The IGBT module is mainly composed of a number of hybrid IGBT chips, which are electrically connected through aluminum wires. In a standard IGBT package, a single IGBT is also combined with a freewheeling diode, and then a large amount of silicone gel is poured on the chip, and finally packaged with a plastic case. The IGBT unit stack structure is shown in Figure 1-1.
From top to bottom, it consists of three parts: chip, DBC (Directed Bonding Copper) and metal heat sink (usually copper). DBC is composed of three layers of materials, the upper and lower layers are metal layers, and the middle layer is an insulating ceramic layer. Compared with ceramic substrates, DBC has better performance: it has a lighter weight, better thermal conductivity, and better reliability.
IGBT package failure mechanismThe reliability of a power device refers to the ability of the device to perform a specified function under specified conditions, and is usually expressed in terms of service life. Since semiconductor devices are mainly used to switch currents and will produce large power losses, the thermal management of power electronic systems has become a top priority in the design. In the working process of power electronic devices, the first thing to deal with is the thermal problem, which includes the steady-state temperature, temperature cycle, temperature gradient, and the matching problem of the packaging material at the working 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 increasing the operating speed of the device. But precisely because of the adoption of this structure, the reliability of the IGBT has been questioned. It is not difficult to imagine that the failure of the IGBT module packaging level mainly occurs at the junction of the bonding wire, the chip welding place, the substrate welding place and the substrate.
In the usual power cycle or temperature cycle, the chip, solder layer, substrate, base plate and package shell will experience different levels of temperature and temperature gradients. Coefficient of Thermal Expansion (CTE, Coefficient of Thermal Expansion) is an important performance index of materials, which refers to the ratio of the increase in line size to its length at 0 degrees for every 1 degree increase in temperature within a certain temperature range. Figure 1-2 is the thermal expansion coefficient of commonly used materials in 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 joints between interconnected 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. Investigations have shown that for every 10°C increase in operating temperature, the failure rate caused by temperature doubles.
Figure 1-3 shows the points where the IGBT module is prone to fatigue loss during the working process, specifically:
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, adding one-thousandth of an alloy to pure aluminum, such as silicon-magnesium or silicon-nickel alloys, will greatly increase the hardness of aluminum and control its corrosion resistance. Due to the disproportion to the length and the slight dependence on the temperature of the substrate, the current capacity of the bonding wire will decrease. 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 pressure buffer, it will withstand large temperature changes, and the IGBT module is composed of materials with different thermal expansion coefficients, and there will inevitably be significant thermal fatigue during operation. This kind of fatigue will become more and more obvious as the working time goes by, the ohmic effect of the wire itself will become more and more obvious, and finally cracks will be produced at the root of the bonding wire.
In the thermal cycle test, the mismatch of the thermal expansion coefficient will cause the bonding surface to periodically squeeze and pull up, and this effect 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 peristalsis, particle sliding, dislocation and other forms. The reshaping of aluminum will lead to a reduction in the effective area of ​​the contact surface, which leads to an increase in sheet resistance. This also explains why Vce also shows a linear increase trend with periodic testing.
Solder fatigue and solder voidsCracks in the solder layer between the chip and the substrate due to the difference in thermal expansion coefficient will increase the contact resistance of the wire, and the increase in resistance will lead to the enhancement of the ohmic effect, so the positive temperature feedback will make the cracks more intense, and ultimately lead to the device’s failure. Invalidate. The voids in the solder layer will affect the temperature thermal cycle, and 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. In the continuous temperature cycle, the shape of the material changes in real time, which in turn 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 reason for the failure of the module.
Wafer and ceramic cracksIn the seven-layer structure of the IGBT, the mismatch of the thermal expansion coefficient will bring a very large mechanical
stress. In the case of temperature differences, the deformation of each layer of material is different, and different parts of the same layer of material will also cause different degrees of deformation due to the difference in temperature distribution, so that there is inevitably the problem of excessive local stress, thus Lead to cracking of the material.
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