0 Preface
Display applications have long been the main appeal of LED lighting components, and LEDs are not required to have high heat dissipation. Therefore, LEDs are mostly packaged directly on conventional 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.
At the same time, the rapid popularization of digital home appliances and flat-panel displays, coupled with the continuous decline in the cost of LEDs, has led to the increasing range of applications for LEDs and the willingness to adopt LEDs. Among them, LCD panel manufacturers are facing the dangers of the European Union. RoHS (Restriction of Hazardous Substances Directive), so it is necessary to develop a mercury-based cold cathode fluorescent lamp (CCFL: Cold Cathode Fluorescent Lamp) in the future. The demand is even more urgent.
Technically, high-power LED packaged products have become a very difficult problem in terms of heat dissipation during use. In this context, they are cost-effective, and similar to the development trend of high-heat-dissipation package substrates such as metal-based substrates, and become another device after LED high efficiency. The focus of attention.
Next, this article will introduce the development trend of the metal-based substrate for LED packaging and the heat-dissipation design technology of the ceramic-based package substrate.
1 development history
Figure 1 shows the evolution of the application field of LEDs. When using high-power LEDs as shown in Figure 2, the heat generated by the LEDs is discharged into the air through the package substrate and the cooling fan.
Figure 1 LED application areas change
Figure 2 The necessity of high heat dissipation substrate
In the past, the output power of LEDs was small, and the substrate can be packaged with a glass epoxy resin such as a conventional 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, but the unit The area of ​​heat is very large.
As mentioned above, automotive, lighting and general livelihood operators have begun to actively review the applicability of LEDs (Figure 3). The characteristics expected by high-power LEDs for general consumer industry are power saving, high brightness, long life and high color reproducibility. This means that high heat dissipation is an indispensable condition for high-power LED package substrates.
Figure 3 Characteristics required for high power LED package substrates
Generally, the heat dissipation limit of the resin substrate only supports LEDs of 0.5 W or less, and LED packages of more than 0.5 W are mostly replaced with metal-based and ceramic-based high-heat-dissipating substrates. The main reason is that the heat dissipation of the substrate directly affects 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 (Fig. 4). The hard substrate is a conventional metal substrate, and the thickness of the metal substrate is usually more than 1 mm. The hard substrate Widely used in LED luminaire modules and lighting modules, it is technically an extension of high thermal conductivity in the same grade as aluminum substrates, and is expected to be applied in high power LED packages in the future.
Figure 4 Structure and characteristics of various metal substrates
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 by thinning the aluminum substrate. A high-power LED package substrate that combines high thermal conductivity and flexibility.
2 Characteristics of hard substrate
Fig. 5 is a basic structure of a hard metal-based package substrate, which utilizes a conventional resin substrate or a ceramic substrate to impart high thermal conductivity, processability, electromagnetic shielding, and thermal shock resistance, and constitutes a new generation of high-power LEDs. Package substrate.
Figure 5 Basic structure of a hard metal package substrate
As shown in the figure, the copper foil is adhered to the surface of the metal substrate by an epoxy resin-based adhesive, and the combination of the metal substrate and the insulating layer material can be changed to form an LED package substrate for various purposes.
High heat dissipation is an indispensable basic feature of high-power LED package substrates. Therefore, the above-mentioned metal-based LED package substrates are made of materials such as aluminum and copper, and the insulation layer is mostly filled with a filler ring filled with a highly thermally conductive inorganic filler (Filler). Oxygen resin.
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, power supply boards for communication equipment, etc. Aluminum substrates are also suitable for high power. LED package.
Fig. 6 is a comparison of the characteristics of various metal-based package substrates. Generally, the equivalent thermal conductivity standard of the metal package substrate is about 2 W/m‧K, which is required to meet the high power of the customer 4~6 W/m‧K. A metal-based package substrate having an equivalent thermal conductivity of more than 8 W/m‧K has been introduced.
Figure 6 Comparison of characteristics of various metal-based package substrates
Figure 7. Heat-dissipating diaphragm analysis of high-power LEDs (secondary element)
Since the main purpose of the hard metal-based package substrate is to support the packaging of high-power LEDs, 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. Figure 7 and Figure 8 show the temperature distribution characteristics of the 2W/m ‧K general package substrate and the 8W/m‧K ultra-high heat conduction package substrate under normal use when the heat output of the LED chip is 1W.
Figure 8 Substrate heat sink diaphragm analysis results
It can be seen from Fig. 8 that the substrate is packaged using a highly thermally conductive insulating layer, and the temperature of the LED chip can be greatly reduced. In addition, the heat dissipation design of the substrate, through the combination of the heat dissipation film and the package substrate, is also expected to extend the service life of the LED chip.
The disadvantage of the metal-based package substrate is that the metal thermal expansion coefficient of the substrate is very large, and the ceramic-based chip assembly with low thermal expansion coefficient is susceptible to thermal cycling when soldered. If the high-power LED package uses aluminum nitride, the metal-based package substrate may be Inconsistent problems occur, 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 alleviating the thermal stress and improving the reliability of the package substrate.
3 characteristics of flexible substrate
Most of the main uses of the flexible substrate are concentrated on the wiring substrate. In the past, high-heating components such as high-power transistors and ICs have hardly used flexible substrates. In recent years, liquid crystal displays have been required to meet high luminance requirements, and are strongly required to be flexible. The substrate can be set at a high density with high-power LEDs. However, the heat generation of the LEDs causes the LEDs to have a reduced service life, which is a very difficult technical issue. Although the aluminum plate reinforcement plate can improve the heat dissipation, there are limitations on cost and assembly. Solve the problem at all.
Figure 9 is a cross-sectional structure of a highly thermally conductive flexure substrate. The metal foil is adhered to the insulating layer. Although the basic structure is identical to the conventional flexural substrate, the insulating layer is filled with a high thermal conductivity inorganic filler. The material has the same thermal conductivity as the hard metal-based package substrate of 8W/m‧K, and also has soft flexibility, high heat transfer characteristics and high reliability (Table 1). In addition, the flexible substrate can also be used. According to customer needs, the single-sided single-layer panel is designed as a single-sided double-layer, double-sided double-layer structure.
Figure 9: Sectional structure of the deflection of a highly thermally conductive substrate
Table 1 Comparison of physical properties of various substrates
The main feature of the high heat conduction flexure substrate is that a high heat generating component can be disposed and assembled in three dimensions, that is, it can exhibit free bending characteristics, thereby achieving high assembly space utilization.
Figure 10 shows the results of heat dissipation experiments when a 1W high-power LED is placed on a high-heat-conducting flexure substrate and a conventional Polyi-mide flexure substrate. The thickness of the polyimide substrate is 25 μm, and the heat dissipation of the substrate is natural convection.
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 problem that the temperature causes the LED life to be reduced 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 confined 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 new structure of the edge-emitting LED backlight module of Figure 11 is matched with ~3W/. The m‧K thermal conductive diaphragm can effectively improve the heat dissipation of the LED module and the assembly workability of the LED module.