New fields of high power inverter systems such as hybrid cars, hybrid trucks, and off road vehicles require new ways of power electronics integration and packaging. The stringent requirements in size and weight, reliability, durability, ambient temperature, and environment can be fulfilled by a careful consideration of IGBT and Diode chip selection, novel packaging technologies and consequent integration of passive components. In this paper the authors will discuss how the environmental conditions of automotive applications drive silicon power device selection and packaging technologies. Extreme cooling conditions require a package design that needs to work on the thermal and electrical limits of the components without making any compromise in reliability and durability. A high power Integrated Inverter Module (IIM) for 600V, 150kVA, integrating all necessary system components in a single package, has been realized. [1].
I. Environmental Requirements
The environmental requirements for electric drive systems in hybrid or electric vehicles are very demanding in terms of ambient temperatures and temperature cycling capabilities. In general each manufacturer has its own requirements for drives at different positions in the vehicle. Figure 1 gives an overview of the most common conditions. In single loop cooling circuits the water temperature can go as high as 105°C for regular operation and up to 120°C with power derating. Therefore the maximum ambient air rating for the power electronic components is 125°C.
II. Lifetime, and Temperature Cycling
When talking about the lifetime of automobiles, one must distinguish between the operational and the non operational life. While the non operational life is 15 years the operational life is only about 10.000h, corresponding to a mileage of about 300.000km with an average of 30km/h. The operational life is generally split in several phases, e.g.: Coolant temperature < 90°C for 95% of the active life Coolant temperature > 90°C for 5% of the active life Assuming a lifetime of about 15 years and two cold starts per day, meaning the coolant is heated up twice from 5°C to 105°C and cooled down again, all components involved have to sustain 10.000 passive temperature-cycles with a shift of 100K over their product life.
Figure 1. Environmental requirements of automotive drive systems
On top of the passive temperature cycles comes the active power cycling stress of the power semiconductors and their interface materials like bond wires, substrates, solder, etc. Depending on the vehicles mission profile each acceleration and braking cycle is transferred in a temperature change. Approximately 3.000.000 power cycles with 40K shift has to be assumed, depending on the vehicles mission profile.
These harsh conditions have to be considered when designing an integrated power module for the automotive environment.
III. Thermal considerations and construction principle of power modules
Power semiconductor modules are characterized by the separation of the paths for current and for heat. Different materials – insulators, conductors and of course semiconductors – have to be connected together. Because of the differences in the Coefficient of Thermal Expansion (CTE) stress is created between the joined materials during temperature and power cycling. This stress is proportional to the differences in the CTE of the materials, the length of the rigid connection and the temperature excursion ΔT. Changes in temperature give changes in the mechanical strain and with it fatigue of the connection between the parts which limits the life time of the device. A construction of the module has to be chosen that is adequate to the thermal and mechanical stress demand in the application. Conventional modules have a massive base plate, normally made of 3 mm copper (figure 2).
Figure 2. Cut through a conventional power module with base plate (Housing not Shown)
Figure 3. Coefficient of Thermal Expansion (CTE) of the main Module Materials
As can be seen in figure 3 the CTEs of the used materials are quite different. Most critical is the difference in the CTE of copper (base plate) and DBC substrate, not only because of the great amount of the difference but also because of the large area solder connection between DBC and base plate. This joint is most critical at passive thermal cycling. The failure mechanism is solder fatigue which will cause an increase of thermal resistance and early module failure. This failure mechanism is well known and for the given temperature cycling requirement of 100K temperature swing a failure can be expected already after less than 2000 cycles. [2], [3] To improve this reliability problem it is common for heavy duty traction modules to replace the copper with an AlSiC base plate which matches the CTE of the DBC substrate and therefore reduces the stress on the large area solder joint. Because of its high price and the relative poor thermal performance AlSiC baseplates are not an option for the cost sensitive and high power density requirements of the automotive applications.
An alternative construction principle for power modules is the SKiiP (SEMIKRON integrated intelligent Power) technology. Here no base plate is used. This eliminates the large area solder connections totally and replaces them by a pressure contact (figure 4). The large area joint between DBC substrate and heat sink is not rigid and the substrate has the ability to “move” on the heat sink.
Figure 4. Power module without Base Plate in SKiiP technology
The terminals are pressed to the DBC and at the same time the DBC is pressed to the heat sink. Because pressure is applied to the DBC at many locations (also near to the power semiconductor chips) a very intimate, reliable thermal contact to the heat sink is provided.
The solder connection between silicon chip and DBC substrate is less critical to thermal cycling because of the relative small CTE mismatch. This joint is common for any module design. Here the high active power cycling stress can cause failures and a careful selection of DBC material and silicon chip size is important in order to keep the mechanical stress low. [4]
Figure 5. Principle of heat spreading in a module
An important issue in thermal considerations is heat spreading (figure 5). By materials with high thermal conductivity the heat conducting area is enlarged and so the thermal resistance lowered. One effect of heat spreading is that the chip temperature is much higher in the middle of the die area than at the edges (figure 6). The consequence is a higher temperature excursion during power cycling in the chip center which influences the reliability of the connection chip to substrate.
Figure 6. Temperature distribution over an IGBT chip 12 mm x 12 mm under typical working conditions
The bigger the die, the higher the temperature difference between middle and edge of the die. To reduce this negative effect on the power cycling capability and to reduce at the same time the thermal resistance between the dies and the substrate significantly the single chip can be replaced by paralleled smaller power semiconductors.
Figure 7. Simulation of the chip temperature in a module without base plate (SKiiP-technology). Heat sink temperature 20°C. Power loss density in the chip(s): 2 W/mm². Total chip area 144 mm²: a) 1 chip 12x12 mm², b) and c) 4 Chips 6x6 mm²
Figure 7 shows how the maximum silicon temperature can be lowered by using 4 small chips instead of 1 big. The total chip area and the current density are the same in all cases. Figure 8 demonstrates the reduction of chip temperatures by replacing a single die 12 x 12 mm² by 4 dies 6 x 6 mm² in dependence on the distance between the dies. This is not only true for modules without base plate but also for standard modules. It may be surprising at the first moment that base plate modules have higher chip temperatures although the copper base plate should assist heat spreading.
Figure 8. Chip temperature for modules with and without base plate for single die and paralleled die configuration (total die area 144 mm², losses 2 W/mm²)
This can be explained by the fact that the rigid base plate is never flat over the whole operating temperature range (bimetal effect between base plate and DBC substrate) and is only pressed to the heat sink at the edge of the module by screws (see figure 2). To circumvent voids in the thermal paste between heat sink and base plate a minimum thermal paste thickness of 50 μm is necessary. The SKiiP construction on the other hand (figure 4) has a flexible DBC base which is pressed to the heat sink at many positions. A thermal paste thickness of only 15 μm applied by an automated silk screen process is sufficient, which improves the over all thermal resistance significantly.
IV System Components
Similar reliability considerations need to be addressed for all other drive system components like controller PCB, current sensors, DC-Link capacitors, packaging and sealing. Since large DC-Link capacitors are not common in the automotive environment a detailed stress analysis is necessary. Because of the high ambient temperatures, the need for high ripple currents and the limited space high temperature Polypropylene (PP) foil capacitors were selected. Another positive feature of the foil capacitor is its self healing effect and the fail open mode which becomes important for vehicle safety considerations. The relative high CTE index of the foil capacitor (143*10-6 K-1) which is about 8 times higher than the copper busbars makes it again important to have a flexible interface to withstand the temperature cycling requirements. Figure 9 shows the displacement and stress simulation in the connections of the DC-Link PP foil capacitor and the copper busbars that are soldered to it. In order to reduce the total change of length, the capacitor was already divided in two equal parts. With a ΔT = 70 K there is already a displacement of up to 1mm which will cause solder fatigue. In order to keep the stress within acceptable limits it is necessary to split the capacitor into several smaller units in parallel connection.
Figure 9. Displacement magnitudes of capacitors and busbar in μm with ΔT = 70 K.
V. Electrical Performance
Since the DC-Link voltage of a 600V IGBT module can go as high as 450V during regenerative braking it is important to reduce the parasitic inductance of the package to a few nH. The simulation of the current densities and parasitic inductance during commutation has been the key for the layout of the DBC substrate and the DC bus bars, allowing high switching speed without creating critical over voltage spikes. In figure 10 the simplified model for simulating the current densities in the DBC substrate, the chips and the DC link during commutation from the top diode to the bottom IGBTs is displayed. In this configuration a parasitic inductance of only 4nH has been achieved.
Figure 10. Current density during commutation when the bottom IGBT is turned on
Figure 11 shows the principle of multiple access to the DBC substrates for the Semikron 600V SKAI module. The IGBTs and free-wheeling diodes for the top and the bottom switch are positioned on one substrate; two IGBT chips are sharing always symmetrical one diode which allows high switching speeds without high over voltages.
Figure 11. Low inductive DBC layout with multiple busbar connections
An additional benefit of this design is the equal current distribution between the paralleled power switches. The multiple busbar connections are pressure contacts providing next to the low inductive connection also an excellent thermal contact of the dies to the heat sink.
VI. Summary
The Semikron SKiiP pressure contact technology meets and exceeds the reliability, low cost and power density requirements of Automotive applications. Since the requirements are pushing the limits of traditional power module designs mechanical, thermal and electrical simulation tools are becoming more and more important. As a result of these fundamental investigations an Integrated Inverter Module (IIM) without basplate has been realised utilising pressure contacts and high temperature PP foil capacitors. The 600V, 150kW Semikron SKAI 4001GD06 module (figure 12) has reached the weight requirements of 20kW/kg and exceeded a power density of 20kW/liter. All standard reliability tests have been passed; end of life testing is still ongoing.
Figure 12. Semikron SKAI 600V IIM
Size: 400mm x 215mm x 100mm
Volume: 6,8 liter
Weight: 8 kg
Reference List
[1] W. Tursky, P. Beckedahl: “Advanced Drive Systems”, Proc. IEEE PESC/CIPS 2004, pp. 4499-4502
[2] G. Lefranc, T. Licht, H.J. Schultz, R. Beinert, G.Mitic: Reliability testing of high-power multi chip IGBT modules, Microelectronics Reliability 40 (2000), 1659-1663
[3] J.J. Mikkelsen: Failure Analysis on Direct Bonded Copper Substartes after Thermal Cycle in Different Mounting Conditions, Proc. PCIM Nuremberg 2001, pp. 467-472
[4] U. Scheuermann, U. Hecht: Power Cycling Lifetime of Advanced Power Modules for Different Temperature Swings, Proc. PCIM Nuremberg 2002, pp. 59-64
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