Developer can improve LED efficiency and lifetime by effective thermal management ,but JADE BRIDEGES explains : about careful selection thermal materials and methods of application are very importance.
LED industry is one of the fastest improvement technology industries. Although it hasbeen used in many electronic devices for many years ,but recently more high-power LED has been used in varieties of lighting ,automotive, signature and home appliance products. For LED lighting is very reliable, but reliability depends on good thermal design around the components, there this article will show you how to carefully select thermal materials and get to know how to use them.
As an alternative to Halogen Lamp, Incandescent lamp and fluorescent lighting systems , the development of lighting market will be impressive. The growth of LEDis attributed to its superiority over traditional lighting on terms of adaptability, lifetime and efficiency, LED has more design freedom ,provide a very long lifetime ,and also efficiency ,it can convert most parts of energy into light ,then reduced the heat emitted in maximum limit .
Produce heat
However , LED still make obvious heat at the semiconductor structure. This heat can make bad effect to LED ,so it must need to dissipate heat to make sure realize the real advantage of solid-state lighting. LEDs are usually classified by color temperature ,there are many different color varieties on the market.
If LEDs working temperature changes, the color temperature also will change. For example ,the sun light temperature increase can cause the LED to emit a warmer CCT.Besides, if there exist the chip temperature changed on the LEDs in the same array, it may emit a certain range of color temperature , affecting the quality and appearanceof the end-lighting products.
As you seen from below chart, maintaining the correct LED chip temperature is not only can extend its lifetime, but also can produce more light, therefore, it just needfewer LEDs can achieve the desired results. The increased Working temperature may have a negative influence on LED performance, but this effect is recoverable. However ,if the junction temperature is exceeded, especially above the maximumworking temperature of LED(120-150℃), then it may occur unrecoverable affectswith complete failure.
For example, the characteristic of Cree XLamp LED show, LED performance changes with the temperature.
In fact ,the the working temperature and led lifetime have the directly relation, when the temperature is higher, the LED lifetime is shorten ,see from the below figure ,the led driver is same as it, its lifetime is determined by the life of electrolytic capacitor. By calculation, we can certain when the working temperature is decreased by10 ° C and the lifetime of capacitor is doubled. Therefore, ensuring the effective thermal management provides the LED array with consistent quality, appearance and lifetime, it will open up opportunities for future applications in a growing industry.
The curve records the relationship between junction temperature and lifetime of Cree XLamp LED at the 350mA.
Thermal conductivity mitigation
There have many methods to improve the thermal management of LED products, we must choose the right type of thermal materials to make sure that realized the desired cooling effect. In the material field, products ranges is range from thermal conductive encapsulation resin that can provide cooling and environmental protection to heat thermal interface material for improved the heat conduction efficiency .
Thermal interface materials are compound designed to fill the gap between device and heat sink, reducing the heat resistance between the two. This material will accelerate heat loss and decline the equipment working temperature. Curing products can also be used as bounding materials. For example include Silicone RTV (room temperature vulcanization) or epoxy compound. The choose material is usually decided by required bond strength and working temperature ranges .
The another option of heat conduction is the use of thermal conductive encapsulation resin. These products are designed to provide protection for equipment ,and alsoallow the heat within the equipment to dissipate the heat into surrounding environment. In this situation ,the encapsulation resin becomes a heat sink and conducts heat away from the equipment. These products can be used in LED devicesand can help to light extraction from within the cell according to the chosen color.
Encapsulation resin includes the use of thermal conductive filler, however, the base resins, hardeners and other additives used can be varied to provide a wide range of options, including epoxy, polyurethane and silicone chemicals. Different chemical materials will provides a series attributes, each of these should take into consideration of the application requirement.
Packaging material options
For example , Polyurethane material provides excellent flexibility ,especially at lowtemperature ,it is a major benefit to epoxy resins. Silicone resin also can provide this flexibility at low temperature and provide excellent high temperature performance over other existing chemical components, Silicone products are usually expensive.
Epoxy resins are very strong and can provide excellent protection in variety of bad environment. They are rigid materials with a low coefficient of thermal expansions and in some cases can add some degree of flexibility to the product. The addition of encapsulating resin can produce a large products with customized properties for any applications; therefore, it is advises to discuss the application with the relevant material supplier.
Application properties
No matter what types you choose of the cooling product, there some key propertiesmust be considered. These could be quite simple parameter, such as the operatingtemperature of equipment, electrical requirements or any other limited condition, for bounding , cure time and so on.
Other parameters are more important to equipment ,but only one date may not enough to select correct products. Thermal conductivity is a main example. Thermal conductivity in Watts/meter degrees （W／m ·K） ,it stands the Thermal conductivity of material. Stacked thermal conductivity values are based on most products datasheets and reflect the expected lever of heat transfer, then can compared with different materials.
But it does not necessarily be the most efficient heat transfer for just rely on stacked thermal conductivity value.
The unit of thermal resistance is K·m2 ／W, which is the reciprocal of thermal conductivity and it consider the interface thickness. Although the measure standardsdepends on the contact surface and applied pressure, some general rules can be followed to ensure the thermal resistance is minimized to maximum the heat transfer efficiency.
For example, compared with a heat transfer compound used at the interface, a metal heat sink has a higher thermal conductivity, so only need to use a thin layer of this compound. In this case, increasing the thickness will only increase the thermal resistance. Using the below formula, you can compare the difference of thermal resistance with a thermal adhesive of 50 μm and 0.5 mm thick thermal pad. So the lower interface thickness and higher thermal conductivity can maximize heat transfer.
This graph shows. Knowing the thermal resistance is necessary to choose the best thermal materials.
Application ways
We need to consider another important factor in the product selection – the application of thermal management material. Either the package chemical compound or interface material, any gap in the heat transfer medium will cause the reduction of heat dissipation rate.
The key to success is ensuring the resin can flow around the cell for thermal packaging resin, including into any small gap. This uniform flow can help to remove any air gaps and ensure there no heat in the entire unit. In order to achieve this application, the resin needs the correct thermal conductivity and viscosity. Usually as the thermal conductivity of resin increases, the viscosity will also increases.
For the interface materials, the viscosity of product or the possible minimum thickness of application can have a big impact on thermal resistance. Therefore, compared with the products which have a lower stacked thermal conductivity and lower viscosity, the high thermal conductivity and high viscosity compounds although not spread evenly to the surface, but has higher thermal resistance and lower viscosity. To maximize heat transfer efficiency, user need to solve stacked thermal conductivity, contact resistance, applied thickness and process.
As the Cree X Lamp LED graph show, the thermal interface material properties directly affect on LED temperature device.
We can see clearly from the graph ,when higher area thermal conductivity of 12．5 W／m ·K is compared with the lower 1.4 W / m • K that does not necessarily have more effective cooling. This reason may be the processing method is not suitable for the product ,the product is not easy to apply ,or it is not designed for specificapplication. Whatever the reason ,it highlights the importance of product applications and product selection, through it to find this too can realize the highest heat conduction .
With the rapid development of electronic fields ,and more actually in LED application ,material technologies also must be satisfied with ever-higher cooling demands. Now the technology is transferred to package chemical compounds to provide products with filler loading to improve thermal conductivity and improve fluidity.
