introduction
According to the survey, 4% of the power modules currently in use are used in automotive applications. In the next few years, this market is expected to grow by 20% annually. Inverters for hybrid and electric drives can already be seen in trucks, buses and agricultural vehicles, as well as automotive and racing applications. Since different applications have different needs, the main concern in all cases is to develop reliable packaging technologies for power modules. The most common packaging solutions today are soldered modules with and without substrates, and substrate-less modules that have recently adopted sintering technology. These packaging technologies have different advantages and disadvantages, which is why service life design requirements require evaluation of these technologies for hybrid and electric vehicle applications. For example, under the cooling water cycle, the changing ambient temperature is the cause of the passive thermal cycle. In addition, the power loss generated in the power semiconductor produces a short-term (5 ~ 20s) temperature rise of t = 40 ° C ~ 60 ° C. Here, power semiconductors are heated from a cooling water temperature of 70 ° C to over 110 ° C to 130 ° C, after which they fall back to the cooling water temperature. Because the materials used have different coefficients of thermal expansion, every temperature change will cause mechanical stress. This is the cause of material fatigue in soldered and bonded connections, and ultimately leads to component failure.
Avoid welding connections
In a substrateless module that uses crimping technology, there are several ways to improve the reliability of the module. By continuously avoiding solder connections, solder fatigue, the main failure mechanism of this power module, can be completely eliminated. Here, the solder connection between the chip and the insulating dbc ceramic substrate is replaced by a highly stable sintered layer, and the conductive connection is carried out using pressure bonding technology. There are many benefits of removing the substrate: first, the thickness of the thermally conductive coating between the module and the heat sink can be reduced. Thermally conductive coatings are one of the main factors that affect the total thermal resistance in power modules, which is why the thinnest thermally conductive coatings are used. In a module with a substrate, a thermal conductive coating of 75 to 150 μm is needed to compensate for the bending of the substrate. In the module without substrate, the main problem to be dealt with is how to compensate for the roughness of the surface of the heat sink and the dbc ceramic substrate, which is why a 20 to 30 μm thermally conductive coating is sufficient. Removal of the substrate means that a major factor causing thermal stress is removed.
The removal of solder joints eliminates solder fatigue, a common failure mechanism in power modules. The removal of the substrate also eliminates most of the thermal stress. The accelerated passive thermal shock test at 40 ° C / 125 ° C shows that the temperature conduction stress is effectively reduced and the reliability is greatly increased: in the case of a substrate-less sintered module, the number of possible thermal shocks is increased by 15 times. A further advantage of removing soldered interconnects and substrates is that in substrate modules, the area of ​​the soldered dbc substrate should be reduced to a minimum to reduce the fatigue of the solder joint material; here, the high thermal conductivity of the substrate ensures the required heat spread. In contrast, when designing a module without a substrate, the area of ​​the dbc substrate can be larger, as shown in Figure 1.
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