Operating in harsh conditions, traction converters demand ever-increasing reliability from power modules. The SEMITRANS 20 meets this challenge with the latest packaging technology for up to five times power cycling compared to standard traction power modules. The SEMITRANS 20 is a low inductance power module, which offers a solution to multiple-sourcing strategies down to the chip level. The low inductance design and layout make the SEMITRANS 20 a simple, flexible solution to the power electronic stack design.
Structure of a SKiiP 4 module
The SKiiP 4 module is based on a baseplate-less design and is pressed directly onto the top of the cooler. Between them is a pre-defined thin layer of highly effective thermally conductive material. Fig. 2 shows the key components in the SKiiP 4 half-bridge configuration.
Chip sintering to DBC
In order to mount the chips to the DBC, the common practice is to solder. Inevitably, microscopic voids heat and grow, causing small cracks over time. This aging continues until the solder joint reaches failure. SEMIKRON eliminates this failure mode by implementing SKiNTER technology, providing two to three times higher power cycling compared to a solder connection. SEMIKRON originally launched SKiNTER technology in 2007, which uses silver sintering to connect chip to DBC. In order to make this connection, high pressure is applied with a middle layer of fine silver powder at a temperature of around 250°C. This creates a silver sinter layer of very low porosity, stably connecting chip and DBC surfaces up to a melting point of silver at 962°C.
As shown in Figure 2, the SEMITRANS 20 implements the latest technology to increase reliability, including:
- Chip sintering to DBC
- Aluminium clad copper (AlCu) bond wires
- Silicon Nitride (Si3N4) ceramic
- Aluminium Silicon Carbide (AlSiC) Baseplate
Aluminium Clad Copper Bond Wires
In order to connect the chips, bond wires are ultrasonically welded to the top side. Compared to copper, however, aluminium has a higher coefficient of thermal expansion. This movement creates more strain on the aluminium bond wire’s connections compared to AlCu during heating and cooling. Copper’s decreased movement in combination with its higher flexibility increases its power cycling capability compared to aluminium. Since chips typically have aluminium metallization, processing methods are also based on aluminium. Rather than using more costly methods, aluminium clad copper bond wires combine the benefits of copper’s reliability with standard processing methods.
Aluminium Silicon Carbide (AlSiC) Ceramic
Compared to ceramics such as Aluminium Nitride, Silicon Nitride is much harder, diminishing the possibility of isolation cracks. Additionally, Silicon Nitride offers great thermal conductivity and heat distribution. An AlSiC baseplate has a coefficient of thermal expansion (CTE) much closer to Aluminium Nitride, reducing stress on connection points during power cycling.
Increased Power Density
In addition to a reliable package design, the SEMITRANS 20 increases power density with the capability of future increases. Previously, standard modules required numerous screws for mounting and only used a little over half of the baseplate with active chip area. The SEMITRANS 20 uses about 72% of the baseplate based on a 4-screw mounting method. This not only simplifies manufacturing, it also decreases the heatsink area, thereby increasing power density.
Since this increases the heat in a smaller area on the heatsink, a low thermal resistance from chip to heatsink is required. Therefore, SEMIKRON recommends using the new High Performance Phase Change Material (HP-PCM). This is a phase change material used as a thermal interface between power module and heatsink. Since it is solid during the manufacturing process, the phase change material is easier to handle, even allowing particles to be brushed away. Historically, phase change material have had a higher thermal resistance compared to the best paste options. With the upcoming HP-PCM, the thermal resistance is even as low as our High Performance Thermal Paste.
Multiple Chip Sourcing
Due to supply chain challenges, SEMIKRON understand the need for multiple-sourcing strategies. In addition to developing a module based on an industrial standard design, SEMIKRON also offers multiple chip options inside. The SEMITRANS 20 contains the R8 IGBT from Renesas as well as the E4 IGBT from Infineon. Both chips are offered in 1700V for the traction market.
Simple Manufacturing
As shown in Fig. 3, paralleling is simple with the SEMITRANS 20. While such a layout allows a low external connection to the DC capacitors, the module internal inductance is only 10 nH. This is achieved by overlapping the DC+ and DC- terminals inside the module. This low inductance allows fast switching without concern about overvoltage due to stray inductance, also perfect for Silicon Carbide in future versions.
To achieve an output current up to 1000ARMS, the SEMITRANS 20 includes three terminals for the AC connection. An isolation rating of up to 6kV can satisfy even higher voltage requirements, especially for 3-level applications. For Active Neutral Point Clamp (ANPC) topologies as an example, the SEMITRANS 20 offers a simple, compact layout. Such a layout keeps stray inductance to a minimum, shown in Figure 4.
Conclusion
The SEMITRANS 20 increases power cycling by up to five times compared to standard technology. Such an increase in power cycling improves reliability of the overall system. This enables traction inverters to last longer while operating in harsh conditions. In addition, the SEMITRANS 20 will increase the power density and scalability of traction inverters to realise compact and flexible traction drives that meet the requirements of future mobility.