Introduction Since <br> <br> long display application has been the primary focus of the LED, the LED for the heat requirements of the situation is not very high, the resin using a conventional multi-LED package substrate.
After 2000, with the development of LED high-luminance and high-efficiency technology, coupled with the significant improvement of blue LED luminous efficiency, LED manufacturing costs continue to decline, so that the scope of LED applications and the willingness to adopt LED industry will continue to expand. Including LCD, home appliances, automotive and other companies, they are also actively considering the possibility of applying LEDs. For example, consumers expect high-power LEDs to achieve power saving, high brightness, long life and high color reproducibility. This represents the ability to achieve high heat dissipation and is an indispensable condition for high-power LED package substrates.
In addition, the LCD panel industry faces the EU RoHS regulations, and it is necessary to face the environmental pressure of completely non-silvering of cold cathode lamps, which has made the market demand for high-power LEDs more urgent.
In addition to protecting the internal LED chip , the LED package also has the functions of electrical connection and heat dissipation between the LED chip and the outside.
The characteristics of epoxy resin are not in line with the demand of high-power LED. One LED can reach several hundred lumens. This is basically not a big problem. The main problem is how to deal with heat dissipation. How to maintain brightness after generating such a large lumen? The stability and sustainability, this is another important issue. If the heat treatment is not done well, the brightness and life of the LED will drop rapidly. For LEDs, how to achieve effective reliability and heat conduction is very important.
In the past, LEDs were packaged using low thermal conductivity resins, but this is considered to be one of the causes of heat dissipation characteristics. In addition, the heat resistance of epoxy resins is relatively poor, and it may happen that the life of the LED chips themselves is not Before the arrival, the epoxy resin has been discolored, so improving heat dissipation is an important key.
In addition, not only the thermal phenomenon will change the epoxy resin, but even the short wavelength will cause problems for the epoxy resin. This is because the white LED light spectrum also contains short-wavelength light, while the epoxy resin is equivalent. It is easily damaged by short-wavelength light in white LEDs. Even low-power white LEDs can make epoxy resin damage more serious. Moreover, high-power white LEDs emit more short-wavelength light, which is more natural than low-power models. Fast, even some products have a service life of only 5,000 hours or even shorter after continuous lighting! Therefore, instead of constantly overcoming the discoloration caused by the old packaging material "epoxy", it is better to seek a new generation of packaging. Material efforts.
Figure 1: Epoxy resin has poor heat resistance, and the epoxy resin has changed color before the life of the LED chip itself reaches.
The metal substrate into a new focus of recent years, therefore <br> <br> gradual conversion of high thermal conductivity ceramic, metal or resin package, in order to solve the heat dissipation, and efforts to strengthen the existing properties do. Common methods for high-power LED chips are: large-scale chips, improved luminous efficiency, high-efficiency light-emitting packages, and high current. In this type of practice, although the amount of current illuminance will increase proportionally, the amount of heat will increase.
For high-power LED packaging technology, due to the problem of heat dissipation, it has caused a certain degree of trouble. In this context, the cost-effective metal substrate technology has become another new concern after the high efficiency of LED. development of.
In the past, due to the small LED output power, the use of traditional FR4 and other glass epoxy resin packaging substrates does not cause too much heat dissipation, but the high-power LED used in lighting has a luminous efficiency of about 20% to 30%. Left and right, although the chip area is quite small, the overall consumption  The power is not high, but the heat per unit area is very large.
In general, the heat dissipation of the resin substrate can only support LEDs of 0.5 W or less, and LEDs of more than 0.5 W are often packaged with metal or ceramic high heat dissipation substrates. The main reason is that the heat dissipation of the substrate directly affects the LED lifetime. Performance, so the package substrate has become the focus of development of high-brightness LED products.
Figure 2: LED chips are mostly made up of large-scale chips, improved luminous efficiency, high light extraction efficiency, and high current to achieve high brightness targets.
Substituted <br> <br> resin material of high power LED package on a metal substrate to accelerate thermal design board, can be divided into the current, the thermal conductivity of the LED chip to the package, the package and to convey the two parts of external heat. When a high-heat conductive material is used, the temperature difference inside the package becomes small. At this time, the heat flow does not locally concentrate, and the heat flow generated by the LED chip as a whole flows radially to each corner of the package, so that the high heat conductive material can be used to improve the interior. Thermal diffusivity.
As far as the improvement of heat conduction is concerned, it is almost entirely dependent on material improvement to solve the problem. Most people believe that with the development of large-scale, high-current, and high-power LED chips, metal packaging will be accelerated to replace traditional resin packaging.
As far as the metal high heat dissipation substrate material is concerned, it can be divided into two types of hard and flexible substrates. The structure, the hard substrate belongs to the traditional metal material, the metal LED package substrate is made of aluminum and copper, and the insulating layer is mostly used. Filled with high thermal conductivity inorganic fillers, with high thermal conductivity, processability, electromagnetic shielding, thermal shock resistance and other metal characteristics, thickness is usually greater than 1mm, most of them are widely used in LED luminaire modules, and lighting modules, etc., technically It has the same high thermal conductivity as the aluminum substrate, and is capable of serving as a high-power LED packaging material under high heat dissipation requirements.
Each package substrate company is actively developing flexible substrates
The emergence of flexible substrates, which were originally expected to be applied in the thinning requirements of LCD backlight modules for car navigation, and the high-power LEDs can be produced under the requirements of three-dimensional packaging, and the substantially flexible substrate is made of aluminum. Aluminum has high thermal conductivity and light weight characteristics, and is made into a high-density package substrate. After being thinned by an aluminum substrate, it has flexible properties and can also have high heat conduction characteristics. Generally, the thermal conductivity of a metal package substrate is about 2W/m?K, but due to the higher thermal effect of high-efficiency LEDs, in order to meet the thermal conductivity of 4~6W/m?K, there are metal package substrates with thermal conductivity exceeding 8W/m?K. Since the main purpose of the hard metal package substrate is to satisfy the packaging of high-power LEDs, various package substrate manufacturers are actively developing technologies that can improve thermal conductivity. Although the use of aluminum plate reinforced plates can improve heat dissipation, there are cost and assembly constraints that cannot solve the problem at all.
Figure 3: After being thinned by an aluminum substrate, it has flexible properties and can also have high heat transfer characteristics.
However, the metal package substrate has the disadvantage that the metal has a large thermal expansion coefficient and is easily affected by the thermal cycle when soldered to the low thermal expansion coefficient ceramic chip. Therefore, when the aluminum nitride package is used, the metal package substrate may be uncoordinated. It is necessary to overcome the difference in thermal stress caused by various thermal expansion coefficient materials in the LED and improve the reliability of the package substrate.
The high-heat-conducting flexure substrate is a metal foil adhered to the insulating layer. Although the basic structure is identical to that of the conventional flexural substrate, in the case of the insulating layer, the soft epoxy resin is used to fill the high thermal conductivity inorganic filler, so that it has 8W. /m?K's high thermal conductivity, combined with soft flexibility, high thermal conductivity and high reliability, in addition to flexible substrate can also be designed according to customer needs, single-sided single-layer board can be designed as a single-sided double-layer Double-sided double-layer structure. According to the experimental results, when the high heat conduction deflection substrate is used, the temperature of the LED is reduced by about 100 degrees Celsius, which represents a problem that the temperature of the LED is degraded, which can be greatly improved by changing the substrate design.
In fact, in addition to high-power LEDs, high-heat-conducting flexure substrates can be applied to other high-power semiconductor components for environments with limited space or high-density packaging. However, relying solely on the package substrate often fails to meet the actual needs. Therefore, the mating of the peripheral material of the substrate is also important. For example, a heat conductive film of 3 W/m?K can effectively improve the heat dissipation. (Edit: Technology)
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