Willem van Driel

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.

Automatic diagnosis and control of distributed solid state lighting systems.

This paper describes a new design concept of automatically diagnosing and compensating LED degradations in distributed solid state lighting (SSL) systems. A failed LED may significantly reduce the overall illumination level, and destroy the uniform illumination distribution achieved by a nominal system. To our knowledge, an automatic scheme to compensate LED degradations has not yet been seen in the literature, which requires a diagnostic step followed by control reconfigurations. The main challenge in diagnosing LED degradations lies in the usually unsatisfactory observability in a distributed SSL system, because the LED light output is usually not individually measured. In this work, we tackle this difficulty by using pulse width modulated (PWM) drive currents with a unique fundamental frequency assigned to each LED. Signal processing methods are applied in estimating the individual illumination flux of each LED. Statistical tests are developed to diagnose the degradation of LEDs. Duty cycle of the drive current signal to each LED is re-optimized once a fault is detected, in order to compensate the destruction of the uniform illumination pattern by the failed LED.

Degradation of light emitting diodes: a proposed methodology

Due to their long lifetime and high efficacy, light emitting diodes have the potential to revolutionize the illumination industry. However, self heat and high environmental temperature which will lead to increased junction temperature and degradation due to electrical overstress can shorten the life of the light emitting diode. In this research, a methodology to investigate the degradation of the LED emitter has been proposed. The epoxy lens of the emitter can be modelled using simplified Eyring methods whereas an equation has been proposed for describing the degradation of the LED emitters.

A novel lifetime prediction for integrated LED lamps by electronic-thermal simulation

In this paper, an integrated LED lamp with an electrolytic capacitor-free driver is considered to study the coupling effects of both LED and driver’s degradations on lamp’s lifetime. An electrolytic capacitor-less buck-boost driver is used. The physics of failure (PoF) based electronic thermal simulation is carried out to simulate the lamp’s lifetime in three different scenarios: Scenario 1 considers LED degradation only, Scenario 2 considers the driver degradation only, and Scenario 3 considers both degradations from LED and driver simultaneously. When these two degradations are both considered, the lamp’s lifetime is reduced by about 22% compared to the initial target of 25,000h. The results of Scenario 1 and 3 are close to each other. Scenario 2 gives erroneous results in terms of luminous flux as the LED’s degradation over time is not taken into consideration. This implies that LED’s degradation must be taken into considerations when LED and driver’s lifetimes are comparable.

Progress in Understanding Color Maintenance in Solid-State Lighting Systems

In this paper, progresses of color maintenance, also known as color shift, in solid-state lighting (SSL) systems are thoroughly reviewed. First, color shift is introduced and a few examples are given from different real-life industrial conditions. Different degradation mechanisms in different parts of the system are also explained. Different materials used as lenses/encapsulants in light-emitting diode (LED)-based products are introduced and their contributions to color shift are discussed. Efforts put into standardization, characterizing, and predicting lumen maintenance are also briefly reviewed in this paper.

Establishment of the coarse grained parameters for epoxy-copper interfacial separation

Atomistic coarse grained parameters were calculated from a non-equilibrium molecular dynamics simulation of the separation of an epoxy-copper interface. The methodology to determine the interaction energy and the equilibrium distance between the interfacial materials at a minimum energy is established. The traction-displacement relations of the separation under the influence of time taken for atomic interaction, displacement step, and molecular size have been studied. The study illustrates that the control of the time step in the molecular dynamics models is important to ensure a proper separation simulation. The result shows close matching with the thermodynamics work of adhesion. An analytical scheme to determine the coarse grained parameters from the relations is discussed. The proposed methodology contributes to the interpretation of interfacial adhesion beyond the continuum framework.

Board level reliability of the advanced RF power packaging

As the market demand of high power, high frequency and high efficiency, the advanced RF power packaging and assembly technology is facing the challenge of new material and new design. Improving the thermal conductivity of the heatsink (flange) is one of the effective ways to obtain low thermal resistance (Rth) component. Compared to the silicon transistors, the low-cost, high thermal conductivity material exhibits lower structural stiffness than the flange material which is widely used. A good understanding of the potential failure mechanism in board level reliability is an essential for a robust packaging development. This paper will focus on the board level reliability and modeling technique for bolt down assembly process. A modified three-point bending (3ptB) test method is also used to charcterised the structural stiffness of the packaging. Hence, the model is able to predict the ringframe crack of the particular package design which is under the board level thermal cycling test. Afterwards, impact of the flatness of the application board, flatness of the flange and the pitch of the mounting screws will be described.

Effects of crosstalk and simultaneous switching noise on high performance digital system packages

The non-ideal nature of package level interconnects gives rise to issues such as crosstalk induced noise and simultaneous switching noise which affect the reliable operation of high performance digital systems. We examine the causes of various signal losses that occur in the package level interconnect, and with an equivalent circuit model, highlight the effects of crosstalk induced noise and detail methods to mitigate its effects. The causes of simultaneous switching noise are also examined and techniques for better design of the power delivery network are suggested with the use of decoupling capacitors and shielding lines.

Thermal modeling for advanced high power packaging development and on-line performance monitoring

As the market demands for high power and high efficiency power electronics, the industries has developed advanced IC technology and conceptual configuration. However, in order to guarantee the performance and reliability of the power electronics, the challenge of the packaging arises.

A Reliability Prediction for Integrated LED Lamp With Electrolytic Capacitor-Free Driver

This paper studies the interaction of catastrophic failure of the driver and LED luminous flux decay for an integrated LED lamp with an electrolytic capacitor-free LED driver. Electronic thermal simulations are utilized to obtain the lamp’s dynamic history of temperature and current for two distinct operation modes: constant current mode (CCM) and constant light output (CLO) mode, respectively. Driver’s mean time to failure (MTTF) and the LED’s lifetime in terms of luminous flux are calculated. Under CLO mode, the LED’s current increases exponentially to maintain the constant light output. As a result, the junction temperatures of LEDs, MOSFETs, and power diodes in driver rise significantly, leading to a much shorter MTTF and faster luminous flux depreciation. However, under the CCM, the junction temperatures of LEDs, MOSFETs, and diodes change modestly; therefore, the driver’s MTTF and LED’s luminous flux decay are not affected much by the variation of temperatures during LED’s degradation process.