Luqiao Yin

Study on the reliability of high-power LEDs under temperature cycle

High-power LEDs have been widely used and have a big potential market due to the many advantages compared to conventional light sources. Reliability is one of the most important issues for the application of LEDs. In order to study the degradation mechanism and the stability of LED withstanding cyclical exposures to temperature variations, life test was carried out under temperature cycle testing. The experiment was designed basing on the EIAJ ED-4701/100. Four groups of 1 W high power LEDs adopting different material and structures were utilized. The temperature cycle accelerated life test under the condition of −25°C/125°C. After the accelerated test, the optical performance of the four kinds of devices were compared and analyzed, and the thermal performance of the LEDs was also tested, analyzed and discussed. At last, the reliability differences of high-power LEDs were analyzed and degradation mechanism of high power LEDs under temperature cycling was discussed.

Multi-chip integrated high-power white LED device on the multi-layer ceramic substrate

In order to solve the thermal issue and improve the thermal resistance of LED, a device with multi-chip packaged on multi-layer composite ceramic substrate is successfully packaged. The top layer of the substrate is low temperature co-fire ceramic (LTCC), while the base layer is aluminum nitride (AINx) ceramic. 18 chips(one watt) are used in the device, and all of them are simultaneously packaged on a composite ceramic substrate which is 55 mm in length and 20 mm in width. The 18 chips are designed into two rows, and three chips in series for six parallels. At the same time the same chips are packaged on aluminum (Al) substrate and alumina (AlOx) substrate. After wire bonding photoelectric parameters of the three package modules are tested, the results show that the properties of the AINx-based package has the best performances, the thermal resistance of the device is far smaller than the other devices packed on the Al substrate and the AlOx substrate. After phosphor coatings the AINx-based multi-layer ceramic substrate package has 896.1 lm (43.4 lm/W) loading 350 milliampere (mA) to each chip, and has 1081.3 lm (35.4 lm/W) loading 500 mA. The multi-chip module has little light degradation adding 500 mA to each chip after working in the room temperature for one afternoon. At last the module is simulated by ANSYS software, the highest junction temperature of the chips in the module is about 70.8degC when 1.8 watts electric power is loaded to each chip, this temperature is far lower than 125degC which will damage the chip, and he highest simulation temperature of the aluminum heat slug is about 39.3degC. The temperature of the aluminum heat slug is about 41.0degC which is tested by thermocouple in the experiments, and it is closely to the simulation temperature 39.3degC. At last the relationship between pn-junction temperature and thermal conductivity coefficient of the silver (Ag) paste , the relationship between pn-junction temperatures and convection coefficient have been researched, the results show that the pn-junction temperature can be decreased a lot while increase the thermal conductivity coefficient of the adhesive layer or increase the convection coefficient a little.

Effects of die-attach materials on the optical durability and thermal performances of HP-LED

Die interconnection layer is one of the main factors to improve the HP-LED thermal issue as it is the main thermal transfer media from the HP-LED chip to the substrate or heat sink. Silver epoxy is now widely used for LED interconnection, but it becomes more and more difficult for efficient heat dissipation as the HP-LED electric power density becomes higher and higher. Nowadays more and more attentions have been moved to metallic eutectic interconnection, especially for solder paste and AuSn20 eutectic interconnection which have big potential to be used for HP-LED. In order to further understand the thermal characteristics and reliability of HP-LED interconnected by different die-attach materials, the AuSn20 eutectic, solder paste and silver epoxy were applied to HP-LED respectively. The transient thermal resistance, steady state junction temperature and optical durability of the HP-LEDs are tested and studied. For HP-LED interconnected by AuSn20 eutectic, solder paste and silver epoxy, when applied 450 mA working current, the transient thermal resistance of die attach layer are 2.5 K/W, 3.2 K/W and 4 K/W respectively, and the junction temperature variation are 0.4%, 9.6% and 6.6% respectively after about 1180 hours 85 °C/85 H aging. The results show the AuSn20 eutectic has better thermal performances and optical durability than the solder paste and silver epoxy.

Effect of the viscosity of organic carrier on the quality of laser-assisted glass frit bonding

Localized laser bonding using a glass frit intermediate layer is an ideal technology to hermetically package devices without heating the function components. The glass frit paste is mainly composed of functional fillers, low temperature vanadate glass powder and organic carrier. The viscosity of glass frit has an important influence on screen printing, pre-sintering and laser bonding quality. The content of thickener in organic carrier is a significant reason to affect the viscosity of the paste. In this paper, the effect of organic carrier viscosity in glass frit was investigated in detail. The research shows that the organic carrier viscosity has direct influence on the on screen printing, pre-sintering and laser bonding quality. Shear force test were used to evaluate the bonding quality. After a series of experiments and analysis, we found that adding 8% of ethylcellulose can get a better result of screen printing, pre-sintering and laser bonding.

Effect of die-attach materials and thickness on the reliability of HP-LED

Different materials and thickness of die-attach layers can directly affect the heat dissipation of LED devices, so the influencing mechanism of interconnect materials and thickness to the thermal performances and optical durability were studied respectively. In the experiments, chip-on-board LED devices interconnected by gold-tin(Au80Sn20), solder paste and silver paste were prepared and evaluated respectively in terms of thermal resistance and reliability under wet and temperature life test. First, finite element analysis software ANSYS was used to simulate the influence of the die-attach layers thermal conductivity and the thickness on the junction temperature. The results show the junction temperature becomes lower when the thermal conductivity of the interconnect layers become higher and the thickness become thinner. At the same time, the thermal stress of different die-attach layers were simulated. By the thermal resistance test with the T3Ster instrument, it was found that the thermal resistance of the Au80Sn20 interconnected LED device was the lowest. Finally, 12 pieces of LED devices of each die-attach material packaged HP-LED were selected and aged in 55°C/55% RH condition for 2200 hours. The HP-LED interconnected by Au80Sn20 got the highest luminous flux durability. During the aging period, the change of the thermal resistance of LED device interconnected by sliver paste was more obvious than those interconnected by Au80Sn20 and solder paste.

Improved Light Extraction of GaN-based LED with Patterned Ga-doped ZnO Transparent Conducting Layer

ABSTRACT In this study, patterned Ga-doped ZnO transparent conducting layers were made to increase the light-extraction efficiency of GaN-based LEDs. The patterns were designed by different side length, spacing and depth of the concave squares. Simulations and experiments were taken to study the light output powers of LEDs made by patterned transparent conducting layers. The area of the vertical sidewalls of the patterns is thought to have important effect on the light extraction of LEDs. With bigger sidewalls, the escape possibility of photon can be increased. Compared with conventional LEDs with planar Ga-doped ZnO transparent conducting layers, the light output power can be increased by 8.9% via using 10μm side length, 5μm spacing and 400 nm depth concave square patterns, which is basically consistent with the simulated results.

Effect of CuO on laser absorption in glass to glass laser bonding

Localized laser bonding using a glass frit intermediate layer is an ideal technology to hermetically package miniature devices without heating the function components. It is important to describe the laser energy transmission and absorption in glass frit when modeling heat transfer in transmission laser bonding of glass to glass. Excess energy leads to over-burning of the glass material and deficit energy produces weak bonding. In this paper, the effect of CuO as additive in glass frit is investigated. The research shows that the content of CuO is confirmed to have direct influence on the laser absorption and the bonding quality. Shear force test, SEM and EDX analysis are used to survey the bonding quality. After a series of experiment and analysis, we found that adding 15% CuO can get a best result of encapsulation. Within the scope of this add, the glass frit could meet the needs of packaging.

Study on the influencing factors of thermal spreading resistance of HP-LED package

The optical performance and reliability of High power light emitting diode (HP-LED) devices can be affected by the junction temperature, while thermal resistance is one of the main factors which influence the p-n junction temperature, especially for the thermal spreading resistance. So, Thermal analysis was conducted by differential structure functions combining with finite element analysis and calculation of thermal spreading resistance of HP-LED devices. The results show that thermal spreading resistance plays an important role on total thermal resistance of LED devices. The thermal spreading resistance shows a declined and then increased trend with the increase of substrate thickness; the contact area between the heating source and substrate is the main factor which influences thermal spreading resistance; It is beneficial to decrease the thermal spreading resistance when contact area ratio be increased. Thermal-convection resistance becomes the main factor that influences total thermal resistance of HP-LED devices with the changing of substrate bottom area, while the influence of thermal spreading resistance is weak. The eccentric distance between chip and substrate plays a distinct impact on thermal spreading resistance and junction temperature. The results of finite element simulation are in good agreement with calculation of thermal spreading resistance.

Recent progress in the studies of low melting Sn-based Pb-free solders

Limited by the toxicity of Pb and Pb-containing compounds and the implementation of the mandatory legislations, such as WEEE and RoHS, the Pb-containing solders have been banned and accordingly the Pb-free solders have become popular in electronic industry. However, the novel developed Pb-free solders could cause a series of packaging defects such as delamination, cracking, and charring etc. due to the relatively high melting temperatures of these Pb-free substitutes compared to the former Sn-Pb

Synthesis of graphene quantum dot/metal–organic framework nanocomposites as yellow phosphors for white light-emitting diodes

A new luminescent composite material GQD@ZIF-8 was obtained by incorporating edge-sulfonated graphene quantum dots (GQDs) as an active fluorescent species into nanocrystals of the zeolitic imidazolate framework (ZIF-8) as both a stabilizer and a carrier by a simple, rapid, and eco-friendly approach. The as-synthesized GQD@ZIF-8 composite not only exhibits excellent fluorescence properties in aqueous solutions but can also emit bright fluorescence in the solid state due to the effective encapsulation by ZIF-8. The GQD@ZIF-8 nanocomposite was used as a novel yellow phosphor in the fabrication of white light-emitting diodes (WLEDs) based on a blue-light LED chip with a proximate CIE coordinate (0.33, 0.33) of highly pure white light and a relatively high color-rendering index.