LED Technology
The powerful impact of protecting future LED lighting applications
Jade Bridges, global technical support manager at Electrolube, describes applications of LEDs and looks at how modern protective compounds and thermal management materials are enhancing the performance and prolonging the life of LED lighting units exposed to challenging indoor and outdoor environments, with positive powerful results
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t is hard to ignore the phenomenal growth of the LED market, spurred on by new applications, advertisements for energy saving and company policies to switch to more efficient lighting systems. In conjunction with the design freedom and application possibilities now offered by the technology, LEDs are predicted to take almost a 70 per cent share of the lighting market and be the most commercially viable lighting technology by 2020. The outdoor lighting market is a typical area where LEDs have opened up new opportunities for architectural designs in a host of applications, including under-floor lighting in heavily trafficked areas. This is a particularly demanding environment for LED lighting, yet it is increasing in popularity thanks to the long life and compactness of LED arrays, which make them the better option for hard-to-access spaces. Protecting lighting installed under a walkway involves the use of encapsulation resins which fully encase the LED array. The resin has to resist abrasion from pedestrian footfall, be UV stable and must not affect the colour temperature of the light emitted. When such tough resins are required, epoxy based systems are often considered, but standard epoxy resins are not UV stable and will yellow over time. Moreover, tests have shown epoxy resins to be vulnerable to abrasion; due to their hardness, they tend to scratch and, where a clear resin is concerned, leave white marks on the surface. Clear polyurethane systems, such as Electrolube’s UR5634, offer good UV
stability and a degree of flexibility, which means that when the surface is scratched or scuffed, it does not mark in the same way as an epoxy resin and thus preserves and prolongs the cosmetic appearance of a lighting unit to which it is applied. The graph in Figure 1 shows how the potting depth of UR5634 affects the colour temperature shift. As the shift only happens as a result of the interaction with the LED, once the first layer is potted it is possible to pot again with more resin to ensure adequate protection is achieved without further affecting the colour temperature of the light emitted.
Algal cultivation Algae produce organic food molecules, using just carbon dioxide and water, via the process of photosynthesis. It is possible to monitor and control the
environmental conditions for algae growth by varying the wavelength of light used, simulating specific conditions for particular strains. Traditionally, high pressure sodium- metal halide/fluorescent lighting has been used for this purpose, but the toxic materials contained within these units present a contamination hazard, should they be damaged during harvesting. Moreover, the wavelength of the light emitted is not optimised for photosynthesis, rendering the process inefficient. An alternative approach is to use LEDs, which are safer to use in a food production environment. Importantly, it is possible to control the wavelength of the light emitted by LEDs, leading to increased growth rates and process cost reductions.
It is important to note that LEDs generate heat; any air voids in the resin close to the surface of the LED arrays will cause hot spots to develop, increasing the ambient temperature in the vicinity of the LED and thus reducing its operating lifetime. Appropriate resin choice is important here, as certain properties like viscosity will affect void formation. What about ancillary equipment, such as power supplies and control units? Controllers are often fitted with internal temperature sensors, such as positive temperature coefficient (PTC) thermistors, which may require the use of thermally conductive compounds to ensure that all air gaps are minimised to improve heat transfer to the sensor surfaces. Thermal management pastes, room temperature vulcanizing (RTV) compounds and gap filling products are often used for such applications, the final choice depending on the geometry of the sensor and correspondingly the size of any air gaps that may be present. The chosen thermal management material must also remain stable at the applied thickness in order to achieve the best thermal transfer to the sensor. Products such as the recently introduced
Electrolube HTSX can provide low thermal resistance and excellent stability over a wide operating temperature range, but as a thermal interface material, it is designed to be applied in thin films.
For power supplies an encapsulation
resin can serve two purposes: protection from the environment and heat dissipation. For example, Electrolube UR5633 provides very high levels of thermal conductivity combined with excellent resistance to water and tolerance of low temperatures. However, the viscosity of this two-part polyurethane resin is very high and so it may not be suitable for space-limited applications where air voids are likely to be formed. Remember, air voids in an
40 February 2019 Components in Electronics
encapsulated unit can create hot spots, leading to reduced performance and shorter lifespan. Lower viscosity resins, such as Electrolube’s two-part ER2221 epoxy resin may be more appropriate in this case, while high performance resins such as Electrolube’s ER2225 offer additional useful properties, including chemical resistance and tolerance of high temperatures.
Lighting the way to better animal welfare
LED based lighting technology is now also helping to improve the health, wellbeing and yield from agricultural livestock. Many studies have shown that improving the quality of light in animal enclosures, pens and barns reduces stress, while utilising LEDs of a specified wavelength can tackle other issues, such as the attraction of flies to the light source. The colour temperature of the light has also been shown to have an impact on the growth and positive behaviour of chickens. Not surprisingly therefore, the poultry industry is gearing up to retrofit as many as 600,000 barns with LED lighting over the coming years. Lighting in these environments is
required to meet IP66 rating for dust tightness and resistance to ingress from high pressure water jets. It must also be resistant to corrosion (particularly corrosive gases that can be present in poultry barns), high humidity and potential impact damage. Protection compounds, such as conformal coatings and encapsulation resins, provide the long-term protection required to meet the rigours of these demanding environments. As with all electronics applications, the
correct choice and implementation of conformal coatings, encapsulation resins and thermal management materials is critical to the performance and life expectancy of an LED product, and these issues should be considered at the earliest stages of the design process.
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