Another way to integrate power semiconductors into a system is to embed them in a printed circuit board or ceramic substrate. Zechun Yu from Fraunhofer IISB presented the embedding of wide-bandgap devices in a ceramic substrate. Embedding into a PCB offers some advantages, e.g. miniaturization (no package), low-inductance design (no bond wires), and double-sided cooling. But embedding into a ceramic substrate offers additional advantages including temperature stability (beyond +200 °C), low thermal resistance, high current carrying capability due to thick copper, immunity to corrosion, and smaller difference in the coefficients of thermal expansion (Fig. 9; )
For a test, the researchers used a SiC diode with 3.3 kV with a current density of 100 A/cm². The various process steps were evaluated, for which soldering as well as double-sided sintering is suitable. The diode worked without any problems in the test up to +250 °C.
Thomas Gottwald from Schweizer Electronic presented how to embed it into a printed circuit board. By using an OptiMOS 5 from Infineon Technologies as an example, he illustrated how much better this is compared to a discrete packaged component. While this 80 V transistor in a discrete TOLL package has an on-resistance of 1.2 mΩ and a peak drain current of 300 A, in the embedded Smart p² Pack these parameters are 0.9 mΩ and 460 A respectively - using the same MOSFET! The thermal resistance also drops by 30 percent (Fig. 10; ).
Compared to mounting them onto an Al2O3 substrate, the Smart p² Pack offers at least ten times the number of load cycles at higher temperature swings (approx. 100,000 cycles with 80 K vs. > 1 million cycles with 120 K). Schweizer is going to develop corresponding solutions for SiC and GaN for higher voltages by 2020.
At the interface between ceramic substrate and base plate, excessive mechanical stress can occur at certain corners, if the substrate is not coplanar with the base plate after soldering. This causes the delamination process to start earlier, decreasing the cycle stability of the power module. To prevent this, Khartik Vijay provided the solution InForms from Indium Corporation. This is a honeycomb-shaped metal grid structure into which the solder is applied. This grid structure acts as a spacer between the substrate and the base plate. In terms of handling, it is similar to an insulation pad, but much larger.
Another advantage is that the soldering process does not have to be modified at all, but is rather a drop-in replacement. For traction applications, various modules were subjected to temperature cycles, whereby the final temperatures of –50 °C and +150 °C were maintained for 30 minutes each and then traversed from one final temperature to the other within 30 seconds. While the first cracks and delamination were observed in conventional modules after only a few hundred cycles, the researchers could observe neither cracks nor delamination in the module soldered with InForm even after 2000 cycles.
Hideo Nakako from Hitachi Chemicals reported on the robustness of sinter connections with copper pastes. These are particularly interesting with regard to the wide-bandgap semiconductors with their higher possible junction temperatures of over +200 °C. Hitachi has developed two copper pastes for this purpose: one that cures without applying pressure, but under a hydrogen atmosphere, and another one that requires a contact pressure of at least 1 MPa under a nitrogen atmosphere. The latter is also suitable for larger chips with a footprint of 15 mm × 15 mm. Both pastes were tested, and it was found that the one that cures under contact pressure and nitrogen atmosphere had approximately half the porosity of the other paste. In a temperature cycling test, the fine-grained copper paste showed the highest robustness; it is 16 times more robust than soldered connections and 1.6 times more robust than sinter connections with silver paste.
To conclude the workshop, Thomas Stoll from the University of Erlangen-Nuremberg presented the opportunities offered by additive manufacturing (3D printing) in power electronics. Nowadays, these possibilities exceed by far just rapid prototyping. It is now possible to manufacture parts with demanding mechanical requirements and add mechatronic properties to circuitries (additive mechatronization). One example is three-dimensional circuit carriers with embedded electronic components. Other objectives that can be addressed with additive manufacturing are the manufacturing of user-specific three-dimensional micro heatsinks, the elimination of a number of process steps compared to conventional module manufacturing, and the elimination of the thermal interface to improve heat dissipation (Fig. 11; ). The reduction of weight and material is also a target.
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