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Sep 04, 2019

Reliability

In the last decade power electronics has gained importance with climate targets set to cut greenhouse gas emissions; therefore increasing renewable energy consumption. The new generation is aware of the environment and pollution challenges that our society is facing, motivating and attracting young engineers to study power electronics. However, this was not always the case. Not so long ago, power electronics could not compare to microelectronics which had been in favor for many years. While printed circuit boards were becoming a commodity and their mass production transferred to Asia, Rogers used its more than 30 years of power electronics experience to begin selling direct bonded copper (DBC).

The Eighties: Start of a New Era

Between 1978 and 1982 research groups at Stanford University, GE Corporate R&D Center, Siemens AG and RCA Corporation were intensely working on the functional integration of power MOS (metal oxide semiconductor) and bipolar transistor concepts. Eventually, these research activities led to the invention of the insulated gate bipolar transistor (IGBT). That produced unprecedented switching efficiency in the range of a few hundred kilowatts to megawatts; thus a revolution began.

At about the same time, multichip power modules were introduced to the market. Compared to discrete devices in plastic housings, power modules contain multiple power semiconductor dies that are integrated on a substrate and connect to form an electrical circuit. They can accommodate various topologies and power ratings in the same package footprint, for instance multiple dies can be easily connected in parallel. In addition, power modules are often more efficient, smaller in size, provide higher power density and are in many cases more reliable. Moreover, labor-intensive assembly steps and additional assembly materials can be saved. However, multichip power modules come with new and tougher requirements for the substrate that carries the power semiconductors, provides electrical insulation and a path for heat dissipation.

In 1983 curamik electronics GmbH was founded. At the time it was the first company to produce direct bonded copper (DBC) alumina (Al2O3) ceramic substrates on large panels at an affordable cost. Although, module manufacturers quickly assessed and qualified this technology, DBC substrates are still widely used in power modules today; because they efficiently dissipate the waste heat from the power devices and increase the modules´ lifetime. Al2O3 DBC substrates still offer the best price-performance ratio, sufficient thermal and mechanical properties for most common industrial applications in our market.

The Nineties: Up to the Highest Power Ratings

Over the years, IGBT semiconductor technology was further developed to handle larger currents and higher voltages; largely because of its role to support railway electrification with high speed lines operating at very high voltages. In the nineties, high voltage direct currents (HVDC) systems, which are used for the purpose of energy transmission, were also adopting the IGBT technology with multiple devices connected in parallel to achieve the required power level.

Therefore, not only the devices but the modules, particularly the substrates, had to be designed to survive high operating voltages and guarantee safety. With a dielectric strength in the range of typically 20 kV/mm, ceramics provide good electrical insulation and their thickness allows for maximum achievable voltage class, as thermal resistance increases with thickness. Compared to Al2O3 with a thermal conductivity of typically 26 W/mK, new ceramics with better thermal conductivity were required to compensate for the increase of thickness. Though, beryllium oxide (BeO) would have been the best material, due to its excellent thermal properties, aluminum nitride (AlN) with a thermal conductivity of typically 170 W/mK was preferred in avoidance of toxicity issues. Although, AlN could not be easily bonded and therefore had to be actively brazed with copper to form active metal brazed (AMB) substrates. Curamik electronics GmbH was the first, and after acquisition by Rogers Corporation in 2011, remains the only company to combine AlN with copper in a classical DBC process following a special pre-treatment of the ceramic.

Extended Lifetime for the New Millennium

Next to the advancements in device technology and cost, improvements in packaging technology were required to achieve better reliability for the most demanding applications with multiple switching events and resulting temperature swings over their lifetime. Particularly, more power semiconductors were required to drive electric power steering motors and water pump motors in vehicles with internal combustion engines (ICE). Stringent requirements from the automotive industry regarding robustness validation of semiconductor devices were paving the way for new materials and joining technologies to overcome aging mechanisms, such as solder fatigue and wire bond lift offs.

As with a chain, it is only as strong as its weakest link, a module needs an equally strong substrate. Zirconia toughened alumina (ZTA) became available at that time. Seeing its superior mechanical properties, ZTA DBC substrates could resist more temperature cycles than traditional Al2O3 DBC substrates; it was selected to provide an additional robustness margin in critical applications. Another approach was to take advantage of the mechanical properties of this new material to reduce thickness and achieve better thermal performance, without impacting the module reliability. Not only the automotive industry, but renewables and medium power industrial applications began rapidly adopting this new material. However, curamik electronics GmbH was first to market ZTA DBC substrates containing up to nine percent of zirconia.

Increased Power Density in the Last Decade

With the continuous electrification trend for passenger cars, busses and trucks, the semiconductor content per vehicle grew tremendously, and thus grew electric power consumption. For battery electric vehicles a significant amount of electric power has to be made available to drive the powertrain. At the same time robustness became more important, since space is limited under the hood and additional weight is not acceptable. Therefore, reliable packaging and efficient cooling technologies are required to deliver the performance expected from an electric car. Moreover, power electronics solutions have to be easily customizable and scalable to become cost competitive and accelerate the electric vehicles´ market adoption.

The result is a significant increase in power density. Materials for die attach, die interconnections, die encapsulations and their corresponding processes have been improved to cope with these tough requirements. Silicon nitride (Si3N4) AMB substrates are becoming more popular as their ceramic provides the best trade-off between bending strength, fracture toughness and thermal conductivity. Depending on specific requirements and vehicle category they are available with up to 0.8 mm thick copper layers. Rogers Corporation identified this market need early on and added this substrate to its portfolio. As of today, curamik® Si3N4 AMB substrates are qualified for automotive applications. Electric cars rely on our substrates and we predict that there can only be more growth for curamik in future models.

The Race for Efficiency Will Continue in the Coming Years

While we are all concerned with climate change, we can argue that power electronics are contributing to a better world, and the solutions that we are developing can help save energy. The race for efficiency will continue in the coming years. Wide band gap devices are now available and have already shown their potential in this regard, however that comes with new packaging challenges and a necessary solution by the power electronics community. But, these are the kind of challenges that Rogers likes to solve by introducing new materials, new processes and ultimately new products. As a manufacturer of metallized ceramic substrates with a full range of DBC and AMB substrates, Rogers´ PES team is available to help as your development partner. Do you have any questions or require support with the design and selection of a suitable substrate for your application? Please contact us, if you need assistance.