE) among different structure material, warpage and stress would be generated in the module and then influence the reliability and long term usage. This paper investigate a novel power electronic packaging design using integrated base plate (IBP). Module structure and assembly process of IBP-module have been introduced. The thermal and mechanical performance of the IBP-module during reflow process and operating process compared with typical power modules have been studied by the approach of finite element methods, where the Anand constitutive model has been used to describe the time-dependent viscoplastic behavior of soft solder materials. The temperature distribution of modules during operation has been presented. It can be seen that the thermal performance of IBP-module has been improved a lot due to the decrease of thermal resistance. Warpage and stress analysis of novel after reflow process caused by CTE mismatch have been conducted to show the advantage of novel packaging style. Finally, virtual reliability results of thermal cycling swung from -55 °C to 150 °C and the corresponding failure analysis based on IBP structure have been discussed to reveal the advantage in higher reliability of the novel packaging.

Effect of micro-channel on warpage and residual stress due to reflow process in IGBT modules

With the trend to be smaller, more integration and higher power in power electronic technology, the cooling problem has become increasingly prominent for those high power devices. Micro-channel in substrate, as an effective cooling method, has been proposed and investigated by many researchers in recent years. Warpage and stress, naturally existing in all the packaging processes due to the unavoidable mismatch of coefficient of thermal expansion (CTE) between materials, can be problematic on reliability. Compared to the traditional substrate, the substrate integrated with micro-channel maybe result in the increase of the warpage and residual stress after packaging processes. In this paper a 1200V Insulated Gate Bipolar Transistor (IGBT) module as a prototype was investigated to describe effects of the microchannel on warpage and residual stress of the substrate due to reflow process. Finite element analysis method with viscoplastic solder model was used to study the reflow process. The Anand constitutive model, which has been adopted successfully to represent the viscoplastic behavior of solder materials, was used to model the material properties of SAC305 solder layers in this study. The whole reflow process was modeled by element “death and activation” technology, and the traditional substrate model and the model of the substrate integrated with microchannel were taken for comparison. The effect of the size and the arrangement of the micro-channel in substrate on warpage and residual stresses after reflow process were also studied.