With the development of LED and solid-state lighting industry, product developers are applying technology to harsh environments. Now we discuss the need to protect LEDs and circuit components in harsh environments such as ocean applications. Coatings and other protection techniques can prolong the life and performance of solid-state lighting systems.
With the rapid development of LED market, the correct product selection is the key to ensure the performance and service life of LED. In this article, we will emphasize the use of LEDs in a variety of environments and describe how to take appropriate protection in these environments. The development process must ensure reliability while ensuring good optical performance in solid state lighting systems
The optical properties of diffusion (UR5635) and transparent (UR5634) polyurethane resins in solid-state lighting systems vary greatly.
LED applications are becoming more diverse; the design requirements, location, or product functions can prove that the challenges faced by led designers are changing. Like most electronic devices, LEDs perform well until external influences begin to degrade their performance. These effects can include electrostatic attraction, chemical or gaseous contamination, and many other possibilities of dust, damp or corrosive environments. Therefore, the end-use environment needs to be carefully considered to ensure that the right products can be selected.
Opportunity and application
LED lighting is due to the development of LED in the adaptability, longevity and efficiency of the lamp advantage than the traditional lighting form. Therefore, it is easy to understand why LED lighting is widely used in a large number of applications, including household lights, industrial lighting in the factory, marine environment lighting and architectural lighting and design.
Comparing environmental conditions in the application of standard building lighting to the marine environment can help us understand the potential causes of LED deterioration. In architectural lighting applications, the LED itself may be covered by the design of the illumination unit, or the direction of the LED may only be exposed to general changes in ambient temperature and humidity. In marine environments, LED lights may be splashed or soaked in brine. In addition, in all cases, the most useful life of a lamp is working in a salt mist environment. High salt conditions can lead to corrosion of printed circuit boards (PCB), which reduces performance faster than general humidity changes. Typically, conformal coatings and encapsulation resins provide high levels of protection in these environments.
Conformal coating is the first protective measure we discussed. The coating is usually in accordance with the PCB outline of the thin paint, providing good protection while not significantly increasing the weight or volume of the circuit board. They are usually 25-75 microns thick and easily accessible by spraying or impregnation techniques.
In order to protect the top of the LED, the coating used must be of good transparency and remain clear throughout the life of the product in the desired environment. If used outdoors, the coating may require a good UV stability. Therefore, the best type of conformal coating is based on an acrylic chemical material that provides clarity and color stability as well as excellent moisture and salt mist protection. Fig. 2 depicts the salt spray test, in which acrylic coatings provide excellent protection.
Coatings based on different chemical substrates can provide varying degrees of performance when exposed to salt spray tests.
Generally, the acrylic conformal coating is a solvent based product, wherein the solvent is a liquid carrier that deposits the resin film on the base material. The solvent used is volatile organic compounds (VOC). This is not a long-standing problem for most systems because this solvent only exists for a few minutes in the application phase. In some cases, led manufacturers have specific requirements for the use of VOC-containing products and other specific chemicals, which are listed in the LED manual. Generally, chemical compatibility checks help to determine whether solvent-based conformal coatings are suitable for specific LEDs.
Color temperature problem
In addition to considering the impact of the coating on the LED, but also need to understand the impact of the coating on color temperature. Figure 3 depicts the associated color temperature (CCT) bands that are common in LED lighting. The color temperature changes over time, also known as color maintenance, is a problem when considering the type of protection media. It is understood that no matter what material placed directly on the LED Crystal, will lead to interaction, resulting in color temperature offset.
The LED is in some typical temperature spectrum bands.
CCT changes are usually from warm temperatures to cooler temperatures and vary between different led types and color zones. In addition, it will vary depending on the protective material applied. This is another area where acrylic conformal coatings are more advantageous than other chemical materials and product types.
Figure 4 depicts the typical color temperature offset of a warm white led. To highlight possible variations in color temperature, it has been included in different thicknesses and curing mechanisms to highlight possible variations in color temperature. The red line indicates the specific type of color temperature boundary of the LED used, that is, when the LED is purchased, its color temperature may be anywhere between these lines.
The thin coating and the thick coating represent the minimum and maximum thickness level of the conformal coating, i.e. 25 and 75 microns. By applying the film, the color temperature offset is minimized and can be controlled within the boundaries given by the LED manufacturer.
Ideally, conformal coatings can be used in all led applications, as conformal coatings are easy to use, small in size and weight of the lighting unit, versatility of use, and effects on color temperature offsets. However, as we all know, a solution is generally not available for all applications. As mentioned earlier, conformal coatings provide excellent protection levels in humid and salt mist environments, but the conformal coating does not provide the highest level of protection in environments that are often immersed in water, chemically splashed, and corrosive gases. In this case, we recommend that we consider sealing resins to provide a higher level of protection.
Sealants are also present in many different chemical types, including epoxy resins, polyurethane and silicone resins. In general, epoxy resins provide more robust protection for mechanical effects, but they do not have the flexibility of other chemicals, which can lead to problems during the heat cycle. In addition, standard epoxy systems do not provide clarity and color stability for other systems.
Silicone resins do have excellent transparency and perform well at extreme temperatures, while polyurethane resins provide good flexibility, transparency and high levels of protection in harsh environments. Fig. 5 shows that three types of resin are exposed to ultraviolet light 1000 hours, by examining the differences in the color of the resin and the transparency of the chemical types of three resins, thereby highlighting the stability of each resin under outdoor conditions. In this diagram, silicone resins and polyurethane resins are clearly superior to the standard epoxy system.
Compare the performance of various products in harsh environments, and also allow users to choose their products according to their final use conditions. For example, fig. 6 shows the effects of corrosive gas environments on acrylic conformal coatings, polyurethane resins and silicone resins. By exposing the three to the mixed gas environment, the luminous flux percentage of the LED is reduced. These results clearly illustrate the importance of choosing the right product for the environment. Although the conformal coating in corrosive gas environment surface insulation resistance will not deteriorate, but for the LED, it can not fully protect the LED, because it allows the gas through the thin coating and penetrate the LED, so that over time, its performance is reduced.
Silicone resins have similar effects; however, in this case, although the protective layer is quite thick (2 mm compared to 50 microns), the gas is still able to pass through the resin and affect the LED. When you compare the results of silicone and polyurethane materials, it is clear that the properties of these two chemical types are different because silicone resins are permeable to gases, while polyurethane resins of the same thickness are not. In this case, optically transparent polyurethane resins, such as Electrolube UR5634, are the most appropriate protective material to prevent corrosive gases from affecting LEDs.
Polyurethane resins are considered to be suitable resins for the protection of LEDs in many different environments. In addition, they can be modified to provide additional advantages, such as a coloring system for covering the PCB but not exceeding the LED height. This resin for the protection of the PCB, not only to make the surface pleasing, but also by the light from the PCB to reflect and increase the light output, thereby improving the performance of lighting lamps. There is also a special resin used to diffuse led light. Resins like Electrolube UR5635, for example, offer two of solutions: free from ambient influences and the spread of light, so you may not need to buy a cooling lampshade or lamp cap.
Protection Material Formula
It is clear that the sealant can provide a high level of protection in a range of environments and can be adapted to the application requirements by choosing a chemical type or a specific resin formulation. However, in the front, we discussed the effect of thin film conformal coating on color temperature is very small. When comparing the thickness of the conformal coating with the sealing resin, it is obvious that the protection level of the resin is increased partly because it can achieve more coating. The resin can be increased at depths of 1-2 mm or deeper, but this depth will also have an effect on the observed color temperature level.
Fig. 7 shows the typical color temperature offset of LEDs with different thickness of polyurethane resin. Obviously, the thickness is directly related to the degree of color temperature offset, so this is another important consideration when choosing the right protective material. We know that the color temperature offset will occur, but take into account the repeatability of the LED offset. If the offset persists, you can reconsider the original led color temperature.
This article discusses the considerations needed to select the protection of LED systems. Evaluating the environment is critical to the successful selection of the product, whether it is the final use performance or the applicability of the production process. Conformal coatings are not only easy to use and fit for design, but also have excellent protection levels in humid and salt mist environments. Because of their low thickness, they also have little effect on the color temperature.
When conditions become more challenging, it is recommended that you convert to a sealed resin. In this case, the choice between chemical types will be determined by the final use condition and the specific environmental impact. In addition, the thickness of the resin to be added should be considered to ensure adequate protection while minimizing the effect on color temperature changes.