Internal Connection Design
To achieve an efficient and highly reliable module, a well-designed internal connection system is essential (Figure 2). In the SKiM, the connections were designed with the following design parameters in mind:

- Solderless, low-inductance connections between the power terminals and the chips.
- Ability to handle high currents with minimal losses.
- Symmetrical current paths, resulting in good current distribution among the parallel chips.
- Pressure points positioned close to the chips for low thermal resistance.

The sandwich structure with parallel current paths on each chip ensures very low internal parasitic inductance. The LCE inductance between the DC and AC connection terminals is less than 10 nH, and the total inductance between DC+ and DC- is less than 20 nH. 

A finite element analysis has shown that most of the inductance is due to the distance between the DC+ and DC- terminals. During the design phase, FEM simulations were performed, allowing for design optimization and a reduction in inductance of up to 30% (-10 nH). Further improvement is not possible because the sandwich structure, which generates less inductance, cannot be used here due to the requirement to maintain isolation between DC+ and DC-. The only way to achieve even greater inductance reduction would be through the use of parallel connections to the capacitor bus.

The benefit of this design for users includes, among other things, the low switching surge generated, which allows the module to operate at higher voltage levels and to shut down smoothly, even in the event of a short circuit. This switching without high surges ensures low switching losses and low electromagnetic noise.
Recent developments in semiconductor technology have made it possible to increase power density across the entire package range. The rated current of the chips in the 600 V SKiM 93 modules is 900 A, almost double the rated current of standard modules. This current value also exceeds the limit established due to the current capacity of the power terminals of the IGBT modules. The material used in the SKiM module dies gives the module a total resistance rcc'-ee', including contact resistance, of only 300 µW, which is half the value of conventional modules. On the other hand, the high contact forces guarantee low contact resistance. 

Furthermore, the generated losses are rapidly transmitted across the DBC surface and to the heatsink via the numerous contact points.
When operating as an inverter, the highest currents pass through the AC terminal. For this reason, this terminal is located at the bottom of the sandwich structure, as this point offers the best cooling conditions. The module is designed for effective AC output currents of 600 A at a heatsink temperature of 70 °C. This value exceeds the highest output current for which the chips were designed (see Table 1). All of this ensures that even with power losses of around 2000 W, the terminal temperature remains below 125 °C (see Figure 2).

DCB Layout:
The DCB design and chip placement are extremely important for switching behavior and semiconductor thermal resistance. An asymmetrical design can easily cause a non-homogeneous current distribution of 10% or more between the different chips. If this occurs, the total output current is limited by the component with the highest losses. 

Voltage variations due to parasitic inductances can cause oscillations between different chips in parallel and lead to them switching at different speeds. To ensure synchronous switching, the inductances must be as small as possible, and more importantly, they must have the same influence on all chips. A design based on a structure with two IGBTs on the left and right sides and the freewheeling diode in the center guarantees low inductance and good switching performance. The switched current path between the IGBT and the diode is as short as possible and has the same length for the top part of the switch (TOP) as for the bottom part of the IGBT branch (see Figure 3).
Figure 4 shows the switching behavior of an SKiM 63 module at 600 A and 900 Vdc. The switching losses, surges, and di/dt are virtually identical for the top and bottom IGBTs. This does not occur in all modules, since in many cases there are differences caused by the different parasitic inductances in the current path.

Similarly, to fully utilize the components' capabilities, a good current distribution between the parallel chips in the module is essential. The DC+ and DC- current path impedance must be the same for all chips, and the current's influence on the trigger circuit must affect all chips equally.
The first condition is met by using a sandwich-type board system. The magnetic field remains virtually unchanged when the current switches from DC+ to DC-. The power terminal inductances are coupled and therefore negligible. The impedance is identical across all parallel chips.
The second requirement has also been considered in the design of this module. All IGBTs have the same gate-emitter voltage, even under dynamic conditions. In the IGBT-diode-IGBT configuration, the di/dt-induced voltage drops cancel each other out, meaning that all chips are affected equally by the voltage drop across the bond wires. The result is a very good current distribution, even under extreme short-circuit conditions.

Thermal resistance (Rth):
The new generation of chips, with low forward voltage drop and high junction temperatures (up to 175°C), allows for very high current ratings in these modules. The current density can even exceed 2 A/mm². By selecting the appropriate chip size, an optimal balance between current rating, cooling requirements, and cost can be achieved.

The thermal resistance (Rth) depends on the chip size, but also on the distance between chips (see Figure 5). Large chips exhibit a long temperature gradient across their area, resulting in poor heat conduction within the module. By placing several smaller chips, with the same effective area but slightly separated from each other, an improvement in Rth is achieved. If this separation between chips is small, there is a reheating effect between the chips. On the other hand, if the separation is large, the thermal resistance is lower. The SKiM family achieves the optimal compromise between the highest thermal performance and the maximum effective area, as it features an area of ​​between 60 and 80 mm² and a distance between chips of 3 mm.
Pressure contacts on both sides of the chips prevent the ceramic base from warping. This helps reduce the required thickness of the thermal grease, from 80-100 µm in conventional modules to 20-30 µm in SKiM, improving the thermal resistance of the contact between the module and the heatsink. Furthermore, the excellent heat-conducting properties of the thin sintered silver layer that bonds the chip to the ceramic sole further improve the Rth of the SKiM module compared to conventional modules where the chip is soldered to the ceramic sole.

Reliability:
The traditional solution of using power modules with a soldered copper sole is unsuitable for the demanding thermal cycles found in automotive applications. The different coefficients of thermal expansion of the materials involved generate mechanical stresses at the joints, leading to fatigue failure. A pressure-contact module without a copper sole is the alternative to traditional solutions. Furthermore, the low thermal resistance and homogeneous distribution of hot spots reduce the temperature difference between the different parts of the module, increasing its lifespan. 

To improve resistance to load cycles, even when operating at high temperatures, the SKiM module family uses low-temperature sintering technology to bond the chips to the ceramic baseplate (DCB). If this bond were soldered, as in other industrial modules, aging would occur, leading to increased thermal resistance and, eventually, module failure. The sintered bond is achieved using a thin layer of silver with high thermal conductivity. Silver's melting point is 900 °C, well above the chip's maximum permissible temperature of 175 °C. In tests performed with sintered modules, no fatigue effect was observed (see Figure 6). By eliminating this failure mechanism, the overall system reliability is significantly increased. 

Thanks to the use of pressure and spring contact technology and the elimination of copper soles with soldered chips, SKiM modules eliminate 100% of soldering. Adding to this the fact that the new IGBT chips used can withstand maximum temperatures of up to 175 °C, SKiM modules enable a compact power converter design, as well as unparalleled power density, reliability, and thermal cycling resistance.

Author:

Dr. Arendt Wintrich, Application Manager SEMIKRON

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