Analysis of the packaging structure of high-power LEDs

长久以来显示应用一直是led发光元件主要诉求，并不要求LED高散热性，因此LED大多直接封装于一般树脂系基板，然而2000年以后随著LED高辉度化与高效率化发展，尤其是蓝光LED元件的发光效率获得大幅改善，液晶、家电、汽车等业者也开始积极检讨LED的适用性。

Today, digital home appliances and flat-panel displays are rapidly becoming popular, and the cost of LED monomers continues to decline, which has led to the continuous expansion of the scope of LED applications and the scope of industries willing to adopt LEDs. Among them, LCD panel manufacturers are facing the Restriction of Hazardous Substances Directive (RoHS) regulations issued by the European Union, and have successively proposed the development policy of making mercury-based cold cathode fluorescent lamps (CCFL) completely mercury-free in the future. As a result, the demand for high-power LEDs has become more urgent.

Technically, it is very difficult to solve the heat dissipation problem of high-power LED packaged products during use. In this context, the development trend of products with high cost efficiency and high heat dissipation packaging substrates such as metal substrates has become another focus of attention after the high efficiency of LEDs.

Epoxy resin no longer meets high power requirements

In the past, the output power of LEDs was relatively small, and traditional glass epoxy resin packaging substrates such as FR4 could be used. However, the luminous efficiency of high-power LEDs for lighting is only 20%~30%, and the chip area is very small. Although the overall power consumption is very low, the heat generated per unit area is very large.

汽车、照明与一般民生业者已经开始积极检讨LED的适用性，业者对高功率LED期待的特性分别是省电、高辉度、长使用寿命、高色彩再现性，这意味著散热性佳是高功率LED封装基板不可欠缺的条件。

The heat dissipation limit of resin substrates mostly only supports LEDs below 0.5W. LED packages above 0.5W mostly use metal and ceramic high-heat dissipation substrates. The main reason is that the heat dissipation of the substrate has a direct impact on the life and performance of the LED. Therefore, the packaging substrate becomes a very important component when designing high-brightness LED product applications.

Metal high heat dissipation substrates are divided into two types: rigid and flexible substrates. Rigid substrates are traditional metal substrates. The thickness of the metal substrate is usually greater than 1mm. They are widely used in LED lamp modules and lighting modules. Technically, they are an extension of the same level of high thermal conductivity as aluminum substrates. In the future, they are expected to be used in high-power LED packaging.

The emergence of flexible substrates is to meet the requirements of thinning medium-sized LCD backlight modules such as car navigation systems and high-power LED three-dimensional packaging. By thinning the aluminum substrate, the packaging substrate is given flexible properties, thereby forming a high-power LED packaging substrate with high thermal conductivity and flexibility.

Caption: Epoxy resin has poor heat resistance and will begin to change color before the life of the LED chip ends.

High-efficiency metal substrates are gaining attention

Hard metal packaging substrates use traditional resin substrates or ceramic substrates to give them metal properties such as high thermal conductivity, processability, electromagnetic wave shielding, and heat shock resistance, forming a new generation of high-power LED packaging substrates.

高功率LED封装基板是利用环氧树脂系接著剂将铜箔黏贴在金属基材的表面，透过金属基材与绝缘层材质的组合变化，制成各种用途的LED封装基板。

High heat dissipation is an indispensable basic characteristic of high-power LED packaging substrates. Therefore, the above-mentioned metal-based LED packaging substrates use materials such as aluminum and copper, and the insulating layer mostly uses epoxy resin with high thermal conductivity inorganic fillers (Filler). Aluminum substrates are high-density packaging substrates made by applying the high thermal conductivity and lightweight characteristics of aluminum. They have been used in the fields of inverters for air conditioners and power substrates for communication equipment, and are also suitable for high-power LED packaging.

Generally speaking, the equivalent thermal conductivity standard of metal package substrates is about 2W/mK. In order to meet customers' needs for high power of 4~6W/mK, the industry has launched metal package substrates with equivalent thermal conductivity exceeding 8W/mK. Since the main purpose of hard metal package substrates is to support high-power LED packaging, various package substrate manufacturers are actively developing technologies that can improve thermal conductivity.

The main feature of the hard metal package substrate is high heat dissipation. The high thermal conductivity insulation layer package substrate can significantly reduce the temperature of the LED chip. In addition, the heat dissipation design of the substrate, through the combination of the heat dissipation film and the package substrate, is expected to extend the service life of the LED chip.

金属系封装基板的缺点是基材的金属热膨胀系数非常大，与低热膨胀系数陶瓷系芯片元件焊接时情形相似，容易受到热循环冲击，如果高功率LED封装使用氮化铝时，金属系封装基板可能会发生不协调的问题，因此必须设法吸收LED模块各材料热膨胀系数差异造成的热应力，藉此缓和热应力进而提高封装基板的可靠性。

Caption: Most LED chips achieve high brightness by making the chip larger, improving luminous efficiency, using high light-collecting efficiency packaging, and increasing current.

Packaging substrate manufacturers are actively developing flexible substrates

The main use of flexible substrates is mostly concentrated in wiring substrates. In the past, high-power transistors and ICs and other high-heat generating components rarely used flexible substrates. In recent years, in order to meet the demand for high brightness, liquid crystal displays have strongly required flexible substrates to be able to set high-power LEDs at high density. However, the heat generated by LEDs reduces the service life of LEDs, which has become a very difficult technical issue. Although the use of aluminum plate reinforcement plates can improve heat dissipation, there are cost and assembly limitations, and the problem cannot be fundamentally solved.

High thermal conductivity flexible substrates have metal foil attached to the insulating layer. Although the basic structure is exactly the same as that of traditional flexible substrates, the insulating layer uses soft epoxy resin filled with high thermal conductivity inorganic fillers. It has the same level of thermal conductivity of 8W/mK as hard metal packaging substrates, and is soft and flexible, has high thermal conductivity and high reliability. In addition, flexible substrates can also be designed to be single-sided single-layer panels into single-sided double-layer or double-sided double-layer structures according to customer needs.

The main feature of the high thermal conductivity flexible substrate is that it can be equipped with high heat generating components and assembled in three dimensions, that is, it can give full play to the free bending characteristics, thereby achieving high assembly space utilization.

According to the experimental results, when using a high thermal conductivity flexible substrate, the temperature of the LED is reduced by about 100°C, which means that the reduction in the service life of the LED caused by temperature is expected to be improved. In fact, in addition to high-power LEDs, high thermal conductivity flexible substrates can also be used to set other high-power semiconductor components, which are suitable for areas such as cramped space or high-density packaging that require high heat dissipation.

Regarding the heat dissipation characteristics of similar lighting LED modules, the packaging substrate alone often cannot meet the actual needs, so the coordination of the substrate's surrounding materials becomes very important. For example, combining a 3W/mK thermal conductive film can effectively improve the heat dissipation and assembly workability of the LED module.

Caption: By thinning the aluminum substrate, it can be flexible and have high thermal conductivity.

Ceramic package substrates are very good for thermal skew

如上所述白光LED的发热随著投入电力强度的增加持续上升，LED芯片的温升会造成光输出降低，因此LED封装结构与使用材料的检讨非常重要。以往LED使用低热传导率树脂封装，被视为影响散热特性的原因之一，因此最近几年逐渐改用高热传导陶瓷，或是设有金属板的树脂封装结构。LED芯片高功率化常用方式分别包括了：LED芯片大型化、改善LED芯片发光效率、采用高取光效率封装，以及大电流化等等。

虽然提高电流发光量会呈比例增加，不过LED芯片的发热量也会随著上升。因为在高输入领域放射照度呈现饱和与衰减现象，这种现象主要是LED芯片发热所造成，因此LED芯片高功率化时，首先必须解决散热问题。

In addition to protecting the internal LED chip, the LED package also has the functions of electrical connection between the LED chip and the outside, heat dissipation, etc. LED packaging requires that the light generated by the LED chip can be efficiently taken to the outside, so the package must have high strength, high insulation, high thermal conductivity and high reflectivity. Surprisingly, ceramics have almost all of the above characteristics. In addition, ceramics are better than resins in heat resistance and light degradation resistance.

Traditional high heat dissipation packaging places the LED chip on a metal substrate and then covers it with resin. However, the thermal expansion coefficient of the metal in this packaging method is quite different from that of the LED chip. When the temperature changes greatly or the packaging operation is not done properly, thermal distortion is very likely to occur, which in turn causes chip defects or reduces luminous efficiency.

As LED chips face the development of larger sizes in the future, thermal distortion is bound to become a problem that cannot be ignored. To address the above problem, ceramics with a thermal expansion coefficient close to that of LED chips can be said to be a very advantageous material for thermal distortion countermeasures.

High power accelerated ceramic resin material

Ceramic materials used for LED packaging are divided into alumina and aluminum nitride. The thermal conductivity of alumina is 55 times that of epoxy resin, while that of aluminum nitride is 400 times that of epoxy resin. Therefore, most of the current high-power LED packaging substrates use aluminum with a thermal conductivity of 200W/mK, or copper metal packaging substrates with a thermal conductivity of 400W/mK.

The bonding agents for semiconductor IC chips use epoxy bonding agents, glass, solder, gold eutectic alloys, etc. In addition to the above-mentioned high thermal conductivity, LED chip bonding agents also require low-temperature bonding and low Young's modulus from the perspective of reducing thermal stress during bonding. The bonding agents that meet these conditions are epoxy resin filled with silver and Au-20%Sn of the gold eutectic alloy system.

The area covered by the adhesive is almost the same as the area of ​​the LED chip, so we cannot expect heat diffusion in the horizontal direction, and can only rely on high thermal conductivity in the vertical direction. According to the simulation analysis results, the temperature difference of the LED joint is better than the low heat dissipation silver-filled epoxy adhesive.

The heat dissipation design of LED package substrate is roughly divided into two aspects: heat conduction from LED chip to frame, and heat transfer from frame to the outside.

热传导的改善几乎完全仰赖材料的进化，一般认为随著LED芯片大型化、大电流化、高功率化的发展，未来会加速金属与陶瓷封装取代传统树脂封装方式，此外LED芯片接合部是妨害散热的原因之一，因此薄接合技术成为今后改善的课题。

提高LED高热排放至外部的热传达特性，以往大多使用冷却风扇与热交换器，由于噪音与设置空间等诸多限制，实际上包含消费者、照明灯具厂商在内，都不希望使用上述强制性散热元件，这意味著非强制散热设计必须大幅增加框体与外部接触的面积，同时提高封装基板与框体的散热性。

Specific countermeasures include: coating the surface of the high thermal conductivity copper layer with a flexible heat dissipation film that uses far-infrared rays to promote heat radiation. Experimental results have confirmed that the heat dissipation effect of the heating element using the flexible heat dissipation film is almost the same as that of a cooling fan with an area close to that of the heat dissipation film. If the flexible heat dissipation film is pasted on the packaging substrate or frame, or the coating layer is directly coated on the packaging substrate or frame, the heat dissipation can theoretically be improved.

Regarding the packaging structure of high-power LEDs, it is required to support the fine wiring technology of LED chip epitaxial bonding; regarding the development of materials, although aluminum nitride has high thermal conductivity, the interactive relationship between high thermal conductivity and reflectivity has become another thorny issue. It is generally believed that if the thermal conductivity of aluminum nitride can be improved in the future, it will have a positive effect on the packaging materials of high-power LEDs.