Improvement design of heat dissipation and lifespan of high power white light LED

Among many environmentally friendly light source application solutions, LED is a light source technology that is more energy-saving and easy to assemble and design than other light source solutions. Among them, high-power white light LED is the most frequently used light-emitting component in lighting light source applications. However, although white light LED has made research and development progress in terms of luminous efficiency and single-chip power, in fact, white light LED still has problems such as luminous uniformity and packaging material life. In particular, the application limitation of chip heat dissipation is the first problem that must be improved in the development of LED light source applications...

High-power white LEDs are used for daily lighting purposes. As environmentally friendly light sources have become increasingly valued, they have become the primary choice for developing environmentally friendly light sources. However, there are still many technical bottlenecks in white LEDs that need to be overcome. Currently, there are related improvement plans to strengthen the design bottlenecks of white LEDs in terms of luminous uniformity, packaging material life, heat dissipation enhancement, etc., and to improve key functions and performance.

The demand for environmentally friendly light sources increases, and high-power white light LED applications are emerging

The main reasons why LED light sources are favored are long product life, high light-to-electricity conversion efficiency, and material properties that allow them to be embedded in any plane. However, in the development of daily lighting sources, due to the need to meet practical "lighting" needs, LEDs originally used for indication purposes cannot directly correspond to lighting applications. They must be enhanced in terms of chips, packaging, substrates, manufacturing technology, and external circuits to achieve the high-power, high-brightness lighting effects required for lighting purposes.

From the perspective of market demand, white light LEDs developed for the lighting application market can be said to be a product with a higher usage in the future. However, in order to achieve the use effect, white light LEDs must be improved in key functions for lighting applications. One is to strengthen the LED chip, for example, increase its light-to-electricity conversion efficiency, or increase the chip area so that the light output (luminous flux) of a single LED reaches its design limit. The second is a more compromised design solution. If it is difficult to continue to increase the area of ​​a single LED chip, multiple LED chips can be packaged in the same light source module, which is also a practical technical solution that can achieve a method close to the above method.

Multi-chip packaging meets low-cost, high-brightness design requirements

In terms of industry practical needs, due to the flexibility of mass production, design difficulty and control of product yield/cost issues, the continued increase in the size of LED chips will encounter cost and yield design bottlenecks. The design difficulties that may be encountered by blindly increasing the chip area are not technically impossible, but in terms of cost and benefit considerations, large-area LED chips are more expensive, and the design flexibility for changes in actual manufacturing needs is low.

Instead, it uses an integrated packaging method of multiple chips to arrange multiple small LED chips at equal distances on a carrier board, connect the chips through wire bonding, and use optical packaging materials for the overall packaging to form a light source module product. After chip testing, the multi-chip package can be integrated into a light source module with an equivalent large chip through secondary processing, but it is more flexible in manufacturing than single-chip design LED light source components.

At the same time, the production cost of multi-chip LED chip module solutions can be greatly reduced due to the chip cost, which means that while obtaining the same luminous flux as a single-chip design solution, it has a lower-cost development option.

Multi-chip integrated light source modules still need to consider maximizing cost-effectiveness

Another development direction is to continue to increase the area of ​​LED chips, and obtain high brightness and high luminous flux output effects through large areas. However, too large an LED chip area may also lead to problems that are not as expected by the design. The common improvement plan is to modify the structure of the complex crystal and make improvements on the chip surface; but the related improvement plan is also likely to affect the heat dissipation efficiency of the chip itself, especially in LED modules used as light sources, most of which require driving at high power to obtain higher luminous flux. This will cause the high heat collected by the chip interface during the chip's light-emitting process to be difficult to dissipate, affecting the application flexibility of the module product and the active/passive heat dissipation design plan.

In general design solutions, according to analysis, the luminous efficiency is best when the chip size is 7mm2. However, it is more difficult to control the yield and light performance of the 7mm2 large chip, and the cost is relatively high. On the contrary, using multiple chips, such as 4 or 8 low-power chips, and performing secondary processing on the carrier board with packaging materials to form an LED light source module is a design solution that can more quickly develop LED light source module products with the required brightness and power performance.

For example, Philips, OSRAM, CREE and other light source product manufacturers have launched LED light source module products that integrate 4, 8 or more small LED chip packages. However, this type of high-brightness component solution using multiple LED chip architectures has also caused some design problems. For example, the combination package of multiple LED chips must be equipped with built-in insulation materials to prevent short circuits between individual LED chips. This process has many more steps than the single-chip design. Therefore, even if it can save costs compared to the single-chip solution, the cost gap between the two solutions will be narrowed due to the additional insulation material process.

Improvements in chip surface processing can also enhance LED light output

In addition to increasing the chip area or number, which is the most direct method, there is also another way to improve the luminous efficiency of the chip's own material properties. For example, an uneven surface structure can be made on the LED sapphire substrate, and this irregular surface design can be used to enhance the LED light output, that is, to establish a textured surface crystal structure on the chip surface.

OSRAM has used this solution to develop Thin GaN high-brightness products. It first forms a metal film material on the InGaN layer and then performs a lift-off process, so that the surface after lift-off can indirectly obtain a higher light output! OSRAM claims that this technology can allow the same chip to achieve 75% light extraction efficiency.

On the other hand, Japan's OMRON's development thinking is quite different. It is also committed to squeezing out the light extraction efficiency of the chip. OMRON attempts to use planar light source technology and a LENS optical system to reflect, guide and control the chip light source. In response to the common light loss problem of LED products with traditional cannon-shaped packaging structures, it further improves its design structure and uses the double-layer reflection effect to control and enhance the light extraction of the LED. However, this packaging technology is relatively more complex and costly, so it is mostly only used in LCD TV backlight module design.

LED lighting applications still need to improve component light decay and life issues

If we expect LED light sources to be introduced into daily lighting applications, there will be more problems to overcome! Because daily lighting sources are used for a long time, they are often used for several hours or even dozens of hours in a row. If the LED is turned on for a long time, the high heat of the components will cause the chip's luminous decay and shorten its life. The components must have better solutions for heat treatment to slow down the premature occurrence of light decay problems and affect the product experience.

Another major problem with the introduction of LED light sources into daily applications is that, for example, traditional fluorescent lamps can maintain the same luminous efficiency after being used for more than dozens of hours, but LEDs are different. This is because the luminous efficiency of LED light-emitting chips will decrease due to the high heat of the components, and this problem is true for both high-power and low-power LEDs. However, low-power LEDs are mostly used for indicative purposes and have little impact on users. However, if LEDs are used as light sources, the problem of decreasing light output will become more severe when the driving power of a single component is increased to increase brightness. Generally, the brightness will decrease after a few hours of use, and heat dissipation design must be improved to meet the light source application requirements.

LED packaging materials need to be improved to cope with high temperature and short wavelength light

In the design of light sources, the driving current is often increased in exchange for a higher light output of the LED chip, but this will cause the heat generated on the chip surface during the light-emitting process to continue to increase. The high temperature of the chip tests the durability of the packaging material. Continuous operation at high temperatures will cause the packaging material with high thermal durability to deteriorate, and material degradation or quality changes will further cause a decrease in light transmittance. Therefore, when developing LED light source modules, it is also necessary to consider using high heat-resistant materials for packaging materials.

There are many ways to increase the heat dissipation of LED light source module components. Improvements can be made from the chip, packaging materials, module heat conduction structure, PCB substrate design, etc. For example, if the heat dissipation conduction speed can be enhanced between the chip and the packaging material, it is also a method to quickly dissipate the core heat source through the surface of the packaging material. Or through the contact between the chip and the substrate, the high heat of the chip core can be directly dissipated through the direct conduction heat source of the material to the substrate to dissipate, and the high heat of the LED chip can be improved. In addition, the PCB adopts a metal material combination and a close assembly design with the LED chip, which can also reduce the thermal resistance of heat conduction and achieve the design goal of quickly dissipating the high heat of the core of the light-emitting component.

In terms of packaging materials, most LED components were previously packaged with epoxy resin. In fact, the heat resistance of epoxy resin itself is not high. Often, before the LED chip reaches the end of its service life, the epoxy resin will deteriorate, deteriorate, and change color due to long-term high-temperature operation. In the design of LED modules for lighting applications, this situation will accelerate the degradation of the packaging material due to the high-power drive of the chip, and even affect the safety of the component.

It's not just a heat problem. Plastic materials like epoxy resin are highly sensitive to light, especially short-wavelength light, which can damage the epoxy resin material. High-power LED light source modules have more short-wavelength light, which can accelerate the deterioration of the material.

Regarding the design of LED light source applications, most companies tend to abandon epoxy resin packaging materials and use packaging materials that are more resistant to high temperatures and short wavelength light. For example, silicone resin has higher heat resistance than epoxy resin. In terms of material properties, silicone resin has the advantage of not changing color even in an environment of 150~180°C.

In addition, silicone resin can also disperse blue light and ultraviolet rays. Silicone resin can inhibit the degradation of packaging materials due to high heat or short-wavelength light, and slow down the decline in light transmittance due to deterioration of packaging materials. As for LED light source modules, silicone resin also has the advantage of extending the service life of LED components, because silicone resin itself has the advantages of high heat resistance and short-wavelength light resistance. The packaging material can resist the continuous high heat and light exposure generated by long-term use of LEDs. The life of the material is relatively long, and the service life of LED components can also exceed 40,000 hours.