**3. External affect factors**

Among the factors that may affect the clamping force distribution within PP IGBTs, we defined the factors from the applications or should pay attention in the applications or the factors outside of the PP IGBTs as the external affect factors. In the application of PP IGBTs, there exist two external factors that may affect the clamping force distribution a lot: external clamping modes and the design of disc spring.

#### **3.1. External clamping modes**

The press pack packaging style for IGBTs is learned from thyristors, IGCT, and so on, thus the clamping fixture to supply the needed clamping force in application is also the same. A force spreader is used to transmit the clamping force to the heatsink and then transfer from the heatsink to the surface of the PP IGBTs as shown in **Figure 6**. The disc spring is also needed to compensate the displacement during the clamping phase, and this parameter will be analyzed in the next part.

The force spreader is quite important in the clamping fixture for the press pack packaging style devices to transmit the uniform clamping force on the devices. Different from those devices with the whole wafer, like thyristors, IGCT, and so on, PP IGBTs contain multi-chips which are connected in parallel to improve the current rating. The uniform of the clamping force on the surface of PP IGBTs will greatly influence the clamping force within PP IGBTs and lower the reliability. Thus, the design of the force spreader is quite important to ensure its basic functions and improve the reliability of the whole system.

According to the basic principle of elastic mechanics, there are two boundary conditions to solve the elasticity problems: force load and displacement load. Force load is the most used boundary condition in the finite element simulations. Most of the studies that focus on the clamping force distribution within PP IGBTs used the force load principle. However, the PP IGBTs clamped by the clamping fixture are restricted by the prescribed displacement. Which boundary condition is suitable for the mechanical analysis of PP IGBTs is unclear. Those two boundary conditions directly applied on the surface of the PP IGBT are compared based on the finite element model mentioned earlier. Nine silicon chips are used as shown in **Figure 7**, and the clamping force of each silicon chip is shown in **Table 1**.

**Figure 5.** Boundary conditions of the mechanical model.

78 Design, Simulation and Construction of Field Effect Transistors

with the exception of the specified models, are set as in **Figure 5**.

**3. External affect factors**

**3.1. External clamping modes**

clamping modes and the design of disc spring.

the boundary conditions are set as follows. In the application of PP IGBTs, a force spreader or a clamp stack is used to transmit the clamping force to the heatsink and then transfer from the heatsink to the surface of the PP IGBTs. A disc spring, usually changing several mm under the clamped phase, is needed to compensate the physical movements, usually several μm, during the process of clamping and thermal expansion. To approximate the working conditions, a prescribed displacement, which is equivalent to the rated clamping force, is applied to the disc spring on the surface of the heatsink of the PP IGBT collector side. A fixed support is placed on the surface of the heatsink of the PP IGBT emitter side. The simplified diagram of the boundary conditions for the mechanical model is shown in **Figure 5**. The mechanical model should also consider the frictional interconnections among the different layers. The interconnections between heatsinks and PP IGBT are set as the bonded interconnection, and the interconnections between multilayers within PP IGBT are set as a frictional contact. A friction coefficient of 0.5 is assumed for the contact layers within the PP IGBT [21, 22], because the friction coefficient has little influence on the pressure distribution [23]. All finite element models used in this chapter to analyze the clamping force distribution within PP IGBTs,

Among the factors that may affect the clamping force distribution within PP IGBTs, we defined the factors from the applications or should pay attention in the applications or the factors outside of the PP IGBTs as the external affect factors. In the application of PP IGBTs, there exist two external factors that may affect the clamping force distribution a lot: external

The press pack packaging style for IGBTs is learned from thyristors, IGCT, and so on, thus the clamping fixture to supply the needed clamping force in application is also the same. A force From the results, we can see that chip 5 located in the center of the PP IGBT has a relatively lower clamping force than other chips with the force load boundary condition, and the error is about −30.32%. As the study [19] shows, the thermal contact resistance existed in the contact interface within PP IGBTs depends on the clamping force to a large extent. Thus, the uniform clamping force distribution will influence the characteristic and reliability of PP IGBTs. However, the clamping force distribution within PP IGBT is relative even with the displacement of load condition that the maximum error is about 1.39%. In the real applications, the PP IGBTs are restricted by the prescribed displacement through clamping fixture. However, it is impossible to ensure a uniform displacement on the surface like the displacement load boundary

**Figure 6.** Simplified schematic diagrams for the clamping system: (a) structure diagram and (b) simulation diagram.

**Figure 7.** Finite element model: (a) chip number and (b) schematic diagram for simulation.


Based on these results, it is shown that the design of the force spreader is quite important that the height of force spreader should be larger than the radius of the PP IGBT. Meanwhile, the clamping mode is very important in the finite element simulation. The best way is to apply the

Clamping Force Distribution within Press Pack IGBTs http://dx.doi.org/10.5772/intechopen.75999 81

The disc spring is another quite important parameter in the application of PP IGBTs because it can not only compensate the displacement during clamping process but also absorb the thermal stress generated by the high temperature. Firstly, the importance of the disc spring is explained by the single IGBT chip submodule as shown in **Figure 9** under different conditions. One is the clamping phase that the submodule is just clamped by the fixture and another is the heating phase that the submodule is heated up with a desired clamping force.

**Figure 9.** Simulation schematic diagram of single IGBT submodule: (a) without spring and (b) with spring.

displacement load on the force spreader.

**Figure 8.** Relationship between the height of force spreader and error of chip 5.

**3.2. Design of disc spring**

condition. Furthermore, the clamping force will increase due to the thermal stress when the PP IGBT is heated up, and the force load condition is not suitable anymore in this situation.

The reason why the clamping force distribution within PP IGBT with force load conditions is worse than the displacement is because the displacement on the surface of PP IGBT is uneven. Therefore, the force spreader is designed to transmit the clamping force to the PP IGBT and ensure that the displacement on the surface is uniform. The clamping force distribution within PP IGBTs is related with the height of the force spreader as shown in **Figure 8** with the results of chip 5.

The radius of the studied PP IGBT is 28 mm, and as seen in **Figure 8**, the error of chip 5 trends to be stable when the height of the force spreader will be higher than 30 mm. That is to say, the displacement on the surface of the PP IGBT is relatively uniform and will not change with a higher force spreader. This is very important in the application. The height of the force spreader should be larger than the radius of the PP IGBT to ensure the clamping force distribution. And the error of chip 5 is still higher than the displacement load condition. However, the displacement load condition on the surface of PP IGBT or heatsink is too ideal.

**Figure 8.** Relationship between the height of force spreader and error of chip 5.

Based on these results, it is shown that the design of the force spreader is quite important that the height of force spreader should be larger than the radius of the PP IGBT. Meanwhile, the clamping mode is very important in the finite element simulation. The best way is to apply the displacement load on the force spreader.

## **3.2. Design of disc spring**

condition. Furthermore, the clamping force will increase due to the thermal stress when the PP IGBT is heated up, and the force load condition is not suitable anymore in this situation.

**Table 1.** Clamping force distribution under different boundary conditions.

**No Rated (N) Force (N) Error (%) Displacement (N) Error (%)** 1000 1074.6 7.46 986.06 −1.39 1000 1001.0 0.10 1010.1 1.01 1000 1073.5 7.35 987.06 −1.29 1000 1001.0 0.10 1009.7 0.97 1000 696.84 −30.32 1011.3 1.13 1000 1000.9 0.09 1010.4 1.04 1000 1076.1 7.61 987.39 −1.26 1000 1002.3 0.23 1011.9 1.19 1000 1073.8 7.38 986.09 −1.39

**Figure 7.** Finite element model: (a) chip number and (b) schematic diagram for simulation.

80 Design, Simulation and Construction of Field Effect Transistors

The reason why the clamping force distribution within PP IGBT with force load conditions is worse than the displacement is because the displacement on the surface of PP IGBT is uneven. Therefore, the force spreader is designed to transmit the clamping force to the PP IGBT and ensure that the displacement on the surface is uniform. The clamping force distribution within PP IGBTs is related with the height of the force spreader as shown in **Figure 8**

The radius of the studied PP IGBT is 28 mm, and as seen in **Figure 8**, the error of chip 5 trends to be stable when the height of the force spreader will be higher than 30 mm. That is to say, the displacement on the surface of the PP IGBT is relatively uniform and will not change with a higher force spreader. This is very important in the application. The height of the force spreader should be larger than the radius of the PP IGBT to ensure the clamping force distribution. And the error of chip 5 is still higher than the displacement load condition. However,

the displacement load condition on the surface of PP IGBT or heatsink is too ideal.

with the results of chip 5.

The disc spring is another quite important parameter in the application of PP IGBTs because it can not only compensate the displacement during clamping process but also absorb the thermal stress generated by the high temperature. Firstly, the importance of the disc spring is explained by the single IGBT chip submodule as shown in **Figure 9** under different conditions. One is the clamping phase that the submodule is just clamped by the fixture and another is the heating phase that the submodule is heated up with a desired clamping force.

**Figure 9.** Simulation schematic diagram of single IGBT submodule: (a) without spring and (b) with spring.

As mentioned earlier, the rated clamping force of the single IGBT submodule is obtained through the displacement on a proper designed force spreader. The calculated clamping force of the submodule is used to compare with different conditions, and it is the real clamping force existing in the submodule after the submodule is clamped by the desired displacement.

**4. Internal affect factors**

**4.1. Slim plates**

ence the layout or the matching of silicon chips.

**No Rated (N) I II III**

**Table 3.** Clamping force distribution with different conditions.

mations because of its good softness.

Those factors that mainly existed in PP IGBTs or during the design process are defined as the internal affect factors. The internal factors that may affect the clamping force distribution should be analyzed and optimized because it is quite important for the structure design of PP IGBTs. There are five parameters that may affect the clamping force distribution within PP IGBTs a lot: slim plates, thermal stress, machining accuracy, internal layout, and the design of electrodes. Among those affect factors, thermal stress is not a structure parameter that can be changed during the structure design. But this factor is quite important because it will influ-

Clamping Force Distribution within Press Pack IGBTs http://dx.doi.org/10.5772/intechopen.75999 83

Until now, there are two main packaging styles for PP IGBTs: press pack and StakPak. The StakPak packaging style contains a spring to distribute the clamping force which is patent protected by ABB and the research is very limited. The press pack is widely used by Poseico, Fuji, Westcode, and Toshiba [20, 24]. All the researches are based on the press pack, and there are also some variations between different manufacturers. As shown in **Figure 3** and stated before, a slim plate of silver is proposed to mechanically support the single chip submodule and compensate few defor-

The contribution of slim plate to the clamping force distribution within PP IGBTs is revealed with three different conditions. As it is known, it is difficult to ensure the same high of all submodules due to the machining accuracy of each component within PP IGBTs or some errors during the assembling process. Condition I is that all the submodules in PP IGBTs have the same height and condition II is that one of those submodules is 0.5 μm lower than others. Condition III is that one of those submodules is 0.5 μm higher than others. The finite element model for this simulation is also shown in **Figure 7** with nine silicon chips, and the height of chip 5 is selected to change. The clamping force distribution within the PP IGBT with different conditions is shown in **Table 3**.

**Without With Without With Without With**

 1000 1016.9 1005.1 1043.79 1044.8 987.27 986.74 1000 1005.5 995.96 1043.4 1045.6 965.52 966.94 1000 1013.3 1000.9 1041.79 1046.3 986.31 987.55 1000 1004.5 976.62 1043.81 1046.1 965.52 965.9 1000 924.14 915.56 659.93 630.19 1200 1194.6 1000 1002.9 990.6 1041.46 1046.5 965.8 966.3 1000 1015.1 999.31 1042.57 1045.8 985.74 987.61 1000 1002.5 983.6 1041.03 1047.1 965.83 966.51 1000 1015.1 1001 1042.18 1047.7 987.52 987.39

Furthermore, the selection or the design of the disc spring is also quite important in the applications. The most important parameter for disc spring is the equivalent elastic coefficient. The higher this value, the disc spring is harder to deform which means higher force is needed to obtain the same deformation. And the lower this value, the disc spring can compensate more displacement with the same clamping force. Therefore, the selection of the equivalent elastic coefficient is a tradeoff problem. The rated clamping force of the submodule is designed to 1 kN and the displacement is 1 mm. Therefore, the rated equivalent elastic coefficient is 1e6 N/m. The submodule under the heating phase with different coefficients, range from 1e6 to 1e7, is also analyzed and the results are shown in **Table 2**.

where "without" means no disc spring is applied in the simulation and the value with a unit of N/m is the equivalent elastic coefficient of the disc spring applied in the simulation. The results show that there is no difference in the calculated clamping force of the single submodule whether a disc spring is applied or not during the clamping phase. And the calculated value is almost equal to the rated clamping force of 1 kN. That is to say, the disc spring has no influence on the clamping phase after the submodule is clamped. The disc spring is used to slow down the change rate of the clamping force during the clamping process but it will not affect the final value after the submodule is clamped. Just like the inductance in the circuit, it is used to restrict the change rate of current.

However, the clamping force will increase sharply due to the thermal stress generated by the high temperature when the silicon chip is heated up. And the value is almost 14 times of the rated clamping force. This high clamping force may mechanically damage the silicon chip. The reason is that the submodule is constricted by the clamping fixture. Thus, a disc spring is needed to compensate the displacement or deformation due to the thermal stress, and the calculated clamping force can be controlled to some extent. The increment of the clamping force is also influenced by the equivalent elastic coefficient. The calculated clamping force will be higher with a higher value of equivalent elastic coefficient. And the increment rate of 2.14, 10.65, and 21.06% is also proportional to the value of equivalent elastic coefficient. The reason is that the allowed displacement of the disc spring is 1, 0.2, and 0.1 mm with the same clamped conditions. Actually, the increment of the clamping force cannot be eliminated if the submodule is clamped even if adequate disc spring is applied. And this will not only increase the cost but also will be very difficult to obtain the desired clamping force. Therefore, the design of the disc spring should consider the requirements from application and it is a tradeoff problem.


**Table 2.** Calculated clamping force of the submodule comparison.
