S. Koh

LED system reliability

This paper presents our effort to predict the system reliability of Solid State Lighting (SSL) applications. A SSL system is composed of a LED engine with micro-electronic driver(s) that supplies power to the optic design. Knowledge of system level reliability is not only a challenging scientific exercise but it is also crucial for successful adoption of future SSL systems. Currently, the lifetime of a SSL system provided by the manufacturers is often based on just the life time of the LED emitter but a malfunction of the system in reality is often induced by the failure or degradation of a combination of subsystems/interfaces. Hence, a significant improvement in the future SSL system can be achieved when the system level reliability is well understood by proper experimental and simulation techniques.

Validation of forcefields in predicting the physical and thermophysical properties of emeraldine base polyaniline

We report a molecular modelling study to validate the forcefields [condensed-phase optimised molecular potentials for atomistic simulation studies (COMPASS) and polymer-consistent forcefield (PCFF)] in predicting the physical and thermophysical properties of polymers. This work comprises of two key steps: (1) generating and validating the molecular model in predicting the material properties of the bulk amorphous emeraldine base polyaniline and (2) modelling the glass–rubber transition of the polymer. From all the molecular dynamics simulation results, it clearly shows that the more recent COMPASS forcefield provides a higher accuracy in predicting the polymer properties than PCFF, and it enables a more accurate prediction of condensed-phase properties (density, glass transition temperature, solubility parameters, etc.) in a broad range of temperature for various applications.

Degradation of epoxy lens materials in LED systems

Due to their long lifetime and high efficacy, solid state lighting (SSL) has the potential to revolutionize the illumination industry. The long lifetime claimed by the manufacturers is often based solely on the estimated depreciation of lumen for a single LED operating at 25°C. However, self heating and high environmental temperature which will lead to increased junction temperature and degradation due to electrical overstress can shorten the life of light emitting diode. Furthermore, each SSL system includes different components such as the optical part, electrical driver and interconnections. The failure/degradation of any components wills severely affects the performance and reliability of whole system and hence the weakest component will become the bottleneck for the reliability and lifetime of the module. Literature reviews of the factors influencing the life of LED lamps identified the degradation of the epoxy lens and plastic package due to the junction temperature and voltages as one of the common failure mode. In this research, a methodology to predict the degradation of the epoxy lens has been proposed. In order to correlate the mean time to failure as a function of the junction temperature and the inputted voltage, the simplified Eyring models had been proposed in this research. Since the life of a SSL system is subjected to varying loading condition, another objectives of this research is to present a methodology to predict the life of a SSL under changing condition.

Temperature dependency in performance of solid state lighting drivers

Solid state lighting (SSL) is a new lighting technology based on high brightness light emitting diodes (LED). This technology because of being much more energy efficient, having longer lifetime and design flexibility has attracted the attention of both manufacturers and consumers. This technology looks very promising because of its advantages with respect to conventional lighting systems. Especially for some applications like outdoor lighting and automotive applications, using SSL can be very beneficial. Nevertheless there is still requirement to study about the reliability and performance of these lighting systems in different environments [1]. Each SSL lighting system is constructed by two major parts; optical part and electrical driver. The effect of temperature on performance and reliability of SSL system electrical driver is the focus of this research. In the beginning we will discuss about the fundamental SSL electronic driver building blocks; control/information, switching and electromagnetic energy storage. It will be explained that the last two blocks, switching and electromagnetic energy storage, are the heart of power conversion and thus more sensitive to the temperature. In this paper we focus on electromagnetic energy storage part. Ceramic capacitors are the most commonly used devices as electromagnetic energy storage component in SSL drivers. Thermal studies generally focus on steady state temperature dependency of the devices and they ignore the transient part of the temperature variation. Although in applications like automotive SSL lighting and also outdoor lighting the transient part of the temperature change should be also taken into account. In this paper we show that the transient part of the temperature change can also play an important role in performance of the most common type of the electromagnetic storage part of an SSL driver.

Highly accelerated life testing of LED luminaries

With rapid development of lighting emitting diode (LED) market, more people are focusing on reliability testing method of LED luminaries system. Based on the previous exploration, to assess the whole system reliability with a fast way, authors propose to divide the system into several subsystems, and then carry out step stress accelerated testing (SSAT) to get failure mechanism and reliability distribution of each subsystem, finally calculate the life distribution of the whole system by combining with system statistics analysis methodologies. The first step, the most important step, is to know the stress limit of the subsystem before conducting SSAT testing. The intention of normal highly accelerated life test (HALT) process is to subject the item under test to stimuli well beyond the expected field environments to determine its operating and destruct limits, but without enough attention on those degradation parameters of products, especially like LED products which present their life characteristics on many output parameters, such as lumen maintenance, color rendering index (CRI), chromaticity coordinate, junction temperature and optical efficiency, which included important life information. Therefore, Focused on the LED light source subsystem, it is not practical to accelerate it with HALT by only focused on failure or destructive data. In this paper, different HALT procedure with different objects is introduced, and two types of LED system from separated vendors are conducted proposed HALT before SSAT testing. Test results suggested that it is feasible to observe output parameters of LED products to decide the stress limitation during HALT testing, pseudo junction temperature and Lumen maintenance can be regarded well as key parameters. For other parameters of system, different parameter has changed with different rule, which indicates that key concerned factor should be fixed. Meanwhile, it is found that the lumen maintenance decreases very fast when temp stress over 110 °C. When focused on some failure mechanisms without change even if overloading the stress, a good solution for a fast qualification method will be worked out. Deep study on this found should be carried out in future.

Semi Emprical Low Cycle Fatigue Crack Growth Analysis of Nanostructure Chip-To-Package Copper Interconnect Using Molecular Simulation

ITRS has predicted that integrated chip (IC) packages will have interconnections with I/O pitch of 90 nm by the year 2018. Lead-based solder materials in flip chip technology will not be able to satisfy the thermal mechanical requirement of these fine pitches. Of all the known interconnect technologies, nanostructure interconnects such as nanocrystalline Cu are the most promising technology to meet the high mechanical reliability and electrical requirements of next generation devices. However, there is a need to fully characterize their fatigue properties. In this research, numerical analysis has been employed to study the semi-elliptical crack growth and shape evolution in nanostructured interconnects subject to uniaxial fatigue loading. The results indicate that nanocrystalline copper is in fact a suitable candidate for ultra-fine pitch interconnects applications. This study also predicts that crack growth is a relatively small portion of the total fatigue life of interconnects under LCF conditions. Hence, crack initiation life is the main factor in determining the fatigue life of interconnects.

Low cycle fatigue crack growth in nanostructure copper

ITRS has predicted that integrated chip (IC) packages will have interconnections with I/O pitch of 90 nm by the year 2018. Lead-based solder materials in flip chip technology will not be able to satisfy the thermal mechanical requirement these fine pitches. Of all the known interconnect technologies, nanostructure interconnects such as nanocrystalline Cu are the most promising technology to meet the high mechanical reliability and electrical requirements of next generation devices. However, there is a need to fully characterize their fatigue properties. In this research, numerical analysis has been employed to study the semi-elliptical crack growth and shape evolution in nanostructured interconnects subject to uniaxial fatigue loading. The results indicate that nanocrystalline copper is in fact a suitable candidate for ultra-fine pitch interconnects applications. This study also predicts that crack growth is a relatively small portion of the total fatigue life of interconnects under LCF conditions. Hence, crack initiation life is the main factor in determining the fatigue life of interconnects.

An accelerated lifetime test method for DC or AC supplied electronic control gear for LED modules of outdoor LED lighting products

This paper will propose an accelerated lifetime test method for DC or AC supplied electronic control gear for LED modules of outdoor LED lighting products. This accelerated lifetime test method focuses on major failure modes of DC or AC supplied electronic control gear for LED modules of outdoor lighting products, selects suitable conditions, and thus is able to reduce total test duration.