Thermal Management
Looking beyond thermal conductivity values
There are a variety of ways to improve the thermal management of LED products, one of which is selecting the correct type of thermally conductive material to ensure that the intended heat dissipation is achieved. Jade Bridges explains
W
hether it is during PCB manufacture or for the protection of components or
complete devices specifically designed and formulated chemical products are an essential factor in ensuring the performance and quality of electronic devices.
In terms of LED design the correct die temperature can not only extend the life but also lead to more light being produced. 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 a non- recoverable effect could occur, leading to complete failure. Operating temperature is directly related to the lifetime of the LED; the higher the temperature, the shorter the LED life.
Ensuring efficient thermal management is crucial and the basic principles of thermal transfer need to be addressed: conduction, convection and radiation. Typically radiation only has a very small effect on the heat transfer of LED systems since the surface areas are relatively small and so it is the principles of conduction and convection that are of most interested: conduction refers to the transfer of heat at the LED junction, between the LED and 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
18 February 2014
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 and in LED applications are fixed 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 the heat transfer. There are many ways to improve upon the thermal management of LED products and the correct type of thermally conductive material must be chosen in order to ensure the desired results for heat dissipation are achievable. Thermal interface materials, such as a heat transfer compound, remove any air gaps present
operating temperature for the device. Curing products can also be used as bonding materials; examples include silicone RTVs or epoxy compounds – the choice will 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 available; the silicone products offer a higher upper temperature limit of 200˚C and a lower viscosity system, due to the silicone base oil used. However products based on, or containing, silicone may not be authorised in certain applications. This could be due to a number of factors, including application requirements or where problems exhibited in cleaning or adhesive processes are observed. Such issues are due to the migration of low molecular weight siloxanes; these volatile species can lower the surface tension of a substrate, making them extremely difficult to clean or adhere to. In addition, due to their insulative nature, migration of low molecular weight
absolute minimum. A range of non-silicone products are also provided for critical applications.
Another option for managing the transfer of heat away from electronic devices is to utilise a thermally conductive encapsulation resin. These are designed to offer protection of the unit from environmental attack whilst also allowing heat generated within the device to be dissipated - the resin becomes the heat sink and conducts thermal energy away from the device. Such products can be used to encapsulate the technology behind and attached to the LED device and can also assist with the reflection of light back from within the unit, depending on the colour chosen. Encapsulation resins also incorporate the use of thermally conductive fillers however the base resin, hardener and other additives used can be altered to provide a wide range of options, including epoxy, polyurethane and silicone chemistries. The different chemistry options will
between mating surfaces and improve the efficiency of heat conduction at the LED junction. Such compounds are designed to fill the gap between the device and the heat sink reducing the thermal resistance at the boundary between the two. This leads to faster heat loss and a lower
Components in Electronics
siloxanes can lead to failures in electronic applications. Electrolube formulates products from raw materials specifically designed for the electronics industry. As a result silicone containing products are only used where the low molecular weight fractions are monitored and kept to an
provide a range of properties and each should be considered depending on the end application requirements. For example, a polyurethane material offers flexibility, particularly at low temperatures, while epoxy systems are very tough and offer protection in a variety of harsh environments. Regardless of the type of thermal management product chosen, there are a number of key properties that must also be considered. These can be simple parameters, such as the operating temperatures of the device, the electrical requirements or any processing constraints - viscosity, cure time, etc. Other parameters are more critical - thermal conductivity is the primary example. Measured in W/m K, thermal conductivity represents a materials’ ability to conduct heat. Bulk thermal conductivity values give a good indication of the level of heat transfer expected, allowing for comparison between different materials. Relying on bulk thermal conductivity values alone will not
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