With the development of high-luminance and high-efficiency LED technology, coupled with the significant improvement in the luminous efficiency of blue LEDs, and the continued decline in LED manufacturing costs, the range of LED applications and the willingness to adopt LEDs are expanding, including LCDs. Household appliances, automobiles and other companies have also begun to actively consider the possibility of applying LEDs. For example, consumers expect that high-power LEDs can achieve power saving, high brightness, long life and high color reproducibility. Achieving high heat dissipation capability is an indispensable condition for high-power LED package substrates.
In addition, LCD panel companies are facing the EU RoHS regulations, and they need to face the environmental pressure of completely non-silvering of cold cathode lamps, which makes 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.
Epoxy properties are no longer in line with high power LED requirements
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 the stability and continuity of brightness after generating such a large lumen, this is another The important issue is that 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 emits short-wavelength light in the luminescence spectrum, but 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 resin, it is better to seek new packaging materials for the first generation. .
Metal substrate into new focus
Therefore, in recent years, the use of high-heat-conducting ceramics or metal-resin packaging structures has been gradually changed to solve the problem of heat dissipation and strengthening of original features. 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 power consumption 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.
High power accelerated metal substrate replaces resin material
The heat dissipation design of the LED package substrate can be roughly divided into two parts: the heat conduction from the LED chip to the package, and the heat transfer from the package to the outside. 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.
Actively develop flexible substrates
The emergence of flexible substrates was originally expected to be developed for the thinning requirements of LCD backlight modules for car navigation, and high-power LEDs can be produced under the requirements of three-dimensional packaging. Basically, flexible substrates are made of aluminum, which is beneficial for applications. Aluminum has high thermal conductivity and lightweight properties, and is made into a high-density package substrate. After being thinned through an aluminum substrate, it has flexible properties and can also have high heat transfer characteristics.
In general, the thermal conductivity of a metal package substrate is about 2 W/mK, but since the thermal effect of a high-efficiency LED is higher, in order to meet the need of achieving a thermal conductivity of 4 to 6 W/mK, a metal having a thermal conductivity of more than 8 W/mK is currently available. Package substrate. Since the main purpose of the hard metal package substrate is to satisfy the package of high-power LEDs, each package substrate company is actively developing a technology that can improve the 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.
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.
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