Improving the Efficiency and Lifetime of LEDs via Effective Thermal Management
Part 1 Improving the efficiency and lifetime of LEDs via effective thermal management
The LED industry is one of the fastest growing markets; despite LEDs being 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 growth of the LED lighting market alone is expected to grow into a $70 billion industry by 2020; a growth from 18% market share to 70% market share in just over 5 years. (Forbes) The growth is attributed to the advantages LEDs offer over traditional lighting forms in terms of adaptability, lifetime and efficiency; they allow more design freedom, offer an exceptionally long life time and 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.
As shown in the table above, 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 to achieve the desired effect. 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 (~120-150˚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 as shown here in the Cree XLamp lifetime graph. This is also true for the LED drivers where the lifetime of the driver can be derived from the lifetime of the electrolytic capacitor and by calculation it can be determined that for every 10º C drop in the operating temperature the lifetime of the capacitor is doubled. (Philips) 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 ever evolving industry.
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. Products range from thermally conductive encapsulation resins, offering both heat dissipation and environmental protection, to thermal interface materials used to 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 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.
Another option for managing the transfer of heat away from electronic devices is to utilise a thermally conductive encapsulation resin. These products are designed to offer protection of the unit from environmental attack whilst also allowing heat generated within the device to be dissipated to its surroundings. In this case, the encapsulation 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 provide a range of properties and each should be considered depending on the end application requirements. For example, a polyurethane material offers excellent flexibility, particularly at low temperatures, a major advantage over an epoxy system. A silicone resin can also match this flexibility at low temperatures as well as offering superior high temperature performance, well in excess of the other chemistries available. The silicone products are also typically more expensive. Epoxy systems are very tough and offer excellent protection in a variety of harsh environments. They are rigid materials with low coefficients of thermal expansion and in some cases, a degree of flexibility can be formulated into the product. The formulation of encapsulation resins can lead to a vast array of products with tailored properties for individual applications and therefore it is advised that applications are discussed in detail with a relevant material supplier.
Regardless of the type of thermal management product chosen, there are a number of key properties that must also be considered. These can be quite 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 to the device and a value alone may not be sufficient to specify the correct product. Thermal conductivity is a primary example of this. Measured in W/m K, thermal conductivity represents a materials’ ability to conduct heat. Bulk thermal conductivity values, found on most product datasheets, 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 necessarily result in the most efficient heat transfer, however.
Thermal resistance, measured in K m2/W, is the reciprocal of thermal conductivity. It takes into account the interfacial thickness and although it is dependent on the contact surfaces and pressures applied, some general rules can be followed to ensure thermal resistance values are kept to a minimum and thus maximising the efficiency of heat transfer. For example, a metal heat sink will have a significantly higher thermal conductivity than a heat transfer compound used at the interface and therefore it is important that only a thin layer of this compound is used; increasing thickness will only increase the thermal resistance in this case. Using the formula given above, some basic calculations can provide some examples of the differences in thermal resistance likely to be seen between a thermal paste applied at 50µm and a thermal pad that is 0.5mm thick. Therefore, lower interfacial thicknesses and higher thermal conductivities give the greatest improvement in heat transfer.
There is however, a concern with using bulk thermal conductivity values alone or comparing thermal resistance values given on the product datasheets. Significant variations in thermal conductivity and thermal resistance values for the same product can be achieved by utilising different test methods or parameters. This can result in bulk thermal conductivity values that appear very high when quoted but in use have a dramatically reduced efficiency of heat dissipation. Some techniques only measure the sum of the materials’ thermal resistance and the material/instrument contact resistance. Electrolube use a version of the heat-flow method that measures both of these values separately, giving a much more accurate bulk thermal conductivity measurement. The test for thermal resistance should ideally be carried out on the actual unit using the normal application, spacing and weight/pressure parameters or alternatively using a comparable method where the pressure is defined. However these tests may be conducted, it is essential that products are compared using the same method to obtain bulk conductivity and thermal resistance values and in all cases, the products should be tested in the final application for a true reflection of effective heat dissipation.
This leads us to another important factor in product selection, the application of thermal management materials. Whether it is an encapsulation compound or an interface material, any gaps in the thermally conductive medium will result in a reduction in the rate of heat dissipation. For thermally conductive encapsulation resins, the key to success is to ensure the resin can flow all around the unit, including into any small gaps. This helps to remove any air gaps and ensure there are no pockets of heat created throughout the unit. In order to achieve this, the resin will have to have the correct combination of thermal conductivity and viscosity; typically, as the thermal conductivity increases, the viscosity also increases. Electrolube offer specialist resins to help reduce viscosity for ease of application, whilst maintaining a high level of thermal conductivity for efficient heat dissipation.
For interface materials, the viscosity of a product or the minimum thickness possible for application will have a great effect on the thermal resistance and thus, a highly thermally conductive, high viscosity compound that cannot be evenly spread onto the surface, may have a higher thermal resistance and lower efficiency of heat dissipation when compared to a lower viscosity product with a lower bulk thermal conductivity value. It is essential that users address bulk thermal conductivity values, contact resistance, application thicknesses and processes in order to successfully achieve the optimum in heat transfer efficiency.
A practical example highlighting the requirement for such considerations is provided in the table below. It shows the potential differences in heat dissipation by measuring the temperature of a heat generating device in use. These results have been based on work completed by an end user, where all products were thermal interface materials, applied using the same method, at the same thickness.
It is clearly evident that a higher bulk thermal conductivity value, in this case 12.5 W/m K, does not necessarily result in more effective heat dissipation when compared to products with lower values, such as the above at 1.4 W/m K. The reason for this could be due to the processing method not being suitable for the product, for the product not being easy to apply or possibly the product was not designed for this particular application and thus is exhibiting a high thermal resistance when compared to the other products tested. Whatever the reason, it highlights the importance of product application as well as product selection and by finding the correct balance of both of these parameters the maximum efficiency of heat transfer can be achieved.
Looking back at the original data for LED performance vs. lifetime and using the above results as an example, a conclusion on the importance of the use and correct selection of thermal management materials can be drawn. Take product #2, this reduces the operating temperature by 20% in this application. If a similar percentage reduction was achieved for the LEDs discussed above it would result in increased efficacy through the reduction in operating temperature from 85⁰C to 68⁰C and similarly, an increased lifetime from 95,000 hours to 120,000 hours; a great improvement. However, when you compare this to product #4 a greater operating temperature reduction is achieved, resulting in an increase in efficacy >3% and an increased lifetime from 95,000 hours to 140, 000 hours. Therefore, by selecting the correct product and using the best process lifetime can be improved by a further 15-20% when using product #4 in place of product #2.
With such rapid advances in the electronics industry and more specifically, LED applications, it is imperative that materials technology is also addressed to meet the ever demanding requirements for heat dissipation. Electrolube have developed specific technologies to improve the ability to process thermal management compounds, easily and effectively. This has resulted in reduced viscosity compounds with higher bulk thermal conductivities and with these two properties combined these products provide maximum efficiency in heat dissipation by minimising thermal resistance. This technology has now also been transferred to encapsulation compounds, providing products with higher filler loadings and thus improved thermal conductivity combined with improved flow. In addition and discussed in Part 2 of this paper, Electrolube also manufacture a range of products other than thermal management materials. Such products include conformal coatings and encapsulation resins in optically clear formats for applications where protection of the entire LED is required, once again re-confirming the importance of continually developing formulated chemical products to meet the rapid and demanding requirements of this popular technology.
Choosing the correct protection media for LED devices to improve lifetime and performance
With the rapid growth of the LED market, correct product selection is imperative to ensure LED performance and lifetime. In Part 1 of this paper we discussed the importance of proper thermal management, including the various ways to ensure maximum heat dissipation in LED systems. The effect of excessive heat generation was discussed in direct correlation to LED lifetime. Similarly, in Part 2 we will also be discussing LED lifetime; in this paper we will highlight the use of LEDs in various environments and introduce how to specify appropriate protection under such conditions.
LED applications are becoming increasingly more diverse; design requirements, location or the function of the product are all elements that prove the challenges that face LED designers are continually evolving. LEDs, like most electronic devices will perform well until external influences start to deteriorate performance. Such influences can include the electrostatic attraction of dust, humid or corrosive environments, chemical or gaseous contamination, as well as many other possibilities. It is therefore extremely important that the end use environment is considered in detail to ensure the correct products can be chosen.
The rapid growth within the LED market can be attributed to the advantages LEDs offer over traditional lighting forms in terms of adaptability, lifetime and efficiency. It is therefore easy to understand why LED lighting is being used in a vast array of applications including domestic lamps, industrial lighting for factories, lighting for marine environments, architectural lighting and designs, to name just a few.
Comparing the environmental conditions in a standard architectural lighting application with that of a marine environment can help us to understand the potential causes of LED deterioration. In an architectural lighting application, it is possible that the LED itself is covered due to the design of the unit, or that the orientation of the LED is such that it is only likely to be exposed to general changes in ambient temperature and humidity. In a marine environment, it is possible that an LED light may be splashed or immersed in salt water and in all cases, it will be in a salt mist environment for the majority of its operating life. Conditions with high salt can cause corrosion on PCBs and thus dramatically reduce performance much faster than general conditions of varying humidity. Typically, conformal coatings and encapsulation resins are used to offer a high level of protection in each of these environments.
Conformal coatings are thin lacquers which conform to the contours of a PCB, allowing good protection without adding any significant weight or volume to the board. They are typically applied at 25-75 microns and are easy to apply by spraying or dipping techniques. For protecting over the top of LEDs, it is crucial that the coating used has good clarity and that it remains clear throughout the lifetime of the product in the desired environment, i.e. the coating may be required to have good UV stability if the product is outdoors. Thus, the best type of conformal coatings are based on acrylic chemistry, offering both the clarity and colour stability combined with excellent humidity and salt mist protection.
Typically, acrylic conformal coatings are solvent-based products, where the solvent used is a carrier fluid to allow a thin film of resin to be deposited on the substrate. The solvents used are classified as VOCs (Volatile Organic Compounds); as this solvent is only present on the LED for a few minutes during the application stage, it is not considered a long term issue for most systems. In some cases, LED manufacturers do have specific requirements regarding the use of products containing VOCs, as well as other specific chemicals, and these will be listed in the LED literature. In general, a chemical compatibility check will assist in confirming if a solvent-based conformal coating is suitable for use with the desired LED; conformal coating manufacturers such as Electrolube can assist with such testing.
As well as considering the effect of the coating applied on the LED, it is also important to understand the effect on colour temperature. Colour temperature shift has been an ongoing issue when considering the type of protection media to use and it is understood that no matter what material is placed directly over the LED lens, it will cause an interaction that leads to a colour temperature shift. This shift is typically from a warm temperature to a cooler temperature and will vary between different LED types and colour temperature bands. In addition, it will also differ depending on the protection material applied. This is another area where acrylic conformal coatings offer advantages over other chemistry and product types. In Graph 2, the results of colour temperature shift of a ‘warm’ light LED are provided. Different thicknesses and cure mechanisms have been utilised in order to highlight the possible changes in colour temperature. The red lines indicate the boundaries of the particular type of LED used; i.e. the colour temperature could be anywhere between these lines when the LED is purchased.
The thin and thick coatings referred to above represent the typical minimum and maximum thickness that conformal coatings are applied, i.e. 25 and 75 microns. By applying such a thin film, the colour temperature shift is minimised and in turn is manageable within the same boundaries given by the LED manufacturer. In an ideal world, conformal coatings would be applied to all LED applications due to their ease of application, minimal effect on volume and weight of the unit, versatility in use and finally, their effect on colour temperature shift. As we all know, it is often not possible to have one solution for all applications, however. Conformal coatings offer an excellent level of protection in humid and salt mist environments, as shown above, however they do not provide the highest level of protection in environments with frequent immersion in water, chemical splashes and also corrosive gas environments. It is in such situations that we advise the consideration of an encapsulation resin to offer the increased level of protection.
Encapsulation resins are also available in a number of different chemistry types, including epoxy, polyurethane and silicone options. Typically epoxy resins offer tougher protection in terms of mechanical influences but they do not offer the flexibility of the other chemistries, which can lead to problems during thermal cycling, for example. In addition, standard epoxy systems do not offer the clarity and colour stability of other systems. Silicone resins do offer excellent clarity and also perform well in temperature extremes, whereas polyurethane resins offer a combination of good flexibility, clarity and a high level of protection in harsh environments. Graph 3 shows the difference in clarity of the three resin chemistry types by examining the colour differences of the resins after 1000 hours UV exposure, thus highlighting the stability of each resin in outdoor conditions. It is evident that the silicone and polyurethane resin outperform the standard epoxy system in this case.
Comparing the performance of various products in harsh environments can also highlight preferential product choice based on the end-use conditions. For example, Graph 3 illustrates the effect of corrosive gas environments on an acrylic conformal coating, a polyurethane resin and a silicone resin by examining the % reduction in luminous flux of the LED after exposure to a mixed gas environment. These results clearly illustrate the importance of choosing the correct product for the environment. Although the conformal coating does not deteriorate in terms of its surface insulation resistance in a corrosive gas environment, it is not an adequate protection for LEDs as it allows the gas to pass through the thin coating and penetrate the LED, thus degrading its performance over time. A similar effect is also seen with the silicone resin, however in this case, despite the protection layer being considerably thicker (2mm vs. 50 microns) the gas is still able to pass through the resin and affect the LED. When you compare the result of the silicone resin to the polyurethane material it is evident that there is a difference in performance exhibited by these two chemistry types as the silicone resin is permeable to the gas whereas the polyurethane resin at the same thickness, is not. In such cases, an optically clear polyurethane resin, such as Electrolube UR5634, would be the most suitable protection media to prevent the corrosive gases from adversely affecting the LED.
Graph 4 – Change in luminous flux after exposure to mixed corrosive gas
Polyurethane resins have been highlighted as suitable resins for the protection of LEDs in a number of different environments, In addition, they can also be adapted to offer additional benefits, such as pigmented systems used for covering the PCB up to, but not over, the LED. Such resins are used for protection of the PCB, offering an aesthetically pleasing finish whilst adding to the performance of the luminaire by reflecting the light off the PCB and increasing light output. There are also specialist resins that can be used to diffuse the light from the LED. Resins such as Electrolube UR5635 can offer two solutions in one; protection from the surrounding environment and diffusion of light, potentially eliminating the need for diffuser covers and caps.
Image 2 – Comparison of clear (UR5634) and diffusing (UR5635) polyurethane resins
Encapsulation resins clearly offer a high level of protection in a range of environments and can be tailored to suit application requirements either by choice of chemistry type or by adaption of the formulation of a particular resin. It is important to return back to the subject of colour temperature shift, however. Earlier in this paper we discussed the minimal effect on colour temperature exhibited by thin film conformal coatings. When comparing the thicknesses of conformal coating to encapsulation resins it is evident that part of the increased level of protection that resins offer is due to the ability to apply a much thicker layer. Resins can be applied at 1-2mm or at much greater depths, however this depth will also have an effect on the level of colour temperature shift observed.
Graph 5 below shows the typical colour temperature shift of LEDs covered with different thicknesses of polyurethane resin. It is clear that the thickness directly correlates to the degree of colour temperature shift, thus highlighting another important consideration when choosing suitable protection media. We do know that colour temperature shift will occur but the important consideration is the repeatability of the shift for the LED used. If the shift is consistent, the change can be accounted for by re-considering the original LED colour temperature band, for example.
Graph 5 – Effect of resin thickness on colour temperature shift
This paper has discussed the various considerations required when choosing protection for an LED system. Evaluating the environment is essential to successfully specifying a product, both in terms of end-use performance and suitability for production processes. Conformal coatings offer the best combination of ease of application and incorporation into the design, with an excellent level of protection in humid and salt mist environments. They also exhibit the lowest effect on colour temperature due to the low thickness applied. When conditions become more challenging, the switch to encapsulation resins is advised. In this case, the choice between chemistry types will be dictated by the end-use conditions and particular environmental influences. In addition, the thickness of resin applied should be considered to ensure sufficient protection is achieved whilst minimising the effect on colour temperature shift where possible. Combining the protection media discussed in this paper with the thermal management products discussed in Part 1, highlight the increasing importance of materials technology in this rapidly expanding market. By ensuring efficient heat dissipation and protection from external environments, the efficiency and lifetime of LED systems can be increased. LED systems can now also be used in a wider range of environments and by offering LED designers support through considered material development, Electrolube are continually providing support for this ever evolving industry.
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