Presently around 4 per cent of all power modules in use are found in automotive applications. Over the next few years, this market is expected to grow by an impressive 20 per cent per year. The application possibilities are vast, and inverters for hybrid and electric drives can already be found in lorries, buses and agricultural vehicles, as well as in automotive and racing car applications. As different as the requirements may be in the different areas of application, the main focus in all cases is to develop reliable packaging technology for the power modules. The most prevalent packaging solutions today are soldered modules with and without base plate, and, most recently, modules with no base plate in sinter technology. These packaging technologies have different advantages and disadvantages, which is why the service life design calls for an evaluation of these technologies with regard to the requirements for hybrid and electric vehicle applications. Changing ambient temperature, for example in the cooling water cycle, are responsible for passive thermal cycles. In addition, the power loss that occurs in the power semiconductors produces brief (5-20 s) temperature lifts of ?T = 40° C to 60° C. Here, the power semiconductors are heated from, for instance, the cooling water temperature of 70° C to over 110° C - 130° C, after which they drop back to the cooling water temperature. Owing to the different coefficients of thermal expansion of the materials used every temperature change that occurs results in mechanical stress. This causes material fatigue in the solder and bond connections and, ultimately, component failure.
Avoiding solder connections
In modules with no base plate featuring pressure contact technology, several paths are pursued to boost module reliability. By consistently avoiding solder connections, solder fatigue – a key failure mechanism in power modules – can be completely eliminated. Here, the solder connections between chips and insulating DBC ceramic substrate are replaced by a highly stable sinter layer and conducting connections in contact pressure technology. The removal of the base plate has a number of benefits: first of all, the thickness of the thermal paste layer between module and heat sink can be reduced. Thermal paste is one of the main factors contributing to the total thermal resistance in the power module; this is why as thin a layer of thermal paste as possible should be used. In modules with base plate, a thermal paste layer of 75-150 µm in thickness is needed to compensate for base plate bending. In modules with no base plate, the main problem that has to be dealt with is how to compensate for the surface roughness of the heat sink and DBC surface, which is why a 20-30 µm thick thermal paste layer is sufficient. The removal of the base plate means the removal of one of the main causes of thermal stress. In Figure 1, temperature-induced stress is effectively reduced and reliability thus significantly increased, as accelerated passive thermal shock tests 40° C / 125° C show: in the case of sintered modules with no base plate, the number of possible thermal shocks was increased by a factor of 15. A further advantage of the removal of solder interconnects and base plate is that, in modules with base plate the soldered DBC areas should be reduced to a minimum in order to reduce material fatigue in the solder joints; here, the high thermal conductivity of the base plate ensures the necessary thermal spreading. When designing a module with no base plate, in contrast, the DBC area can be larger. Figure 2 reflects when the inverter is in operation, conduction and switching losses occur, meaning that the power semiconductors act as local heat sources. With the help of 3D finite element calculations, thermal spread in an inverter module and heat sink for any given operating state can be calculated. Figure 3 shows why in the thermal image, the IGBT positions are seen as strong heat sources. In the case of modules with base plate, the heat is concentrated in the centre of the 3-phase configuration. Owing to the close positioning of the semiconductors and the short distance between the phases, the temperature of the IGBTs is highest at this point. Although in this operating state the freewheeling diodes are subjected to moderate loading only, the IGBTs cause the diodes in the centre of the module to heat up considerably. At the edges of the inverter module, the temperature of the diodes, by way of contrast, is 15° C lower. Despite base plate, the power semiconductors in the edge regions of the inverter module become far less hot than in the module centre, which ultimately leads to non-homogenous heat distribution to the three phases: the mean thermal load on the IGBTs in the centre phase is almost 10° C higher than the mean temperature of the IGBTs of the external (outer, at the module edges?) phase. The difference between the maximum and minimum IGBT temperature is more than 20° C. The centre phase limits the useable electric power in the entire inverter module. This has two consequences: on the one hand, the cooling conditions and the load have to be selected such that the temperatures in the centre DBC do not become too high; on the other hand, temperature-induced damage mechanisms have a stronger effect on the centre phase. This means the design engineer of the power circuitry for the inverter should always factor in the temperature of the centre phase.
Temperature and service life
For actual thermal loading on an inverter in operation, time-dependent loads must be taken into consideration. During the actual running of a hybrid or electric vehicle, various load states occur: during vehicle acceleration the IGBTs are under a particularly high load, while during deceleration where energy recovery takes place and the battery of the electric motor is re-charged, it is the freewheeling diodes that are under the greatest load. To describe the time-dependent heating of the inverter module, the behaviour of the power module has to be investigated for load cycles in the 0.1 s – 30 s region, too. The time-dependent thermal resistance of IGBTs increases for both configurations in line with the duration of the load impulses (Figure 4). The heat begins to flow, spreading from the power semiconductors in the direction of the heat sink, causing the entire module to heat up. If the load impulses last longer than 30 s, the module will fully heat up and the thermal resistance will cease to increase. Figure 4 shows that the time-dependent thermal resistance values can now be used to calculate the thermal load acting on the semiconductor switches and valves during operation. To do this, realistic load cycles, as would occur in actual application, are used to simulate typical load states and load impulse durations. Figure 5 shows the maximum temperature rise of the IGBTs is ?T = 40° C. In terms of module service life, this is equivalent to 6 million load cycles. Just how important homogenous temperature distribution is for inverter service life and design can be seen if one looks at a temperature rise of just 10°C more - ?T = 50°C – where the number of possible load cycles is 3 times lower at just 2 million cycles. All in all, sintered modules with no base plate offer a series of possibilities for boosting the reliability of inverters in hybrid and electric vehicles. The disadvantages of solder connections and expansion caused by the base plate are eliminated. The optimised layout ensures a largely homogenous temperature distribution across the power semiconductors during operation. This means that in the service life expectation calculations all 3 phases may be considered in equal terms, facilitating inverter design. The reliability of the inverter, even under considerable active and passive temperature swings, is clearly improved.
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