The reliability of a power device refers to the ability of the device to complete the specified function under specified conditions, usually expressed in terms of service life. Since semiconductor devices are mainly used to realize the switching of current, which will generate large power loss, the thermal management of power electronic systems has become the top priority in the design. In the working process of power electronic devices, the first thing to deal with is thermal problems, which include steady-state temperature, temperature cycling, temperature gradient, and matching of packaging materials at operating temperature.
Because the IGBT adopts the stacked packaging technology, this technology not only improves the packaging density, but also shortens the interconnection length of the wires between the chips, thereby improving the operating speed of the device. However, because of this structure, the reliability of the IGBT has been questioned. It is not difficult to imagine that the failures at the packaging level of the IGBT module mainly occur at the connection of the bonding wire, the chip welding place, the substrate welding place and the substrate.
During a typical power cycle or temperature cycle, the chip, solder layer, substrate, backplane and package will experience different levels of temperature and temperature gradients. The thermal expansion coefficient is an important performance index of the material, which refers to the ratio of the increase of the line size to its length at 0 degrees for every 1 degree increase in temperature within a certain temperature range. Figure 1-2 shows the thermal expansion coefficients of commonly used materials in the IGBT stack structure. Due to the different thermal expansion coefficients of the respective materials, the thermal strains between different materials are different when the temperature changes, and the bonding between the interconnecting layers will cause fatigue loss due to thermal stress. . Therefore, the thermal behavior of the device is closely related to the structure of the module package. The survey shows that the failure rate caused by temperature doubles for every 10 ℃ increase in operating temperature.
1. Detachment of aluminum bonding wires
The diameter of the aluminum bonding wires in the IGBT is usually 300-500um, and their chemical composition varies from manufacturer to manufacturer. However, in almost all cases, pure aluminum is added with a thousandth of an alloy such as silicon-magnesium or silicon-nickel Alloy, the hardness of aluminum will be greatly improved and the corrosion resistance can be controlled. Due to the disproportion to length and a slight dependence on the temperature of the substrate, the current capacity of the bond wire will be reduced. The maximum DC current is limited by the melting caused by the ohmic heating effect of the wire itself. Since the aluminum bonding wire is directly connected to the chip or the pressure buffer, it will withstand large temperature changes, and the IGBT module is composed of materials with different thermal expansion coefficients. During operation, there will inevitably be obvious thermal fatigue. This kind of fatigue With the passage of working time, the ohmic effect of the wire itself becomes more and more obvious, and eventually cracks are generated at the root of the bonding wire.
2. Reconstruction of aluminum wire
In the thermal cycle test, the mismatch of thermal expansion coefficient will cause the periodic squeezing and pulling effect of the bonding surface, which is far beyond the expansion and contraction range of the material itself. In this case, the pressure will be released in different ways, such as diffusion creep, particle sliding, dislocation and so on. The reshaping of aluminum can lead to a decrease in the effective area of the contact surface, resulting in an increase in electrical resistance. This also explains why Vce also increases linearly with periodic testing.
3. Solder fatigue and solder voids
The cracks in the solder layer between the chip and the substrate due to the different thermal expansion coefficients will increase the contact resistance of the wires, and the increase in resistance will lead to the enhancement of the ohmic effect, so the positive feedback of temperature will make the cracks more and more intense, and eventually lead to device failure. invalid. The voids in the solder layer will affect the temperature thermal cycle, the heat dissipation performance of the device will be reduced, which will also promote the temperature rise, thereby accelerating the damage of the module. In addition, there is a hysteresis phenomenon between stress and strain. During the continuous temperature cycle, the shape of the material changes in real time, which increases the thermal fatigue of the solder. In addition, the voids introduced in the solder due to process problems will affect the thermal cycle during the working process, resulting in excessive local temperature, which is also an important cause of module failure.
4. Wafer and ceramic cracks
In the seven-layer structure of IGBT, the mismatch of thermal expansion coefficient will bring very large mechanical stress to each layer. In the case of temperature difference, the deformation of each layer of material is different, and different parts of the same layer of material will also have different degrees of deformation due to the difference in temperature distribution, so there will inevitably be a problem of excessive local stress. lead to cracking of the material.
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