High-power package substrates increase LED application range

Display applications have long been the main appeal of LED light-emitting components, and LEDs are not required to have high heat dissipation. Therefore, LEDs are mostly packaged directly on general resin-based substrates. However, with the development of high-luminance and high efficiency of LEDs since 2000, especially The luminous efficiency of blue LED components has been greatly improved, and liquid crystal, home appliances, automotive and other companies have begun to actively review the applicability of LEDs.

Nowadays, the rapid popularity of digital home appliances and flat-panel displays, coupled with the continuous decline in the cost of LEDs, has led to the expanding range of LED applications and the willingness to adopt LEDs. Among them, LCD panel manufacturers are facing the guidance of hazardous substances issued by the European Union. (RoHS: Restriction of Hazardous Substances Directive), and the development policy of mercury-free cold cathode fluorescent lamp (CCFL) has been proposed in the future. As a result, the demand for high-power LEDs is more urgent.

Technically, high-power LED packaged products are very difficult to use when dissipating heat. In this context, products with high cost efficiency and high heat dissipation package substrates such as metal-based substrates are becoming more and more efficient. A high-profile focus.

Epoxy resin does not meet high power requirements

In the past, LEDs have low output power and can be packaged with glass epoxy such as FR4. However, the luminous efficiency of high-power LEDs for illumination is only 20% to 30%, and the chip area is very small, although the overall power consumption is very low, The heat per unit area is very large.

Automotive, lighting and general people's livelihood operators have begun to actively review the applicability of LEDs. The characteristics expected by high-power LEDs are power saving, high brightness, long life and high color reproducibility, which means high heat dissipation is high. The power LED package substrate is indispensable.

Most of the heat dissipation limits of resin substrates only support LEDs of 0.5 W or less. Most LED packages of more than 0.5 W are replaced with metal-based and ceramic-based high-heat-dissipating 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 package substrate is a very important component in designing high-luminance LED products.

The metal-based high-heat-dissipating substrate is divided into two types: a rigid and a flexible substrate. The hard-based substrate belongs to a conventional metal substrate, and the thickness of the metal substrate is usually greater than 1 mm, which is widely used in LED lamp modules and The lighting module is technically the same grade of high thermal conductivity as the aluminum substrate, and is expected to be used in high power LED packages in the future.

The flexible substrate is used to meet the requirements of thinning of a medium-sized LCD backlight module such as a car navigation system and the requirement of a high-power LED three-dimensional package, and the flexibility of the package substrate can be imparted through the thinning of the aluminum substrate. Further, a high-power LED package substrate having high thermal conductivity and flexibility is formed.


Caption: Epoxy resin has poor heat resistance. Before the end of the life of the LED chip itself, the epoxy resin has already changed color.

High-efficiency metal substrates are receiving attention

The hard metal-based package substrate is a new-generation high-power LED package substrate by using a conventional resin substrate or a ceramic substrate to impart high thermal conductivity, processability, electromagnetic shielding properties, and thermal shock resistance.

The high-power LED package substrate is obtained by adhering a copper foil to the surface of a metal substrate by an epoxy resin-based adhesive, and changing the combination of the metal substrate and the insulating layer material to form an LED package substrate for various uses.

High heat dissipation is an essential feature of a substrate for high-power LED packaging. Therefore, materials such as aluminum and copper are used for the metal-based LED package substrate, and a high thermal conductivity inorganic filler (Filler) epoxy resin is often used as the insulating layer. The aluminum substrate is made of high thermal conductivity and lightweight properties of aluminum to form a high-density package substrate. It has been applied to converters for air conditioners and power supply boards for communication equipment, and is also suitable for high-power LED packages. .

In general, the equivalent thermal conductivity standard of metal package substrates is about 2W/m•K. To meet the needs of customers with 4~6W/m•K high power, the company has introduced equivalent and thermal conductivity exceeding 8W/m• K metal-based package substrate. Since the main purpose of the hard metal-based package substrate is to support high-power LED packages, various package substrate manufacturers are actively developing technologies that can improve thermal conductivity.

The main feature of the hard metal-based package substrate is high heat dissipation. The high thermal conductivity insulating layer package substrate can greatly reduce the temperature of the LED wafer. In addition, the heat dissipation design of the substrate is combined with the package substrate through the heat dissipation film, and the service life of the LED chip is also expected to be extended.

The disadvantage of the metal-based package substrate is that the metal thermal expansion coefficient of the substrate is very large, similar to that of the low thermal expansion coefficient ceramic wafer component, and is susceptible to thermal cycling. If the high-power LED package uses aluminum nitride, the metal-based package substrate There may be problems of inconsistency, so it is necessary to absorb the thermal stress caused by the difference in thermal expansion coefficient of each material of the LED module, thereby mitigating thermal stress and improving the reliability of the package substrate.


Caption: LED chips are mostly made up of large wafers, improved luminous efficiency, high light extraction efficiency, and high current to achieve high brightness.

Package substrate manufacturers actively develop flexible substrates

Most of the main uses of the flexible substrate are concentrated on the wiring substrate. In the past, high-power transistors and high-heat-emitting elements such as ICs have hardly used flexible substrates. In recent years, liquid crystal displays have been highly demanded to meet high luminance requirements. Flexural substrates can be used to set high-power LEDs at high density. However, LED heat generation causes LED life to be reduced, but it has become a very difficult technical issue. Although aluminum plate reinforcement plates can improve heat dissipation, there are cost and assembly limitations. Can't solve the problem at all.

The high heat conduction flexure substrate adheres to the metal foil in the insulating layer, although the basic structure is exactly the same as the traditional flexural substrate, but the insulating layer is filled with a soft epoxy resin filled with a material having a high thermal conductivity inorganic filler, and has a hard metal system. The package substrate has the same thermal conductivity of 8W/m•K, and has soft flexibility, high heat transfer characteristics and high reliability. In addition, the flexible substrate can also be designed as a single-sided double-layer, double-sided double-layer structure according to customer requirements.

The main feature of the high heat conduction flexure substrate is that high heat generating components can be disposed and assembled in three dimensions, that is, the free bending property can be exerted, thereby achieving high assembly space utilization.

According to the experimental results, when the high heat conduction deflection substrate is used, the temperature of the LED is lowered by about 100 C, which means that the temperature reduction caused by the temperature of the LED is expected to be improved. In fact, in addition to high-power LEDs, high-heat-conducting flexure substrates can be equipped with 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 LED modules similar to illumination, the package substrate alone cannot meet the actual needs. Therefore, the matching of the materials around the substrate becomes very important. For example, the thermal conductive film of 3W/m•K can effectively improve the LED mode. The heat dissipation and assembly workability of the group.


Caption: After being thinned by an aluminum substrate, it achieves flexible properties and can have high heat transfer characteristics.

Ceramic package substrates are very beneficial for thermal skew

As described above, the heat generation of the white LED continues to increase as the intensity of the input power increases, and the temperature rise of the LED chip causes a decrease in the light output. Therefore, it is important to review the LED package structure and the materials used. In the past, LEDs have been packaged in low thermal conductivity resins and are considered to be one of the causes of heat dissipation characteristics. Therefore, in recent years, high thermal conductivity ceramics or resin packaging structures with metal plates have been gradually used. Common methods for high-power LED chip include: large-scale LED chip, improved LED chip luminous efficiency, high light-efficiency efficiency package, and high current.

Although the amount of current illuminating increases proportionally, the amount of heat generated by the LED chip increases. Because the illuminance in the high input field is saturated and attenuated, this phenomenon is mainly caused by the heat generated by the LED chip. Therefore, when the LED chip is high in power, the heat dissipation problem must first be solved.

In addition to protecting the internal LED chip, the LED package also has the functions of electrically connecting the LED chip to the outside and dissipating heat. LED packaging requires that the light generated by the LED chip can be taken to the outside with high efficiency. Therefore, the package must have high strength, high insulation, high thermal conductivity and high reflectivity. It is surprising that ceramics almost all of the above characteristics, in addition to ceramic resistance Thermal and light deterioration resistance are also superior to resins.

The traditional high heat dissipation package is to place the LED chip on the metal substrate and then coat the resin. However, the metal thermal expansion coefficient of this package is quite different from that of the LED chip. When the temperature changes greatly or the package operation is not easy, heat is generated. Skewed, which in turn causes wafer defects or reduced luminous efficiency.

In the future, when LED chips are facing large-scale development, the thermal skew problem is bound to become a problem that cannot be ignored. For the above problems, ceramics with a thermal expansion coefficient close to the LED chip can be said to be a very favorable material for the thermal skew countermeasure.

High power accelerated resin resin material

The ceramic material for LED packaging is divided into aluminum oxide and aluminum nitride. The thermal conductivity of aluminum oxide is 55 times that of epoxy resin, and aluminum nitride is 400 times that of epoxy resin. Therefore, most of the substrates for high-power LED packaging use thermal conductivity. It is 200W/mK aluminum or a copper metal package substrate with a thermal conductivity of 400W/mK.

As the bonding agent of the semiconductor IC wafer, an epoxy-based bonding agent, a material such as glass, solder, or a gold eutectic alloy is used. In addition to the above-described high thermal conductivity, the bonding agent for LED wafers requires low-temperature bonding and low Young's modulus, etc., depending on the viewpoint of lowering thermal stress at the time of bonding, and the bonding agent satisfying these conditions is an epoxy-based cement filling silver, respectively. Epoxy resin, Au-20%Sn with gold eutectic alloy system.

Since the coating area of ​​the bonding agent is almost the same as the area of ​​the LED wafer, thermal diffusion in the horizontal direction cannot be expected, and high thermal conductivity in the vertical direction can only be expected. According to the results of the simulation analysis, the temperature difference of the LED joint portion is shown, and the Au-Sn excellent in thermal conductivity is superior to the low heat dissipation silver-filled epoxy resin cement.

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

The improvement of heat conduction depends almost entirely on the evolution of materials. It is generally believed that with the development of large-scale, high-current, and high-power LED chips, metal and ceramic packages will be replaced in the future to replace traditional resin packaging methods. In addition, LED wafer bonding is a hindrance to heat dissipation. One of the reasons, therefore, the thin bonding technology has become a problem for improvement in the future.

In order to improve the heat transfer characteristics of LED high-heat discharge to the outside, cooling fans and heat exchangers have been used in the past. Due to noise and installation space, etc., consumers and lighting manufacturers do not want to use the above-mentioned mandatory heat dissipation. Component, which means that the non-forced heat dissipation design must greatly increase the area of ​​contact between the frame and the outside, and at the same time improve the heat dissipation of the package substrate and the frame.

Specific measures such as: coating the surface of the high heat conductive copper layer with a far-infrared radiation-promoting heat-dissipating heat-dissipating film, and confirming the heat-dissipating effect of the heat-generating body using the flexible heat-dissipating film, almost the same as the cooling fan having an area close to the heat-dissipating film. If the flexural heat-dissipating film is adhered to the package substrate or the frame, or the application layer is directly applied to the package substrate or the frame, the heat dissipation can be theoretically improved.

Regarding the package structure of high-power LEDs, micro-wiring technology capable of supporting epitaxial bonding of LED chips is required; regarding the development of materials, although aluminum nitride has high thermal conductivity, the interaction between high heat conduction and reflectivity has become another Tough problems, it is generally believed that if the thermal conductivity of aluminum nitride can be improved in the future, it will be positive for the packaging materials of high-power LEDs.

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