Electronics Protection - November/December 2013 - (Page 10)

Feature Thermal Management of LEDs: Looking Beyond Thermal Conductivity Values Jade Bridges, European Technical Support Specialist Electrolube Ltd. Specifically designed and formulated chemical products are widely used in the electronics industry for an array of applications. Whether it is during PCB manufacture, or for the protection of components or complete devices, such products have become essential in ensuring performance and quality of electronic devices. The subject of this paper is to discuss the application of these formulated chemical products for thermal management applications, specifically within the rapidly expanding LED industry. As we are all aware, LEDs have been present in many electronic devices for a number of years. More recent developments in this industry have lead to their vast array of use in all types of lighting, signage and domestic appliance products, to name but a few. In offering alternatives to halogen, incandescent and fluorescent lighting systems for both interior and exterior applications, the possibilities for LEDs are seemingly endless. LEDs offer advantages over traditional lighting forms in terms of adaptability, allowing more design freedom due to the reduced space required and exceptionally long life time, resulting in a cost effective solution for many applications. They are also considerably more efficient, converting the majority of energy to light and thus minimising the heat given off. Although LEDs are considerably more efficient than traditional lighting forms, they do still produce some heat. This heat can have an adverse effect on the LED and therefore must be managed to ensure the true benefits of this technology are realised. Typically categorised by colour temperature, LEDs are available in a huge number of colour variants. With a change in operating temperature of the LED, a change will also occur to the colour temperature; for example, with white light an increase in temperature could lead to a 'warmer' colour being emitted from the LED. In addition, if a variance in die temperatures is present across LEDs in the same array, a range of colour temperatures may be emitted, thus affecting the quality and cosmetic appearance of the device. Maintaining the correct die temperature of the LED can not only extend the life, but also lead to more light being produced and therefore, fewer LEDs may be required. Therefore, an increase in operating temperature can have a recoverable effect on the properties of the LED, however if excessive junction temperatures are reached, particularly above the maximum operating temperature of the LED (~120°C to 150°C), a non-recoverable effect could occur, leading to complete failure. Operating temperature is a directly related to the lifetime of the LED; the higher the temperature, the shorter the LED life. Ensuring efficient thermal management is employed will therefore provide consistent quality, appearance and lifetime of LED arrays and in turn, opens up the opportunity for further applications for this evolving industry. The principles of thermal transfer can be discussed in detail however, for the purpose of this paper, we shall briefly address the basics; conduction (heat transferred through a solid mass via 10 direct contact - Fourier's Law), convection (transfer of heat through the movement of fluids and gases - Newton's Law) and radiation (the heat transferred through an q = -k A (ΔT/s) electromagnetic field). Fourier's law of heat conduction Radiation typically only q is the heat transferred through conduction (W) has a very small effect k is the thermal conductivity (W/m K) on the heat transfer A is the cross sectional area of the material of LED systems since through which the heat flows (m2) ΔT is the temperature difference across the the surface areas are material (°C or K) relatively small and s is the material thickness (m) so it is the principles of conduction and q = h A ΔT convection that we Newton's Law of Cooling - Convection are most interested in q is the heat transferred through convection (W) here: Conduction refers h is the heat transfer coefficient (W/m2 K) to the transfer of heat A is the surface area (m2) at the LED junction, ΔT is the temperature difference typically between the surface temperature and ambient between the LED and air (°C or K) the heat sink, whereas convection refers to the transfer of heat from the heat sink to the surrounding air. Newton's law of cooling states that the rate of loss of heat is proportional to the temperature difference between the body and its surroundings. Therefore, as the temperature of a component increases and reaches its equilibrium temperature, the rate of heat loss per second will equate to the heat produced per second within the component. Since heat is lost from a component to its surroundings at its surface, the rate of dissipation will increase with surface area. This is where heat sinks are used. Varying in size and shape, heat sinks can be designed to offer a significantly increased surface area to maximise heat dissipation. Heat sinks are often used in LED applications and fix onto the back of the component. Ideally, these mating surfaces should be perfectly smooth enhancing the efficiency of heat conduction, but this is not usually possible. As a result, air gaps will be present at the interface of the device and the heat-sink, reducing the efficiency of heat transfer. There are many ways to improve upon the thermal management of LED products and therefore, the correct type of thermally conductive material must be chosen in order to ensure the desired results for heat dissipation are achieved. The first type of thermal management product we shall discuss are thermal interface materials, such as a heat transfer compound, to remove any air gaps between mating surfaces and improving the efficiency of heat conduction at the LED junction. Such compounds are designed to fill the gap between the device and the heat sink and thus reduce the thermal resistance at the boundary between the two. This leads to faster heat loss and a lower operating temperature for the device. Curing products can also be used as bonding materials; examples include silicone RTVs or epoxy compounds - the choice will often depend on the bond strength or operating temperature range required. Solid materials such as gap filling pads and phase changing materials are also a possibility, where a thin film substrate is used at the interface. Therefore, an initial consideration in product selection is whether a curing product is required to help bond the heat sink in place, or whether a non-curing thermal interface material is more appropriate to allow for rework. Silicone and silicone-free non-curing products are also avail- November/December 2013 www.ElectronicsProtectionMagazine.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - November/December 2013

New Features Make F-Series TeraFrame Gen 3 One of CPI’s Most Advanced Cabinets
Silicones for High Reliability and Yield in Electronic Applications
Surge and Transient Protection for Telephone, CATV & Satellite Services
Thermal Management of LEDs: Looking Beyond Thermal Conductivity Values
Understanding NEMA Ratings for Electrical Enclosures
Silent Air Cooling: A New Approach to Thermal Management
VadaTech Releases Rugged Conduction-Cooled MicroTCA Ecosystem
Directable Inverted Blowers Deliver High Volume Air Flow
Cima NanoTech Launches Ultra Low Resistance Sante EMI Shielding Film
Littelfuse Introduces Surge-Tolerant Fuses
ProTek Devices’ TVS Array Provides Circuit Protection in Computing Applications
Reell’s PolyTorq Technology Expands Capabilities For Hinge and Torque Insert Applications
Industry News

Electronics Protection - November/December 2013