**4.2. Thermal stress**

A clamping force is needed to ensure the basic functions, and all the components within PP IGBTs contact well. The silicon chips will produce much heat and increase the temperature under working condition of the PP IGBT. Thus, this high temperature induces thermal stress because all the components are constricted by the clamping fixture, and there is no space to move when they are heated up. The thermal stress during the heating phase will change the clamping force distribution within PP IGBTs to a large extent [20]. The finite element model of the conceptual PP IGBT studied consists of 44 silicon chips (30 IGBT chips and 14 FRD chips), and the chip number is marked as shown in **Figure 10**.

A finite element multi-physics model co-coupled with an electrical field, thermal field, and mechanical field is proposed to predict the clamping force distribution within the PP IGBT. The status of the clamping phase is that the PP IGBT is only clamped by the clamping fixture with a prescribed displacement. And the heating phase is the state where the clamped PP IGBT is heated up caused by the collector current to approximate its working condition. More details for this part can be found in the study [20]. The simulation results are shown in **Figure 11**.

with the contact pressure. The average clamping force of one FRD chip (#2) and three IGBT chips (#3, #8, and #13) located in the axis is extracted (marked with a red block in **Figure 10**)

Clamping Force Distribution within Press Pack IGBTs http://dx.doi.org/10.5772/intechopen.75999 85

**Figure 11.** Contact pressure distributions within PP IGBT: (a) clamping phase and (b) heating phase.

The reason is that the collector electrode presents a warpage when the PP IGBT is heated up, and this leads to an extremely uneven clamping force distribution. However, there also exists a little difference among the silicon chips in the clamping force distribution even in the clamping phase because of the warpage of the collector electrode. Another reason is that the pedestal is too hard to absorb the thermal stress generated by the high temperature of silicon chips. Therefore, a harder collector electrode and a softer pedestal can improve the clamping

As stated before, the PP IGBT has a multilayer structure and all components are stacked. It is impossible to control each component having the same size because of the machining accuracy. Therefore, the difference or error in the size among components is inevitable. Furthermore, the size error existing in each component will also be summed up during the assembling process as the components are stacked. Adding up all factors means that the height of each submodule will not be the same. These existing differences of the height of the submodules will affect the clamping force distribution. The finite element model including 11 IGBT chips

and five FRD chips is shown in **Figure 12**. More details can be found in the study [13].

under different height tolerances as shown in **Figure 14**.

The FRD chip 5 with different height tolerances, ranges from 0 to −3 μm, is selected to predict the clamping force distribution within the studied PP IGBT. The von Mises stress on the surface of the silicon chips (IGBT 4, IGBT 8, FRD2, and FRD5) with different heights is extracted and shown in **Figure 13**. Furthermore, the average clamping force of FRD chip 5 is extracted

The results show that the clamping force of FRD chip 5 decreases sharply while the height tolerances is increasing. The clamping force decreases to less than 100 N, which is much less than

and compared in **Table 4**.

force distribution within PP IGBT.

**4.3. Machining accuracy**

The contact pressure distribution within the studied PP IGBT is relatively uniform under the clamping phase. However, this situation changes a lot when the PP IGBT is heated up. The pressure distribution is extremely uneven and is mainly concentrated in the center. That is to say, the clamping force distribution will also be uneven, and the change trend will be the same

**Figure 10.** Internal layout and chip numbers of the conceptual PP IGBT.

**Figure 11.** Contact pressure distributions within PP IGBT: (a) clamping phase and (b) heating phase.

with the contact pressure. The average clamping force of one FRD chip (#2) and three IGBT chips (#3, #8, and #13) located in the axis is extracted (marked with a red block in **Figure 10**) and compared in **Table 4**.

The reason is that the collector electrode presents a warpage when the PP IGBT is heated up, and this leads to an extremely uneven clamping force distribution. However, there also exists a little difference among the silicon chips in the clamping force distribution even in the clamping phase because of the warpage of the collector electrode. Another reason is that the pedestal is too hard to absorb the thermal stress generated by the high temperature of silicon chips. Therefore, a harder collector electrode and a softer pedestal can improve the clamping force distribution within PP IGBT.
