Discussion and Outlook: Heat Dissipation Technology for High Power LED Packaging

For a long time, when the heat dissipation requirements of LEDs were not very high, LEDs were mostly packaged using traditional resin substrates. However, as the market application areas continue to expand and the demand level continues to increase, traditional resin substrates have gradually become insufficient after the advent of the high-power LED generation. Therefore, the discussion and prospect of high-power LED packaging materials has become a hot topic in the industry.

The principle of LED heat dissipation
Research shows that high-power LEDs can only convert 20% of electrical energy into light energy, and the rest will be dissipated in the form of heat energy. If up to 80% of the heat energy cannot be dissipated in time, the life of the LED will be greatly reduced. How does the heat energy of LEDs dissipate?

The heat dissipation capacity of LEDs is usually affected by the packaging mode and the thermal conductivity of the packaging material. The heat dissipation paths are conduction, convection, and radiation. Since most of the heat energy accumulated in the LED packaging material is dissipated by conduction, the selection of packaging materials becomes particularly important.

Traditional materials can no longer meet the heat dissipation requirements of high-power LEDs

As more and more high-power LED applications appear in the market, we need to consider how to dissipate heat energy while also taking into account the stability and sustainability of LED light emission. If the heat energy of LED cannot be dissipated as quickly as possible, its brightness and life will decrease rapidly. Therefore, for high-power LEDs, traditional epoxy resins are limited by their characteristics and no longer meet their heat dissipation requirements.

Under the conditions of low-power or general-power LED use, the disadvantages of epoxy resin, such as low thermal conductivity and poor heat resistance, have not been fully highlighted. However, if epoxy resin is used as a material for encapsulating high-power LEDs, it is very likely that the epoxy resin will not last long before the LED chip itself reaches its lifespan.
In addition, not only will the heat dissipation phenomenon cause changes in the epoxy resin, but even short wavelengths will cause trouble for the epoxy resin. This is because the emission spectrum of white light LEDs also contains short-wavelength light, and epoxy resin is easily damaged by the short-wavelength light in white light LEDs. Even low-power white light LEDs can aggravate the damage of epoxy resin, not to mention that high-power white light LEDs emit more short-wavelength light, and the deterioration phenomenon is naturally faster and more serious. Therefore, it is urgent to find a new material to replace epoxy resin to encapsulate high-power LEDs.

LED heat dissipation substrate type

At present, common types of LED heat dissipation substrates include: rigid printed circuit boards, high thermal conductivity aluminum substrates, ceramic substrates, flexible printed circuit boards, metal composite materials, and so on.

Rigid printed circuit boards (PCBs) are mostly used for various electronic substrates, but LED controllers cannot withstand the heat dissipated by high-power LEDs. Therefore, their applications are limited to low-power and general-power LEDs, and there is no possibility of extending their applications to high-power LEDs.

High thermal conductivity aluminum substrate (metal core PCB; mc PCB) is to change the substrate under the PCB to aluminum alloy. Generally speaking, although the heat dissipation coefficient of pure aluminum is higher than that of aluminum alloy, it is difficult to use due to the low hardness of pure aluminum. Therefore, only aluminum alloy is used as the material for making the substrate.

There are currently three types of ceramic substrates, namely Al2O3 (aluminum oxide), LTCC (low temperature co-fired ceramics), and AlN (aluminum nitride). In terms of technical level alone, aluminum nitride is undoubtedly the highest, followed by low temperature co-fired ceramics. Although LED substrates made of low temperature co-fired ceramics have better heat dissipation, high temperature resistance, and moisture resistance, their prices are several times higher than traditional substrates, so they are still not the ideal material for making heat dissipation substrates to this day. Of course, if the price factor is not considered, then ceramic substrates are still the well-deserved first choice.

Flexible printed circuit board (FPC) has the advantages of light weight, thin thickness, flexibility, flexible application space, etc., and its thermal conductivity is better than that of traditional PCB substrates and MCPCB substrates, and its application area is larger than that of ceramic substrates. However, the technology is still in the experimental stage, and the yield is low, so it cannot be put into large-scale production.

Metal substrate becomes the first choice for high power LED

Through the above comparison, metal substrates have the highest cost-effectiveness, so they have become the preferred solution for high-power LEDs. Moreover, with the development of large-scale, high-current, and high-power LED chips, metal packaging substrates will replace traditional resin packaging substrates at an increasingly faster pace.

As far as the materials of high heat dissipation metal substrates are concerned, they can be divided into two types: hard and flexible. In terms of the structure of LED controllers, hard substrates belong to traditional metal materials, using metals such as aluminum and copper, and the insulation layer is mostly filled with inorganic substances with high thermal conductivity. This metal substrate has the advantages of high thermal conductivity, high heat resistance, and electromagnetic shielding. Its thickness is usually greater than 1 mm, so it is widely used in LED lamp modules and lighting modules, which greatly contributes to the promotion and popularization of high-power LEDs in street lamps.

Flexible substrates have great potential

Generally speaking, the thermal conductivity of metal packaging substrates is about 2w/m•k. However, since high-efficiency LEDs have higher heat dissipation requirements, in order to meet the thermal conductivity of 4~6w/m•k, metal packaging substrates with thermal conductivity exceeding 8w/m•k have appeared. Since hard metal packaging substrates are mainly used to meet the needs of high-power LED packaging, various packaging substrate manufacturers are actively developing technologies that can improve thermal conductivity efficiency.

However, metal packaging substrates also have the disadvantage of a large metal thermal expansion coefficient. When the LED controller is welded with a ceramic chip with a low thermal expansion coefficient, it is susceptible to thermal cycle shock. Therefore, when aluminum nitride is used for packaging, the metal packaging substrate may become incompatible. Therefore, it is necessary to overcome the thermal stress differences between various materials with different thermal expansion coefficients in the LED and improve the reliability of the packaging substrate.

The emergence of flexible substrates has solved the above problems. High thermal conductivity flexible substrates are made by pasting metal foil on the insulating layer. Although the basic structure is exactly the same as that of traditional flexible substrates, the LED controller uses soft epoxy resin filled with high thermal conductivity inorganic substances in the insulating layer, so it has a high thermal conductivity of 8w/m?k, while also having the advantages of softness, flexibility and high reliability. In addition, flexible substrates can also be designed according to customer needs, from single-sided single-layer boards to single-sided double-layer boards and double-sided double-layer boards. According to experimental results, the use of high thermal conductivity flexible substrates can reduce the temperature of LEDs by about 100 degrees Celsius, which can greatly increase the service life of LEDs.