Packaging power modules for electric vehicles

The wind power and electric vehicle markets today require power modules with high power density, high reliability and mechanical, thermal and electrical ruggedness. These requirements are best met by solder-free pressure contact technology. In a next step, wire bonds will be replaced by a flex foil to overcome today’s limitations on power cycling fatigue.

Superior performance in power density could be achieved by deploying these technologies not only to power modules, but to un-package these modules and design embedded systems by mechanical integration.

A power module can be defined as a device which contains more than one semiconductor chip and which provides a heat flux path separately from the electric path. These modules, which entered the market in 1975, have been a success story. Power modules are one of the key building elements in almost all power electronic systems.

Two of the fastest growing markets for power modules - renewable energies and electric driven vehicles - will be reviewed here in terms of enabling packaging technologies and their impact on these applications.

The power module market share for wind power is only around 5%. However, with 25% annual growth, it has the strongest growth of all market segments. By next year, this market is estimated at US$250 million. Power modules for wind power require a very high intermittent operating lifetime, long-term availability, very high reliability and suitability for harsh environments.

An integrated power module, which is used widely in wind power applications, is mounted on a user-specific heatsink (air or liquid cooled) and mechanically integrated with gate drive and protection, as well as current, temperature and voltage sensors.

The suitability of this module for the wind power market lies in its unique pressure contact system that provides electrical power and auxiliary contacts, as well as thermal contact, which are to a large extent free from fatigue induced by thermal and power cycling.

The motor vehicle market share for power modules is only about 4%, but is growing strongly at about 19% per year. High power applications have a whole set of special requirements which drive the power module requirements.

Technically most demanding are high ambient temperatures and a high number of thermal cycles.

The SKiM module also uses pressure contact. However, for the first time, this module does not contain any solder materials and processes. Instead, IGBT and diode chips are sintered to the direct bonded copper substrate.

In the sintering process, the chips are first placed into a silver paste layer that is pre-applied by stencil printing. Under very high pressure and moderate temperatures (250°C), the silver paste transforms into a solid layer of silver.

After formation, the silver layer between chip and substrate would only melt at 961°C, the melting point of silver. Therefore, the operating temperature of 175°C is only 18% of the disassembly temperature, which means that the silver layer will not fatigue over time in normal use.

This is quite different in the classical chip solder process, where the operating temperature of the device is about 60% of the disassembly temperature, thus leading to the well-known solder fatigue mechanisms in power cycling and thermal cycling.

Power modules require thin aluminium wires to connect to the top side of the chips by ultrasonic bonding. The mismatch of coefficient of thermal expansion of the aluminium wire and the chip causes these bond wires to lift off from the chip after a sufficient number of power cycles, limiting the lifetime of the module. Although great progress has been made in materials, design and process, this fatigue is still a very limiting design factor, aiming at lower cost, higher power density and longer life.

The solution may come in the form of flex foils instead of wire bonds, a technology called SKiN. Bond wires are replaced by welding the chips with a flip chip process to a sandwich layer composed of aluminium, polyimide and copper. The aluminium side provides the load and gate tracks; the copper side can be designed to carry the drive and sense electronics.

Vias through the polyimide layer enable contacts from the upper metal layer to the gates and sensors by thin wire bonding.

One way to overcome the limitations of package materials is to simply leave them out. Instead of producing power modules in the classical fashion, DBC substrates may be used to build an embedded power electronic system ‘from scratch’ by mechanical integration.

This stripping of most of the classical package components such as housings, terminals and base plates is called ‘un-packaging’.

The entire assembly requires only a small number of steps. Substrates are fabricated that contain power MOSFETs (soldered or preferably sintered) in a dual pack configuration, temperature sensors and filter capacitors.

These substrates are mounted on a heatsink together with a frame (made of high-temperature-compatible plastic material) with already moulded-in screw type terminals. Next, a DC and AC bus bar system with integrated DC link capacitor is placed.

A pressure part is mounted on top that contains pre-assembled springs for auxiliary contacts and presses the power terminals to the substrate, thus enabling thermal and electrical contact at the same time. Then a gate drive and electronic sensor board and a controller board are mounted.

Finally, a metal or a plastic hood is placed over the assembly, providing environmental protection.

The resulting motor drive inverter gives benchmark power density, withstands 20 g vibration and 100 g shock, and shows - in absence of any solder contacts - a very high thermal and power cycling capability.

The 200°C junction temperature capability of advanced power MOSFETS can be fully used. Due to its compact design and the integrated filter capacitors, it has little conducted and radiated emissions.

Renewable energies and electric vehicles represent strongly growing markets for power semiconductor modules and systems. Besides the high market dynamics, both application areas have high technical and economical expectations. Emerging technologies, such as IGBT pressure contact modules, flex foils and un-packaged, embedded systems may provide the right answers.

To be successful in the future, it will be important to serve the market perfectly by satisfying demands with existing products and, at the same time, to drive the innovation to inspire these new markets.

By Thomas Stockmeier, CTO Semikron