Package cooling problem and material solution of power device
1. The cooling problem of Power Semiconductor Devices Packaging
Power semiconductor device is the core component of power electronics system, which is a unit of energy generation, transmission, conversion and control. For the high-power semiconductor device field of 600V and above, the main products are silica-based insulated gate bipolar transistor (Si-IGBT) and SIC-based metal-oxide semiconductor field effect transistor (SiC-MOSFET). With it owns higher switching efficiency, lower power loss and higher operating temperature than Si-IGBT, SiC-MOSFET has begun to replace Si-IGBT in the market of the high-power semiconductor device. SiC-IGBT devices which have high current/voltage are also expected to enter the market in the near future.
At present, the size of power semiconductor chips in the market is generally less than 200 mm2, and the maximum current of it can reach about 300 A. The chip will generate a lot of heat in a limited area during work. IGBT module not only realizes the electrical interconnection between chips, the connection of the moving unit and the external main circuit, but also plays the role of support, protection, cooling and so on. The typical structure of power package module consists of power chip, connecting material, bonding lead, copper-covered ceramic substrate, cooling base plate and so on. Packaging materials have different thermal, electrical and mechanical properties. To insure the entire power module can achieve the optimal electrical, heat dissipation, mechanical and reliability comprehensive performance requirements, it is necessary to consider the selection of packaging materials.
2. Selection of Cooling Materials for Power Device Packaging
2.1 Copper clad ceramic substrate
Copper clad ceramic substrate is a kind of cermet composite substrate formed by directly bonding high conductive oxygen-free copper to the ceramic surface at high temperature. It not only has the characteristics of high thermal conductivity, high electrical insulation, high mechanical strength, low expansion of ceramic, but also has the high conductivity and excellent welding performance of copper metal. It can be etched into various graphic circuits like PCB. Copper clad ceramic substrate is a key material for power module packaging in power electronics field which connecting chips and cooling backplane. The copper clad ceramic substrate must meet the comprehensive requirements of power semiconductor devices for packaging materials: ceramics must have excellent thermal conductivity and withstand voltage characteristics; copper layer has extremely high current carrying capacity; metal and ceramic have high adhesion strength and reliability; copper layer has excellent welding performance-good welding performance between the metallized layer base of the chip and aluminum or copper wire.
The copper clad ceramic substrate has a three-layer structure of conductive layer, insulating layer and conductive layer. The upper and lower metal conductive layers are respectively used for the interconnection of chips and base plates. The heat generated by the device is mainly transmitted to the cooling base plate and shell by the copper clad plate. Since oxygen free copper has high thermal conductivity, the thermal conductivity of ceramic copper clad plate is determined by the performance of ceramic lining plate materials. At present, the ceramic substrates used in CCL mainly include three kind of materials, namely aluminum oxide, aluminum nitride and silicon nitride.
Aluminum oxide is the most widely used copper clad substrate material because of its good insulation, chemical stability, good mechanical properties and low price than other materials. However, the application of alumina materials as packaging materials for high-power modules are limited due to it’s low thermal conductivity and low matching coefficient of thermal expansion with silicon. With high thermal conductivity and thermal expansion coefficient matching with silicon and silicon carbide materials, aluminum nitride has replaced aluminum oxide copper clad substrate in the application of larger power modules.
Both aluminum oxide and aluminum nitride have relatively low bending strength and fracture toughness. All of these leads to ceramic cracking because of thermal stress fatigue during thermal cycle after copper layer welding and finally affect the reliability of the entire power module. Silicon nitride ceramics have high thermal conductivity of 100W/(m·K), high strength, low coefficient of thermal expansion, low high temperature creep, good oxidation resistance and thermal corrosion resistance. It is the substrate material with the best comprehensive performance than other materials at present. The test results show that the silicon nitride copper clad substrate can withstand more than 5000 cycles at - 40~150 ℃, which is much higher than that of aluminum oxide 300 times and aluminum nitride 200 times. The fracture toughness and bending strength of silicon nitride are more than twice that of aluminum nitride. The silicon nitride substrate can be used with a thickness of 0.3mm, and can withstand a thicker copper clad layer than aluminum oxide and aluminum nitride. However, aluminum nitride substrate can only be used with a thickness of more than 0.5mm due to its low mechanical strength. The silicon nitride with a thickness of 0.32mm can have a copper coating thickness of 0.5mm. Its thermal resistance is equivalent to the aluminum nitride substrate with a substrate thickness of 0.635mm and a copper coating thickness of 0.3mm, more ever its reliability has improved by an order of magnitude.
2.2 Bonding of chip to copper-coated substrate
The traditional Sn-Pb soft solders have low shear modulus, high wettability, high ductility, high thermal mechanical properties and reliability, which have been widely used as chip bonding materials. However, Sn-Pb solder does not meet the environmental requirements of RoHS and has been gradually abandoned by the industry in recent years. Another chip bonding material widely used in chips is conductive silver adhesive, which is mainly composed of resin matrix and silver powder. However its low conductivity, low thermal conductivity and maximum operating temperature less than 150 ℃ greatly limit its application in the field of high-power device packaging. At present, solid phase atomic diffusion welding based on sintering of nano silver or micro silver particles is emerging. The process temperature is 200-300 ° C, and the melting point is 961 ° C. It has the advantages of high thermal conductivity, high reliability against temperature cycling and power cycling, and has become the preferred connecting material between high-power devices and copper clad substrates. The nano silver solder has much higher thermal, mechanical and electrical properties than the conductive silver adhesive. The use of low-temperature welding technology can ensure the reliability of the power module at the working temperature higher than 250 ℃.
2.3 Wire bonding materials
Wire bonding materials for the encapsulation and interconnection of high-power device modules are usually coarse aluminum. Aluminum is soft, low stress, low cost, high electrical conductivity. Copper has higher thermal and conductive properties than aluminum. Copper-copper bonding processes are beginning to replace aluminum-copper bonding processes for better electrical connection and heat transfer properties. Studies have shown that over-current capacity of copper wire is increased by more than 40% compared with that of aluminum wire at the same temperature.
2.4 Cooling base plate
The materials of cooling base plate for power devices mainly include Cu, AlSiC and WCu alloys. The silicon material or silicon carbide material in IGBT has low thermal expansion coefficient, and need similar thermal expansion coefficient material cooling substrate to reduce the thermal stress of the interface. Cu has high thermal conductivity, but its thermal expansion coefficient is much different from that of ceramic substrate material. It is easy to produce large thermal mechanical stress in work. Both aluminum and silicon carbide have high thermal conductivity, while silicon carbide has lower thermal expansion coefficient than that of aluminum. The aluminum silicon carbide not only have the high thermal conductivity of silicon carbide and aluminum but also achieve high thermal matching degree of high thermal conductivity.
3. Trend of development
3.1 Development of ceramic substrate materials
Zirconia reinforced aluminum nitride composite ceramics is already on the market. The thermal conductivity and mechanical strength of it are between aluminum nitride and silicon nitride, which is a compromise substrate material alternative solution.
U-MAP tried to use its aluminum nitride whisker (Thermalnite) to strengthen aluminum nitride ceramics. The ceramic lining plate made by U-MAP not only has the same strength and toughness as silicon nitride ceramics, but also has the high thermal conductivity of aluminum nitride. It is expected to become a ceramic substrate material for high power devices packaging.
3.2 Copper clip bonding
Because of the limitation of wire diameter, copper wire bonding will produce large Joule heat loss under high working current, and lead to the rise of device junction temperature. In the copper clip bonding mode, the resistance and thermal resistance decrease at the same time, the joule heat decreases, and the heat dissipation capacity increases significantly due to the large increase of the overcurrent section and the reduction of the conduction distance. Another significant advantage of clip bonding is that the high frequency parasitic impedance of the high frequency switching process is reduced, which improve the switching efficiency of the device.
3.3 Double-sided cooling package structure
The double-sided connection structure is that the chip is symmetrically connected with the upper and lower base plates to realize heat dissipation in both side of the chip. Compared with wire bonding, the heat dissipation efficiency can be improved more than 40%. As its inductance and switching loss are reduced, the chip junction temperature is reduced, which improve the overall efficiency of the device module and extend the life cycle of chips.
