Paul Newman, Managing Director at Semikron explores the challenges facing power electronic module development and some techniques for achieving greater efficiency
Low cost and small size have alwaysbeen important factors in power module design. However, inexpensive and readily available energy has often made energy efficiency less important. Now, significantly higher costs and potential scarcity of energy, coupled with implications for the carbon footprint of systems, are making energy efficiency much more important.
This need for better efficiency applies in both the generation and use of electrical energy. For example, motors controlled by modern electronic systems can be up to 30 percent more efficient than conventional motors. However, less than ten percent of the world’s electric motors are controlled in this way, so there is tremendous potential for energy savings and market growth.
Alternative renewable energy sources, including wind, solar, wave and tidal power generation systems, are becoming widely accepted and must clearly provide the highest possible levels of efficiency and reliability.
To achieve maximum energy, cost and space efficiency, coupled with high reliability, we must combine the best silicon, packaging, cooling techniques and control in power modules. Innovative design and tight control of manufacturing processes are also key to meeting these aims.
A number of performance enhancements result from increases in the junction temperature of the semiconductor devices at the heart of the modules. Recent advances in technology have made it possible to achieve these higher chip temperatures.
In the last few years, the maximum junction temperature of certain IGBTs and freewheeling diodes increased to 175°C and products capable of working at 190°C junction temperature maximum will be introduced to the market within the next year. Silicon Carbide (SiC), particularly in freewheeling diodes and MOSFETs, allows junction temperatures of 190°C. Other wide band gap materials are also entering the market and promise size and efficiency advantages.
Although these technologies can produce increases in system efficiency of 20 to 30 percent, higher operating temperatures and current densities have an adverse effect on reliability, in particular power cycling capability.
The most significant problem caused by higher temperatures and larger swings of temperature is delamination of the soldered joints used to connect the silicon to the substrate and the substrate to the baseplate. This problem has been completely overcome by using better jointing materials.
One such solution, employed by company’s such as Semikron, is sinter technology to join the semiconductor chips to the ceramic substrate instead of solder, meaning that higher operating temperatures are possible with increased reliability.
The sinter bond is a thin silver layer that has a superior thermal resistance to a soldered joint and contains far fewer, and smaller, voids. It is not subject to the delamination that affects solder joints, resulting in a low thermal resistance that remains low over many tens of thousands of power cycles. The high melting point of silver also prevents premature material fatigue.
Other issues presented by solder have been overcome by using a combination of different pressure-contact technologies for connections to the direct-bonded copper (DBC) substrate, power terminals and auxiliary connections. The use of mechanical pressure to press the DBC substrate to the heat sink, without soldering, results in a homogenous pressure distribution with a thermal connection between the ceramic substrates carrying the semiconductor chips and the heat sink.
Thermal resistance is 40 percent lower than standard modules and thermal cycling capability is five times higher than a module with base plate. Other advantages of the pressure-contact techniques include ease of manufacturing, significant resistance to damage caused by shock and vibration and lower contact resistance.
Another issue raised by increased junction temperatures is the fatigue of the wire bonds used to join the chips to the substrate. This has been minimised by a number of changes to production techniques, including changing the geometry of welded joints and the introduction of novel stress-relief techniques. Improvements have also been produced by the move to copper bond wires, and the company has now built on this technology by developing its SKiN flexible copper track interconnection system.
In addition to advantages of improved current density and reliability, this technology opens up the possibility of mounting low-power SMD devices on the flexible substrate and using new isolation techniques to achieve a safe extra-low voltage (SELV) interface.
A third problem that will develop, as higher temperatures become the norm is the stability of the plastic materials used to house modules. The company is carrying out research into different materials to overcome this.
Making module layouts symmetrical can also increase efficiency. This ensures that inductance is equally distributed and that all chips share equal amounts of current, and switch symmetrically. Coupled with the use of planar assembly technologies and low stray connections, the over-voltages normally associated with power modules are reduced, producing an improvement in switching efficiency of around 15 percent.
Emerging markets such as renewable energy and electric vehicles present strongly growing demand for power semiconductor modules and systems. Both application areas have very high technical and economical expectations. New technologies, such as IGBT pressure contact modules, flex foils and sintering provide some of the right answers for this.
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.
Semikron
