Encyclopedia of Thermal Packaging

The following sections are included:

• Foreword to the Encyclopedia of Thermal Packaging

• Preface

• Contents

With the recent recgonition of energy and concerns regarding energy consumption, lighting technologies have gained further attention. Lighting products use in excess of 20% of the total energy in consumed in buildings, thus more efficient technologies are needed. Monochromatic light emitting diodes (LEDs) have historically been used for indicators and produced low amounts of light and heat. The introduction of high brightness colored and white LEDs has allowed them to penetrate specialty and general illumination applications. The output of a typical commercial high brightness, 1 mm², LED has surpassed 150 lm at 1 W. This evolution has created new lighting opportunites along with packaging and thermal challenges. This chapter will start with the discussion of conventional lighting technologies and then attention will be turned to energy efficient solid state lighting (SSL).

High brightness LEDs require novel packaging approaches that satisfy the thermal, mechanical and electrical requirements in order to realize the potential of solid state lighting. This chapter will start with a discussion of the various types of LED packages and the packaging evolution. Attention will then be turned to the thermal issues associated with LEDs. In addition, the optical path of an LED package plays a key role in its efficiency. Therefore, encapsulants and phosphor will be discussed. Finally, multi chip packages (MCM) for high lumen LED lighting applications will be reviewed.

Thermal management has become a serious hurdle in many engineering applications. While this problem has been more challenging for electronics systems, SSL lighting systems are also limited by the thermal management. While conventional lighting systems do not require any thermal management, and in fact taking advantage of high temperatures (incandescent lamps), LEDs must be kept below 100°C to ensure reliability, and thermal management has to be lightweight and low cost. In this chapter, we will discuss the thermal issues and possible thermal management techniques. The discussion will start with heat transfer and local temperatures at the chips, and it will be followed by phosphor downconversion and effect of local heat generations at the phosphor layers and particles. Finally, the chapter will conclude with the interaction between thermal and optical performances.

Conventional lighting products generally do not require any cooling scheme, therefore lighting designers have traditionally worried more about light output and aesthetics. However, LEDs being a solid state based technology, requires that designers now pay attention to the temperature of the device in order to maintain lifetime and reliability. To accomplish this a thermal solution must be integrated into the lamp or luminaire. The weight, size, cost and lifetime of the thermal solution needs to be considered and tradeoffs made for the particular application. In this chapter, we will review a number of candidate technologies and discuss the pros and cons of each. The most common solution in passive cooling, heat sinks, will be discussed first, followed by small scale fans. The combination of heat sinks and fans can provide heat transfer coefficients greater than 30/m²-K which is triple that of free convection cooling. Next, two rather new, microfluidic technologies, piezo fans and synthetic jets will be discussed. The compelling advantages of these two technologies are simplicity, low cost, no rotating parts, and the potential for much longer life. Lastly, liquid cooling of LEDs will be presented. While liquid cooling provides much higher heat transfer capability, special requirements are necessary for LED lamps when compared to conventional liquid cooling of electronics. The dielectric property, transparency, refractive index and cost are all major considerations when used in LED systems.

Organic light emitting diodes are another form of LEDs consisting of layers of light emitting polymers. Their main use is in display technology for cameras and cell phones. The trend is towards enlarging the area of these displays and using them as, non addressable, general illumination lighting devices. There are a number of issues to be resolved before large area device deployment becomes reality, especially for general illumination. They will need a much lower price point for end customers, unlimited size, and higher efficiency. The majority of OLED designers are working on glass substrates while a few are pursuing low cost plastic substrates, for roll to roll production. Temperature and humidity are two major obstacles for OLED reliability, making plastic substrates an even tougher challenge. Since both these substrates are low in thermal conductivity, conducting and removing the heat becomes an even greater challenge. Therefore novel methods to remove the heat and improve lumen efficacy as well as lifetime are needed.

A high brightness LED die typically has about 1 mm² surface area. Therefore, it is usually packaged for handling purposes. This usually involves mounting of the die to an intermediate, slightly larger substrate and over-molding with a lens. The intermediate substrate provides electrical connections and a solderable surface for assembly to an interconnect board. In addition, recent light engine packaging techniques have enabled direct chip on board (COB) packaging eliminating the thermal impedance of the package and allowing greater design flexibility, by allowing smaller compact light sources. This chapter will start discussion around single LED package designs and close the discussion with LED lamp design consideration. While there are a number of design options for LED downlights, here we will show an air cooled heat sink design for a 100 W replacement downlight.

Solid State Lighting technologies following Haitz law are progressing rapidly. Today, system efficacies far exceed incandescent and are very similar to CFL. It is expected that their efficacies will double during the next decade. Thermal management is a key part of SSL systems. It affects reliability, cost and efficacy of the system…

The following sections are included:

• Author Index

• Subject Index

• About the Authors

• About the Editor-in-Chief