In view of the heat dissipation problem of high-power LED packaging, people have optimized the thermal system of the device in terms of packaging structure and materials.
The early successful package structure is the AlGalnN power flip chip structure. Since the LED chip is flip-chip connected to the silicon substrate through bumps, the heat does not need to have a poorly conductive sapphire substrate, but is directly conducted to the thermal conductivity. The silicon substrate with a higher rate is then conducted to the metal conduit. Since its active heating zone is closer to the heat sink, the internal heat sink resistance can be reduced.
In a silicon-based flip-chip structure, the thermal resistance is proportional to the thickness of the heat sink. Limited by the mechanical strength and thermal conductivity of the silicon wafer, it is difficult to further reduce the thermal resistance of the internal heat sink by thinning the silicon wafer, which limits the further improvement of its heat transfer performance.
The metal circuit board structure is an effective way to solve the heat dissipation of the LED. This packaging structure uses the good thermal conductivity of metals such as aluminum to package the chip on a PCB with a metal sandwich, or package the chip on a PCB with a few millimeters thick copper electrode, and then package it on a heat sink to solve the problem of heat dissipation. problem. Although the use of this structure can obtain good heat dissipation performance and greatly increase the input power of the LED, the PCB in the sandwich is a poor conductor of heat, which hinders the heat conduction. The research results show that the thermal resistance of the structure reaches 60-70K/W.
The micro-pump structure is also a feasible solution to the heat dissipation problem. A micro-pump system is installed on the radiator. In the packaging system, the water enters the small grooves on the floor of the LED under the action of the micro-pump to absorb heat, and then returns to the small water container to absorb heat through the fan. This micro-pump structure can reduce the external thermal resistance to 0.192K/W, and the cooling effect is very good. However, if the internal interface thermal resistance of this structure is large, its thermal conductivity will be greatly compromised. In addition, the structure is too complicated.
In order to further reduce the thermal resistance of the power LED and improve the thermal conductivity of the system, after determining the appropriate packaging structure, it is necessary to select and optimize substrate materials, adhesive materials and packaging materials. The material for the chip should have low resistivity and high thermal conductivity; a copper or aluminum heat sink can be added to the bottom of the chip, and a semi-encapsulated structure can be used to accelerate heat dissipation.
Paste the material. There are mainly three types of chip paste materials: thermal conductive glue, conductive silver paste and tin paste. Thermally conductive adhesives have poor thermal conductivity. Although they are inexpensive, they are not suitable for power LEDs. Conductive silver paste has good thermal conductivity and high bonding strength, but because it heats up while increasing the brightness, and contains toxic substances such as lead, it does not meet the trend and requirements of lead-free packaging. In contrast, the thermal conductivity and electrical conductivity of conductive tin paste are the best.
Substrate material. The use of alloys such as Cu/Mo plate and Cu/W plate as heat dissipation materials can better solve the problem of electrode lead breakage caused by expansion mismatch between chip material and heat dissipation material, but the production cost of such alloys is too high. Ceramics, aluminum plates and copper plates are the focus of current research and selection as heat dissipation substrate materials. Aluminum-based metal core printed circuit boards MCPCB and composite metal substrates are all available for research and selection. By optimizing the thermal design of the MCPCB or directly binding the ceramic to the metal substrate to form a metal and low-temperature sintered ceramic substrate, a substrate with good thermal conductivity and a small expansion coefficient can be obtained.
Packaging materials. Epoxy resin is currently the main material with excellent electrical insulation and dielectric properties. When selecting, follow the principles of transparency and colorlessness, low impurity content and low viscosity. However, it has the shortcomings of hygroscopicity, easy aging, poor heat resistance, and is easy to change color under high temperature and short-wave light, and has certain toxicity before curing, which will have a certain impact on the life of the LED. At present, some manufacturers have switched to using silicone and ceramics as encapsulating materials instead of epoxy resins to extend the life of LEDs.
In addition to epoxy resin packaging materials, reactive diluents, defoamers, toners, mold release agents, methylhexahydrophthalic anhydride, accelerators, and oxidizers should also be appropriately selected and matched.