LED lighting application trends and heat dissipation problems Due to the continuous advancement of solid state lighting technology, LED luminous efficiency has been improved in recent years, and it has gradually replaced traditional light sources. At present, luminous efficiency has been chasing incandescent lamps and halogen lamps and continues to grow. As shown in Figure 1. Some companies have developed LED components with efficiency exceeding 100lm/W, which makes LED lighting applications more and more widely used. It has not only been applied to indoor and outdoor lighting, mobile phone backlight modules and car direction lights, etc. Applications such as high-wattage projection lights and street lights, large-size backlight modules, and automotive headlights. Due to the advantages of power saving, environmental protection and long life, the trend of LED light source in the future is becoming more and more obvious.
Figure 1 LED luminous efficiency trend comparison
In order to make the LED emit brighter light, it needs to input higher power. However, the current photoelectric conversion efficiency (WPE) value of the high-power LED is still limited, generally only about 15~25% of the input power becomes Light, the other is converted into heat. Since the LED chip area is small (~1mm2), the heat generation (heating density) per unit area of ​​the high-power LED is very high, even more serious than the general IC component, and the junction temperature of the LED chip is also (Junction Temperature). Greatly improved, it is easy to cause overheating problems. Excessively high junction temperature of the wafer will reduce the luminance of the LED, with the red light being most pronounced. It will also cause the wavelength shift of the LED to affect the color rendering, which will cause the LED reliability to be greatly reduced. As shown in Figure 2, the heat dissipation technology has become the bottleneck of the current LED technology development.
Figure 2 Relationship between component lifetime and wafer temperature
Therefore, the design of the thermal design has great challenges. From the wafer level, the package level, the PCB level to the system module level, the heat dissipation design must be highly valued and the best heat dissipation method should be sought. For LED lighting products, the heat dissipation requirements of other levels are more obvious due to the greater heat dissipation limitations at the system side.
For the LED heat transfer problem, the most basic analysis method is to use the thermal resistance network for analysis. That is, the LED is constructed from a heat dissipation path from the heat source of the wafer to the ambient temperature, as shown in FIG. 3, and then the characteristics and magnitudes of the respective thermal resistance values ​​are analyzed, so that the wafer temperature in the ideal condition can be estimated and targeted to heat. Resist the various parts of the network to reduce the thermal resistance. It should be noted that Figure 3 is a thermal resistance network composed of Chip Level, Package Level, Board Level and System Level. In the actual analysis, a more detailed thermal resistance network can be formed according to the system structure, for example, considering the thermal resistance of the interface materials such as Die Attach material and Solder, or the thermal resistance value of the heat dissipation module structure.
Figure 3 LED heat dissipation path and thermal resistance network
Thermal design of Chip Level, Package Level and PCB Level
Due to the poor thermal conductivity of the Sapphire substrate of the LED chip, the thermal resistance value Rjs of Figure 3 is too high. Therefore, the improvement method must replace the Sapphire with a highly thermally conductive material such as copper, or use a flip chip to remove the substrate from the heat transfer path. To reduce the thermal resistance value.
Current heat dissipation designs with better wafer-to-package level performance, including co-fused gold substrates and flip-chip designs, make heat easier to transfer from the wafer to the package. Increasing the size of the wafer to reduce the heat density is also a viable direction. In the package heat dissipation design technology, the heat sink of high thermal conductivity metal (Al, Cu..) is used, as shown in Figure 4, and the design of high thermal conductivity ceramic substrate (AlN, SiC...) can quickly heat the wafer. Diffusion effectively reduces the package thermal resistance value Rsc. In the heat dissipation design of the PCB level, the difference from the traditional PCB is mainly due to the high heat density of the LED. The heat dissipation capability of the traditional FR4+ copper foil layer is limited. Therefore, the thicker metal layer is required to reduce the diffusion heat resistance (Spreading Resistance). ), this structure is called MCPCB (Metal Core PCB).
The basic structure of the MCPCB is shown in Figure 5, including a thicker metal layer, a dielectric layer, and a copper foil layer. The heat of the package can be further diffused and quickly transmitted to the heat dissipating components of the system module to reduce the thermal resistance value Rcb.
Figure 4 High power LED package structure and Heat Slug structure
Figure 5 MCPCB structure diagram
In order to reduce the thermal resistance of the components, some designs currently use the Chip-on-board design to directly design the LED chips on the MCPCB, and reduce the thermal resistance of the packaging materials and the Solder interface materials, thus improving the heat dissipation effect. This design is also used in products (Lamina Inc., Citizen Inc., OSRAM Inc, Avago Technologies...). However, such a design increases the difficulty of optical design and causes process reliability problems, and is complicated in design.