Design Challenges are Mounting for Power Modules

The need for increased energy efficiency is shaping both the generation and consumption of electrical energy. On the generation side, this requirement is helping to grow the market for wind and solar power systems. As for energy use, there are many examples of products specifically developed to save energy. These range from complex systems such as hybrid vehicles to simpler ones such as motors with electronic motor drives.

Because power modules find use in all these applications, their development is being heavily influenced by the demands for greater efficiency. However, efficiency is not the only requirement and there are other factors such as small size, high-temperature performance and reliability that are driving module development as reflected by recent trends in semiconductor and packaging technology.

Improved semiconductor technology has brought about components with finer structures and faster switching speeds. With the introduction of fourth-generation IGBT chips, current density has increased by 25% over the previous generation. One of the reasons for the improvement in efficiency over time has been the substantial decrease in chip thickness. This trend is expected to continue.

Nevertheless, thin-wafer technology, based on the existing mounting and assembly technology, has reached its limit. This limit is reflected in the derating of a key specification for the latest 1200-V trench IGBTs. For these devices, which have 70-µm-thick die, the maximum permissible short-circuit duration had to be reduced from 10 µs to 6 µs. Immense short-circuit power surges can no longer be stored by such thin silicon wafers, and the thermal impedance of the design does not allow for heat to be dissipated quickly.

Another trend in semiconductor technology has been the migration to higher current density and thus higher chip temperatures. In 2007, the maximum junction temperature for 1200-V IGBTs and free-wheeling diodes was increased by 25°C to 175°C. Today that rating is heading toward the 200°C mark. Unfortunately, higher operating temperatures and current densities have an adverse effect on reliability, in particular load-cycle capability. To combat this, improved mounting technology is vital.

One aspect of module mounting technology is the use or omission of a baseplate. In modules with no baseplate, the DBC substrate is mounted directly onto the heatsink. When a 3-mm copper baseplate is used, it increases the thermal capacitance and thermal spreading beneath the chips. But the large soldered area between the insulating ceramic substrate and the copper baseplate clearly reduces the load-cycle capability of the components. That's because the ceramic substrate and the baseplate have significantly different coefficients of thermal expansion, which leads to tension and solder fatigue. Baseplates made from composite materials such as AlSiC or CuMo are an alternative to copper. But these are used solely in traction applications due to their low thermal conductivity and high costs. In the future, graphite composites may gain importance as baseplate materials due to their low cost.

DBC layout is another issue in module design, and efficiency can be increased by ensuring that module layouts are symmetrical, so that all chips share current equally. Coupled with the use of planar assembly and connections with low stray parasitics, symmetrical layouts reduce the overvoltages normally associated with power modules, improving switching efficiency by about 15%.

An emerging factor in module development is SiC technology in the form of free-wheeling diodes and MOSFETs. SiC allows junction temperatures of 200°C. Nevertheless, the effects on the reliability of the assembly technologies and packaging materials have to be watched. When used with the latest IGBTs, SiC devices can increase system-level efficiency by 20% to 30%. However, SiC is still expensive and further development will be needed before it finds uses across a broad spectrum of applications.

As emerging markets in alternative energy and hybrid vehicles continue to grow, power semiconductors and power modules will continue to evolve. Trends toward improved cooling, ever-higher current densities and integrated driver electronics will be ongoing. However, higher operating temperatures and better cooling are only feasible at the cost of reliability. Going forward, the only way to combat this problem will be to develop new mounting and assembly concepts.

Paul Newman joined SEMIKRON in 1990, where he has been involved with the research and development of intelligent power stacks for diverse applications.