Shuzhi Li

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.

Study on the application of thermal interface materials for integration of HP-LEDs

Contact thermal resistance existed in the contact face between light emitting diode(LED) substrate and heat sink, which could hinder thermal conducting from PN junction to heat sinks, when mounting high power LEDs(HP-LEDs) devices on the heat sinks. Thermal interface materials(TIMs) were filled in the contact face to reduce contact thermal resistance. This paper studied the application of TIMs used in cooling LEDs, and three types of TIMs were studied by thermal test and optical-electro test. The experiment results presented that when select TIMs for LED mounting, thermal conductivity, thickness, and the volume of fillers should be considered.