An important characteristic of power semiconductor modules, such as those used in electric drives, wind power and solar systems, is the thermal resistance between the semiconductor and heatsink. A new packaging technology, which replaces solder joints by sintering joints and direct pressure onto the top of the semiconductors, enables considerably higher power densities with resulting cost savings.
Increasing power while reducing installation footprints, extending product lifetimes and reliability while reducing costs, these were all requirements for the new generations of power semiconductor modules. In the case of wind power and solar power plants, this evolution is primarily driven by the higher cost pressure on investments and, in particular, ongoing operating costs. For e-mobility applications, strict requirements are placed on the available space. IGBT and MOSFET semiconductors are also constantly evolving, enabling higher currents to be achieved on smaller chip areas, placing enormous requirements on thermal management. The traditional assembly and connection techniques in which semiconductor components are soldered onto substrates, the chip top-side connections by bond wires, then the entire assembly soldered together, usually onto a copper baseplate, is reaching its limitations and being capable to fulfill the new requirements.
Conflict of objectives - performance and reliability
The thermal resistance of power modules is essentially determined by the materials between the semiconductor chips and heatsinks, their thickness, and the available area to spread the heat. In addition to the mechanical stability and heat dissipation, the electrical insulation of components must be ensured. This results in the need to connect several layers of different materials, which, owing to their different electrical properties, also differ considerably in their thermal expansion coefficients. During operation, temperature changes occur, which lead to a continuously varying mechanical stress between the layers. This in turn, leads to joint degeneration and ultimately to the failure of the entire system. By optimizing the choice of the materials and layer thicknesses, the attempt is made to find a compromise between thermal resistance, durability and costs. In addition, there are process related influences such as solder cavities and fluctuations in solder layer thickness, which also have an influence on the thermal resistance. Thus, the baseplate and chip soldering may contribute up to 25% to the total thermal resistance chip to the heatsink Rth(c-s).
Direct Pressed Die
More power on the same module surface can be achieved by reducing the thermal resistance between the semiconductor chip and the heatsink, allowing more thermal losses to be dissipated at the same semiconductor temperature. If, in addition, it is possible to reduce the stray inductance with an improved layout, the semiconductors can be switched faster, which in turn reduces the losses and allows for additional output power. The new packaging technology "Direct Pressed Die" (DPD) has been developed for this purpose. In contrast to the traditional technique where copper/ceramic substrates (DCB = Direct Copper Bond) are soldered to a baseplate, the substrate with the semiconductor chip on top is purely put on the baseplate or the heatsink. The thermal connection to the baseplate or heatsink is produced by a defined pressure on the chip top sides, thus eliminating all cavities below the semiconductor chips. In addition, the baseplate soldering which negatively influences the thermal resistance, can be removed completely. The mechanical connection formed between the DCB and the baseplate or heatsink, is comparable in terms of thermal resistance to a metal to metal joint. Since, however, the layers do not show a hard connection to one another, they can move laterally with respect to each other during temperature changes, and as a result the mechanical stress within the layers is greatly reduced. This decoupling also prevents the bimetal effect, which increases the thermal conductivity between the baseplate and the underlying heatsink when bolted on.
Sintered flex-layer as a connection level
Since the upper side of the semiconductor must be accessible as a flat pressure surface, the "Direct Pressed Die" (DPD) technology cannot be applied to wire-bonded modules. Instead, a two-layered flex-layer is sintered onto the semiconductor chips, and these are then sintered onto the substrate. The combination of sintering and flex-layer also makes for a considerably longer service life compared to classic aluminium wire bond connections, while at the same time permits a higher maximum chip temperature. The reason for this is the much higher solidus temperature of the sintered material compared to solders and the associated increased robustness of the connection. Commutation paths and control signals can also be implemented with very low inductances, which significantly improves the dynamic switching behaviour. Overall, the thermal resistance can be improved by up to 20% when compared to traditional soldered modules using the same DBC material. In addition, thermally optimized ceramic materials can be used due to the stress-reduced design. While having a substantially lower thermal resistance, such materials unfortunately have reduced mechanical robustness, which usually precludes their use in soldered modules. However, the use in "Direct Pressed Die" modules is possible, which leads to a further improvement in thermal resistance by up to 25%.
Integrated pressure system
The technical requirements for the "Direct Pressed Die" pressure system are extensive. Over a wide temperature range and a long service life, a defined pressure must be maintained and the electrical insulation properties must also be adhered to over various climatic conditions. This places high demands on the design and the materials used. Novel silicones which meet the requirements for high-temperature stability with low mass loss and defined elasticity are suitable here, and can also be implemented at low cost.
Increased lifetime
In essence, the thermo-mechanical stress between bonded material layers and their robustness determines the service life of power semiconductor modules. The mechanical decoupling of these layers allows an increase of lifetime by a factor of 10 to 20, depending on the application and the materials used. In applications such as offshore wind farms, where maintenance is associated with considerable costs, significant economic advantages can be achieved as a result. In addition, converters using "Direct Pressed Die" modules can be more compact, reducing plant size and costs.
Conclusion: Higher performance and longer service life, while reducing space
"Direct Pressed Die”, or DPD technology allows for a significantly higher power density, while maintaining a longer service life than the traditional solder and connection based assembly technology. This opens up new solutions for addressing the increasing demands on power electronics, including modern regenerative energy systems and electrical drives.