Heat generated from the p-n junction and not converted to useful light is transmitted from the junction to the ambient through several layers spanning a long path, from junction to solder, solder to the substrate, substrate to TIM and finally to the ambient via the heat sink. The junction temperature primarily depends on the power of the LED, thermal impedance of materials and the heat sink design.  Since the LED power cannot be controlled to provide a luminosity, the study of thermal dissipation path is of utmost importance. Another important aspect is that when multiple heat sources are packed in a small space, heat tends to accumulate by flowing laterally. This heat accumulation causes thermal crossover effect that further increases the junction temperatures. Since LEDs and microelectronics have space constraints that follow the miniaturization trend, efficient arrangement of heat sources and material selection for components that make up these structures are critical. Hence it is necessary to address the heat spreading requirements in high power LEDs using power electronic substrates for efficient heat dissipation, especially when multiple heat sources are arranged in arrays like in the COB architecture. This project studies the thermal constraints governing heat dissipation in high power LEDs and explores methods to minimize the thermal crossover effect using experiments and simulations. In this study, we identify the problem statement with respect to the current LED technology by studying a LED COB model by CITIZEN. Then a  solution is developed for optimizing the arrangement of heat sources on a  substrate to minimize the thermal crossover effect while maintaining the small size of the module. A 3-way relation between the thermal crossover effect, heat sink thermal resistance and the minimum separation distance between heat sources was obtained. This relation was used to study design parameters like arrangement pattern, heat sources density, silicone encapsulation etc. that could be exploited to minimize device temperatures while maximizing LED  reliability and light output. 

Introduction

LEDs are widely used in many fields including modern vehicle lights, traffic lights, remote controls and liquid crystal display (LCD) displays owing to their long life,  high efficiency, good reliability and low power consumption. One of the major applications of modern  LEDs is general lighting using high power white LEDs. High-power LEDs can reduce the energy cost for lighting by $30 billion per year by 2030 [5]. Furthermore, since LEDs are semiconductor-based, they rugged and resistant to damage when compared to ordinary light bulbs and fluorescent tubes. Junction temperature is the highest operating temperature of a semiconductor in an electronic device [9]. LED is a p-n junction diode, which emits photons when a suitable voltage is applied to the leads [7]. A major amount of energy is converted into heat that results in increased temperature of the p-n junction, also called as the junction temperature. Junction temperatures affect the lifetime and reliability of LEDs.  The light output of a single LED is less than that of incandescent and compact fluorescent lamp and hence multiple LEDs are required for lighting purposes. High power LEDs consumes more than 350 milliwatts of power and most of the electricity is dissipated as heat [11]. Thus,  the use of large LED arrays generates high heat loads that call for efficient heat dissipation as a cooling necessity. These two factors of high power requirement and large LED arrays make thermal management of high power  LEDs a crucial area for research and development. Although the high cost of  LEDs is preventing them from supplanting conventional lighting sources, the advantages justify the costs and with the efficient thermal management of LEDs,  they will eventually replace other lighting sources saving energy in the process.
LED arrays are assemblies of LED packages built using several methods. The intensity and uniformity of light output from an array depends on the methods of LED array manufacturing. LED arrays play an important role of combining LEDs into a  structure bright enough to compete with conventional lighting sources. Compared to lamp packaging and surface mount package, the Chip-on-Board (COB) boasts efficient thermal management and a wider application [16]. Due to the small size of the LED chip,  Chip-on-Board technology allows for a much higher packing density than surface mount technology. Chip-on-Board technology is a semiconductor assembly technology in which the microchip or die is directly mounted on and electrically connected to the final circuit board.  It differs from other packaging methods that employ packaging as an individual  IC. The COB technology includes three major steps of processing, namely die attach, wire bonding and encapsulation. The optical structure involves bonding  LED chips directly into the silicon substrate and attached to the aluminum board using a thermal interface material (TIM). COB technology enables achieving a superior design flexibility and better miniaturization when compared to SMT high-power packages [19].
To maintain an acceptable junction temperature, several methods are used to remove heat from  LEDs. Conduction, convection, and radiation are three means of heat transfer and can be exploited for the efficient thermal management of LEDs. Nearly all heat produced is conducted through the backside of the LED chip through a heat sink,  in contrast to other power electronic substrates where heat can dissipate through the board as well as the heat sink. The materials used for packaging of  LEDs play a major role in thermal dissipation. A typical thermal model can be used to represent the LED package where every layer is given thermal impedance.  Heat generated from the p-n junction and not converted to useful light is transmitted from the junction to the ambient through several layers spanning a  long path, from junction to solder, solder to the substrate, substrate to TIM and finally to the ambient via the heat sink. The junction temperature primarily depends on the power of the LED, thermal impedance of materials and the heat sink design. Since the LED power cannot be controlled to provide a luminosity,  we look at different methods for heat dissipation investigating materials and heat sink design characteristics. To maximize the ambient temperature range for a given power dissipation, the total thermal resistance from junction to ambient must be minimized. Additionally, applied mounting pressure, thickness and surface flatness are important to thermal resistance design.
In the case of LED  arrays, the simple thermal resistant model falls short in incorporating the heat spreading effect from a localized LED source into a bigger volume (MCPCB) [23]. Heat spreading effect produces localized hotspots between heat sources and in turn causes a rise in junction temperatures. This phenomenon is studied in this report and methods to alleviate this thermal crossover are proposed. Analytical models exist to estimate thermal resistant using various heat spreading theories. Some models assume a spreading angle of 45° [24],  some reports use a multi-layer substrate model with fixed bottom temperature [25] while some recent models use a bi-layer substrate with varying coefficient of convection [26]. For modeling of a LED array, the thermal resistance network can be obtained by using parallel resistance theory for multiple heat sources. Heat sink selection can also be done based on the thermal resistance to control the conduction and convection losses. The convective coefficient is sensitive to changes in thermal conductivity, specific heat, density, flow rate and flow type, and dynamic viscosity. For a heat sink, the thermal conductivity, specific heat and density are fixed. Furthermore, the heat sink design factors including spacing of fins,  the number of fins, type of heat sink and total exposed surface area alters the net convective heat coefficient. Heat transfer by radiation can be managed by changing  the emissivity of the heat sink, surface area and sink temperature.
In this project, an  LED COB module was analyzed for its transient thermal response. The heating  pattern showed regions of maximum temperatures corresponding to points of high  thermal crossover effect. A solution was developed for optimizing the  arrangement of heat sources on a substrate to minimize thermal crossover  effect, while maintaining the small size of the module. For developing the  solution, the material properties, power, heat sink thermal resistance and  other parameters governing heat dissipation were studied. Thermal resistance of  heat sink was varied by changing its thermal conductivity alone since the  thickness and surface area of the heat sink was kept constant throughout. For  the LED module dimensions studied, an optimum separation distance and the maximum  number of heat sources were identified that prevented the junction temperature  from rising beyond its safety limit of 85ºC. For a complete analysis of the COB  module from heat sink to the silicone encapsulate, the effect of silicone  covering on thermal crossover was studied and a notable rise in junction  temperatures was observed in the case with a silicone covering as opposed to  the case without the silicone covering. Thus, critical design considerations  were investigated to minimize device temperatures that would maximize light  output and device reliability.

Experiments details

CITIZEN LED Experiments

To understand heat dissipation in an LED COB structure, CLU701-10304C4 model by CITIZEN was studied. Thermal analysis of the LED gave maximum temperature points and experiments conducted later were aimed to reduce thermal crossover effect among multiple heat sources and thus reduce the temperature atthe hotspots. The mechanical dimensions of the COB array are 13.5 x 13.5 x 1.4mm [31]. The absolute maximum ratings for the LED are given in table 4. The LEDs are enclosed in a silicone covering at the center of the substrate. The electrodes occupy 2 opposite ends and the other 2 ends are used to attach the LED module to the PCB substrate.  To model the arrangement of heat sources (LEDs inside the COB configuration), the LED was sliced to give its cross-section and the sample was observed under an optical microscope. The optical microscope gave the cross-sectional dimensions of the materials starting from aluminum substrate at the bottom, solder as the TIM, sapphire substrate, GaN led film, and the encapsulation, which consisted of a phosphor layer and a silicone cover. X-Ray observations gave the distance between adjacent heat sources and their arrangement in the COB structure and the dimensions were used to model the LED package as shown in \ref{769959}.