Innovative Thermal Management Breakthrough Of Advanced Solid State Lighting Module (ThermManSSL)Themes: Micro/Nano System Integration and Reliability
Solid-State Lighting (SSL), powered by Light-Emitting Diodes (LEDs), is an energy-efficient technology for lighting systems. Blue GaN-based LEDs using InGaN quantum wells combined with a broad spectrum yellow emitting phosphorous materials, such as YAG:Ce3+, are generally used to produce white light. In contrast to incandescent lights which obtain high efficiency at high temperatures, the highest efficiency of LEDs is reached at low temperatures. This introduces a new challenge for both optical designer who need to consider the thermal structure and thermal designer who need to optimize the light performance. The thermal management in LED product is then a key design parameter as the high operation temperature directly affects the maximum light output, quality, reliability and life time. Currently, the thermal path within a LED package is designed from the die to the heat sink, where the heat is dissipated to environment by convection and radiation. As the electric input power increases, high-power/high-brightness LEDs have higher power density and consequent higher heat dissipation. This results in higher temperatures if no counter-measures are taken. Solutions are sought in optimizing the thermal path, materials with better thermal conductivity and increasing the performance of convection and radiation. However, the apparent dilemma is to optimize light efficiency and effectively designing thermal management. Two questions arise: Firstly, what is the optimum temperature for maximum light performance? Secondly, what temperature leads to thermal management solutions with less weight, energy consumption, and light hindering? The first question can be answered as “the lower temperature the better light”. For the second question, a high temperature gradient will result in high heat fluxes, which can move away large amounts of excess heat and reduce the complexity of cooling solutions. This means that high LED temperatures would be favorable. This gives a contradiction. Therefore, the thermal management of SSL is not just cool down the temperature but to achieve stable and acceptable temperature for optimum light quality, possibly using in situ monitoring, less weight materials, easy fabrication process, less energy consumption cooling solution. There are two major heat sources in the LED package, the LED die (chip) and the phosphorous layer. The excess heat generated in the die during the conversion from electric energy to blue light and also in the phosphorous layer where blue light is converted into yellow light. The two conversion rates are both temperature dependent. Thus the temperature is the most important parameter from both thermal and optical point of view. In order to be able to optimize the LED temperature, an electrical-thermal-luminous-chromatic (E-T-L-C) model was developed that takes into account the thermal effect on the energy conversion rate from electric power (E) to primary blue light and from blue light into yellow light as a function of the in-situ temperature (T). As the conversion rates of the die and phosphor differ, the white light performance changes both in flux (L) and spectrum (C). The model was successfully verified using three commercially available LED packages. Furthermore, the model was also used to study the effect of layer thickness and particle density variations of the phosphorous layer on the thermal performance and light quality. As the temperature is both critical to light performance and thermal management, knowing the temperature in the LED is indispensable. The diode forward voltage method with pulsed currents has been widely used to monitor the junction temperature (T j) of LEDs. However, this method suffers from a thermal transient effect (TTE) resulting in measurement errors. Using Thermoelectric (TE) physics, these phenomena was explained and a group of experiments was used to study the TTE in T j measurements for high-voltage (HV) LEDs. The measurement uncertainty was more than ± 10 ◦C which is not acceptable for accurate monitoring. Therefore, an improved Pulse-free Direct Junction Temperature Measurement (DJTM) method was applied to HV LEDs to reduce the errors and to achieve an accurate in situ T j measurement using DC currents. This also resulted in a simpler setup and a simpler measurement sequence. Although the T j is the most relevant temperature to know, the LED package or case temperature is much easier to measure and apply. The case temperature is a very good alternative to the T j. If the temperature sensors are positioned well, the sensor temperature will be nearly equal to T j. Besides monitoring the temperature through a sensor, it can be used to control active cooling solutions. A micro-electromechanical-system (MEMS) based, temperature triggered, switch was developed as a cost-effective solution for smart cooling control in SSL systems. The switch was embedded in a silicon substrate and fabricated with a single-mask 3D micromachining process. The device switched on at a designed temperature threshold with a small contact resistance, and switched off when the temperature drops below that limit. Through the embedded MEMS switch, an automatic temperature controller was obtained without adding electrical components to the package. As standard semiconductor manufacturing processes are used, integration and fabrication in future silicon based SSL systems is expected to be straight forward. Now that the thermal management design considering heat and light, using in situ temperature sensors, can be correctly applied, there is still a need for novel cooling solutions. Among the cooling solutions, the traditional package-TIM-heatsink (TIM: Thermal Interface Material) structure has already reached its cooling limit because (i)The low heat transfer coefficient by natural convection cannot be improved without extra volume and weight of heat sink, whereas the force convection is not preferred by the extra energy; (ii) Materials with higher thermal conductivity are too expensive; (iii) TIMs suffer from limited reliability; and (iv) the normal thermal conductive materials are opaque which hinder lights and thus limits the area of application in LED products. Apart from the current thermal solutions of SSL system, liquid cooling may enhance thermal management with simple structure, transparency and small weight as it can replace the same volume of traditional metallic heat sinks. Furthermore, liquids can also act as a heat transporter and thus out-class solids in their heat removal capacity. Liquid cooling has already been successfully applied in many (high end) semiconductor microelectronic applications. Therefore it is an ideal candidate to be used in LEDs. In order to exploit the transport capabilities of liquids, a driving force to move the fluid must be installed in the cooling circuit. This “pump” must be cost-effective and small in size. Two types of liquid drivers that are small in volume, which is used for cooling microelectronic devices, are investigated: a pump and heat pipe (HP). The phase change from liquid to vapor can be used as a driving force to move the fluid in a cooling system. Based on this principle, a cooling solution on MEMS technology was presented. A test vehicle was constructed consisting of a miniaturized evaporator with a fluid channel and an embedded bulk silicon temperature sensor. A commercial HP LED package was mounted on the evaporator, with the goal to achieve maximum light output using a very small coolant flow rate. Results showed that the package obtained high efficiency and correspondingly increased light output by the two-phase cooling. The phase transition of coolant allowed the package temperature to remain below 115 C with LED power up to 2.8 W, as compared to less than 1.5 W under conventional circumstances. Additionally, via numerical simulation the phase change phenomenon and temperature distribution inside the evaporator was further investigated and optimized water flow rates for specific input powers of the package were calculated. Micro or micro-wick heat pipes (HPs) have received considerable attraction in the past decades especially for cooling of electronics in a limited volume. Among the HPs, the micro HP (MHP) and loop HP (LHP) with micro wicks are most preferred for their high efficiency, small dimension, and compatible process with semiconductor devices. Especially, the LHP possess all the main advantages of traditional HPs and next to that they can transfer heat over distances up to several meters at any orientation in the gravity field. Although silicon is one the most favorable materials for MHP and LHPs, polymer based MHP and LHPs are very attractive for further investigation. Therefore, a package, using a silicon substrate with temperature sensors and a polymer based LHP was designed, manufactured and assembled. This package was able to provide low and relatively stable temperature, enabling higher optical power, more luminous flux and less color shift. Whatever cooling configuration is made, a constant element is the heat sink, which eventually dissipates the heat to the ambient environment. Thus, the heat sink design is essential but it needs a case by case approach. Using a thermal design of vertical fin arrays with HPs as passive cooling the design methodology was demonstrated. The HPs may be converted into active/passive liquid cooling as presented previously. As the natural convection and radiation dominate heat transfer in this case, the optimum vertical fin spacing, which is the critical parameter for natural convection, was calculated by the most used empirical correlations. In addition, the fin spacing was further numerical investigated and optimized using Computational Fluid Dynamics (CFD). The design was verified by building a prototype and the experimental and numerical results correlated well. The achieved results show the HPs supply good equivalent thermal conductivity with less weight and volume compared to copper or aluminum base. Furthermore, the HP (liquid cooling) enhanced the natural convection by high thermal conductivity and less obstruction to air flow.