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PCBD-Oct2017

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70 The PCB Design Magazine • October 2017 Does excess heat have an adverse effect on the LED and if so, how can thermal management materials mitigate this? Heat adversely affects an LED by reducing both its efficiency and its life expectancy. By re- ducing the junction temperature by just a few degrees, the lifetime of an LED can be increased by thousands of hours. Moreover, a reduction in the temperature of the LED's surrounding envi- ronment will also impact on the junction tem- perature and, again, the useful life of the LED. Thermal management materials are designed to dissipate heat away from critical areas. In LED arrays, they can help distribute the heat, which reduces the temperature at LED junctions. Ther- mal management materials can either be used beneath LEDs as an interface between the PCB and the outer casing (also sometimes acting as a heat sink), or they can be used to encapsu- late up to the LED surface, spreading the heat both above and below the PCB surface. Which method is ultimately chosen will be dependent upon the design of the assembly. Thermal man- agement materials can also be used to encap- sulate associated electronic circuits such as LED drivers, again reducing operating temperatures and improving the efficiency of the LED unit as a whole. What different types and varieties of thermal interface materials are available, and how do I select the most appropriate one for my LED application? There are many ways to improve upon the thermal management of LEDs. Products range from thermally conductive encapsulation res- ins, offering both heat dissipation and environ- mental protection, to thermal interface materi- als used to improve the efficiency of heat con- duction 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. The design of an LED array will also dictate an appropriate choice of thermal interface ma- terial. For example, if it is a large array, across which temperature fluctuations are likely, it may be beneficial to use a material that is not subject to movement during thermal cycling. A surface-curing thermal paste or one of the latest phase change materials would be ideal choices for these larger LED assemblies. Other applications may require less localised heat dissipation and therefore a resin that cov- ers the whole surface of the PCB may be more efficient at spreading the heat. An encapsulated PCB is also protected from adverse environmen- tal influences such as mechanical shock or high humidity. And let's not forget thermally con- ductive bonding materials; for some applica- tions, you might want both adhesive strength and thermal management, in which case a ther- mally conductive room temperature vulcaniz- ing material would be the appropriate choice. If I am seeking to dissipate heat away from other power components such as LED drivers, what would be an effective solution? Encapsulation resins every time! These pro- vide high levels of protection against adverse environmental factors as well as total heat dissi- pation; they also provide structural stability and flame retardancy. Does the operating environment impact upon my choice of thermal interface material and what are the key considerations? Temperature changes, as mentioned be- fore, can be a key factor. Temperature fluctua- tions can cause the thermal interface material to move, which is clearly undesirable. The gap or distance between LED/PCB and the heat sink also needs to be considered. If it is too large, a non-curing thermal interface material intro- duced between the LED and its heat sink is more prone to movement. LED lighting assemblies are in use just about everywhere these days and it is important to consider the ability of the thermal interface ma- terial to withstand the conditions under which it operates. For example, high ambient tempera- tures, poor or damaged waterproofing, conden- sation, high humidity and so on will all have an impact on its performance. As with any protec- tive compound, you need to understand the op- erating environment and discuss with chemical suppliers to identify the most suitable materials for your application. THERMAL MANAGEMENT FOR LED LIGHTING MANUFACTURERS

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