Failure Modes and Mechanisms of Press-fit and Solder-type IGBTs

The failure rate is the most important evaluation criterion for reliability, so studying the failure mode and mechanism of IGBT has a guiding effect on improving the reliability of IGBT.

   The difference in the packaging form of the crimped IGBT device and the soldered IGBT module ultimately leads to the difference in the failure mode and failure mechanism of the two IGBT devices, as shown in Table 1. This paper analyzes the main failure modes and failure mechanisms of IGBT devices in two different packaging forms.

Table 1 Comparison of failure modes

   1. Welded IGBT module
   The performance of the packaging material is the basis for determining the performance of the module. In particular, the reliability of the packaging material has a very important impact on the reliability of the module. The most important indicator is the coefficient of thermal expansion, followed by electrical conductance, heat capacity and thermal conductivity. The difference in thermal expansion coefficient of materials is often the root cause of module failure.

   IGBT will produce temperature fluctuations under different conditions, and the difference in thermal expansion coefficient of materials will lead to different thermal stresses, which will affect the interior of the device. Therefore, the difference in thermal expansion coefficient of adjacent interface materials should be as small as possible. The thermal expansion coefficient (α) of commonly used materials for soldered IGBT module packaging is shown in Figure 1.

Figure 1 Thermal expansion coefficients of different packaging materials

   1.1 Bonding wire falls off
   Among the failure modes of soldered IGBT modules, the loss of bonding wires is the most likely to occur. Some data show that the loss of leads can account for about 70% of IGBT module failures. As shown in Figure 2, the bonding wire is generally an aluminum lead. After the lead wire is subjected to repeated thermal stress for a long time to a certain extent, an arc flashover occurs when the current flows rapidly, which will cause the bonding wire to peel off. Solder craters are generated on the interface of the contact parts, and solder residue can be detected on the chip.

Figure 2 Bonding wire detachment and wire detachment area

   As shown in Figure 3, in fact, before the welding wire is detached, due to the effect of power cycle, the shear stress is continuously applied to the interface, which will cause cracks in the solder layer due to material fatigue, crack growth and even delamination, voids or bubbles, and Eventually lead to fall off.

Figure 3 Schematic diagram of lead wire falling off

   Improving the welding process, such as using ultrasonic bonding technology and using copper wire bonding technology can significantly improve the adhesion quality of the lead. Utilizing silver sintering technology and coating polyimide on the bonding wire will also achieve good power cycle capability and improve the life of the bonding wire and solder layer to a certain extent.

   1.2 Welding layer fatigue
   Solder layer fatigue is also a common failure mode of soldered IGBT modules. The so-called solder fatigue is due to the fracture or delamination of the solder layer and the contact surface, which increases the thermal resistance of the device and accelerates the failure of the device as a whole, as shown in Figure 4. Figure 5 shows the temperature distribution on the surface of the 1200V/150A IGBT chip during operation. The temperature gradient difference on the diagonal of the chip reaches 40°C. The direct cause of the degradation of the solder interface is the high stress caused by the difference in thermal expansion coefficient. Fractures at the solder interface increase the local thermal resistance in the corresponding chip area, resulting in a local increase in chip temperature.

Figure 4 Schematic diagram of failure of solder layer

Figure 5 Infrared image of 1200V IGBT chip (@150A)

   If the fracture starts from the edge, the temperature of the relatively cool chip area increases, while the highest temperature in the center of the chip remains unchanged. When the fracture starts from the center maximum temperature, the chip center maximum temperature will increase rapidly. This positive feedback loop accelerates the fatigue progress of the entire interface solder layer, thereby reducing the lifetime of the power module.

   1.3 Metallization reconstruction
   Soldered IGBT modules experience a grainy structure in the metallized aluminum layer after repeated temperature fluctuations. At junction temperatures above 110°C, the cyclic stress during the heating phase of the temperature cycle causes the particles to exceed their elastic strain limit, resulting in plastic deformation. This can be well detected using a scanning acoustic microscope (SAM), and Figure 6 shows the effect of various temperatures during power cycling. Figure 6(a) shows 3200000 power cycles, the IGBT surface metallization image at 85-125 °C; Figure 6(b) shows 7250 power cycles, power cycle temperature difference ΔT=131K, θhigh=171 IGBT surface metallization image at ℃; Figure 6(c) shows the surface metallization image of the diode after 16,800 power cycles at 40-200°C.