The new environmental policy focuses particularly on the development of global renewable energy sources, especially wind power and solar power generation, and energy efficiency. These two goals have a considerable impact on power electronics. The original purpose of power electronics was efficiency control and power conversion. Therefore, power semiconductor components must meet new requirements in terms of efficiency, service life, and compactness. Manufacturers are striving to develop new assembly and connection technologies, provide higher current densities and reliable chip temperatures, and utilize new semiconductor materials to meet these very needs.
In the next 20 years, the demand for global primary energy will increase at an average annual rate of about 2%. Demand will increase by 50% by 2030. At present, one-third of primary energy is dedicated to electricity generation. In 2004, the average global electricity consumption was about 12 billion kWh (source: CPES 2004). 40% of it is used for driving - in most cases it is an uncontrolled motor.
Figure 1: Proportion of renewable energy in Germany in primary energy consumption
Rethinking climate policy Today, most of the primary energy needs are met by burning fossil fuels such as oil, natural gas and coal, which greatly accelerate the global greenhouse effect. In recent years, increased awareness of the adverse effects of global warming has created the concept of reducing greenhouse gas emissions. A cornerstone of the new climate policy is the global development and expansion of renewable energy and energy efficiency.
Europe is a pioneer of modern energy and climate policy. Germany is a best example of using new energy technologies. In view of the goal of reducing CO2 emissions by 14% (compared with 2005 emissions) by 2020, the proportion of renewable energy in primary energy consumption will increase to 18% by 2020 (6 in 2005). %). For Germany's total electricity consumption, this means that the share of renewable energy will be doubled by 2020, as shown in Figure 2. Looking ahead, the share of renewable energy planned for 2050 will reach 70%.
Today, wind power is the largest segment of renewable energy. In Germany, wind power has a 45% market share, followed by bioelectricity, hydroelectricity and solar power. (Source: German Ministry of the Environment, March 2008)
Figure 2: Proportion of renewable energy in Germany in total energy consumption
Reduce electricity costs
Parallel to the implementation of political requirements and the provision of financial incentives from net metering projects, the cost of renewable electricity generation is gradually decreasing. Take, for example, solar power generation, which is still considered the most expensive alternative to traditional power generation. In September 2008, a crystalline solar module cost about €3.5/Watt; today, on the contrary, comparable modules cost 35% less. This benefits from excess production capacity, fierce competition, especially from manufacturers in China, the transition to mass production, and last but not least, the relaxation of the silicon raw material market. By the end of 2010, prices may be as low as €1/Watt. Due to these cost factors, the cost of using solar power to generate one kilowatt-hour will enter the cost range of traditional power generation methods (source: Spiegel Online, March 2009; photon).
Benefit from the power semiconductor industry
The power semiconductor industry will benefit in two ways from the upcoming growth of the renewable energy market. First, the energy conversion itself requires a power semiconductor, for example an inverter in a wind power plant. Second, semiconductors are the core components of variable-speed drives, and variable-speed drives are indispensable for wind, solar, and biogas power installations. Such a control driver is used for adjusting the solar tracker of the solar panel according to the sun's movement path or for the adjustment of the optimum blade pitch in a wind turbine. In a biogas plant, the control driver is responsible for the precise input and mixing of biomaterials.
Due to its technical advantages and user-friendliness, the module is mainly used as an electronic switch in renewable power generation applications. A module consists of a silicon chip, an insulated ceramic substrate, and a module housing that provides the necessary power connections. These modules have different versions depending on assembly and connection technology and degree of integration, such as integrated drivers, current sensors, and heat sinks.
Figure 3: Cross section of different power modules; left: standard IGBT half-bridge module; right: intelligent power module (IPM) containing semiconductor chips, insulating materials, drivers with protective sensors, current sensors and heat sinks.
In 2008, the share of power semiconductor modules for renewable energy applications was only 7.5% of the module market. In other words, this market has the fastest annual growth rate of 25%. By 2012, the market is expected to generate $380 million in sales (Source: IMS Quarterly Report, February 2009).
In wind power generation and solar power generation, power supply reliability is a top priority because of the need to ensure economical operation. Followed by high efficiency and system compactness. For power semiconductor manufacturers, this means a particularly difficult challenge: how to meet these conflicting requirements in certain areas. In addition, as the power of the inverter increases, parallel module connection and thermal management will become more and more important. Take a wind turbine with an output of 3MW as an example: A heat loss of about 45 kW occurs in power semiconductors - this value is equivalent to the power demand of heating systems in three private homes.
New challenges for manufacturers
Solder connection
In conventional solderboard power modules, the solder connection is often a mechanical weak point on the module. Due to the difference in thermal expansion coefficient of the material, high temperature fluctuations, and excessive load cycling during operation, the solder layer may be fatigued. This shows an increase in thermal resistance, which leads to higher temperatures. This feedback mechanism will eventually lead to device failure.
Cold solder joints often also present additional reliability issues in solder connections to printed circuit boards.
Figure 4: Sectional view of the module showing the solder connection
2. Substrate
Substrates of large-size, high-output modules can only be difficult to optimize, and/or can be optimized for optimum thermal and mechanical performance at a considerable cost. Single-sided soldered substrate connections can have a bimetal effect, resulting in non-uniform deformation. It is not possible to provide a good thermal connection to the heat sink. The gap between the base plate and the heat sink must be filled with a heat-dissipating coating that has poor thermal conductivity, rather than an optimized thermal connection with a full-material seal. As a result, the thermal resistance of the system deteriorates.
3. Internal module layout
For 200A and above modules, some semiconductor chips must be connected in parallel to the DCB ceramic to achieve a module with a higher rated current. However, it is not possible to design a fully symmetrical DCB due to the mechanical limitations of the substrate module design. Therefore, the difference in switching characteristics and current levels occurs at different chip locations. Therefore, the module's specifications must be based on the weakest chips. Internal wires or connectors with bond wires can make the internal module's resistance and parasitic inductance worse.
4. Chip temperature
In recent years, improvements in semiconductor technology have made silicon structures better, resulting in smaller chip sizes and higher current densities. For example, the size of 150A/1200V IGBTs has shrunk by more than 35% in the past few years. At the same time, the maximum allowable chip temperature rises to 175°C. This means that it is possible to design and manufacture more compact modules. However, one disadvantage of this trend is that there is a greater temperature gradient in the module, which can lead to solder fatigue, the common cause of failure described in Section 1. In other words, the reliability of the overall module is reduced.
Innovative technologies provide solutions
The above issues are all interdependent factors. Therefore, people are made aware of the need to find an integrated solution rather than looking at these issues in isolation.
SkiiP technology has a solution to the problem of substrate and solder connections. It removes the substrate and the large area of ​​fatigue-prone connection to the substrate and uses a patented crimp system. In a crimping system, the substrate is pressed against the heat sink by mechanical pressure. Since the ceramic substrate is flexible and the pressure is applied through several points of mechanical "finger", the contact between the ceramic substrate and the heat sink is guaranteed to be very tight. Therefore, the thickness of the heat-dissipating coating can be reduced by only 20-30 μm. In contrast, the heat-dissipation coating with the substrate module has a thickness of 100 μm.
Figure 5: Comparison of different crimping systems. Crimp technology replaces fatigue-prone welding connections.
This crimping system can adapt to given conditions regardless of the geometry of the module. In the MiniSKiiP module, the pressure contact is located in the plastic module housing itself. In the SKiiP and SkiM modules, the pressure is applied through a suitable pressure element. The main terminal is also connected to the ceramic substrate using the same crimping system. Spring contacts are used instead of soldered gate terminals and load connections up to 20A. Spring contacts have proven to be suitable, especially when the housing is excessively vibrating.
The latest scientific and technological achievements are that the chip connections use silver sintered alloy instead of welding. Table 1 shows the comparison of the main parameters of welding and sintering connections. It is alarming that the sintering junction has a much higher melting point, which means that the ageing rate of the connection at a given temperature swing will be much lower. Therefore, the fatigue of the material and the resulting failure will be closer to the end of its useful life. Using the method described here, the thermal cycling capability of the power module can be increased by a factor of five. Therefore, it is possible to obtain a higher chip operating temperature without any compromise on the reliability of the module.
Table 1: Comparison of the main parameters of welding and sintering connections. Note that the melting point of the sintered joint is much higher.
The last point worth considering is the internal mechanical design of the new SKiM module. Figure 5 shows the layout of the ceramic substrate marked with the chip location. Please pay attention to the height symmetry of the layout. On the right you can see the internal power bus that also acts as a mechanical crimping system. The layered busbars and the current flowing through each chip directly lead to less than 20nH stray inductance between the DC+ and DC terminals. As for the turn-off of the IGBT, no difference was found for chips in different locations.
Figure 6: The layout of the ceramic substrate marked with the chip position. At the same time, the inner rail as a mechanical contact system is shown on the right side.
Renewable energy sector
Despite the current severe economic situation, the renewable energy sector will play an important role in promoting a country’s industrial production and employment rate in the future. The power semiconductor industry seems to have taken on the challenges of the future. Technical requirements from hybrid and electric vehicles, as well as new materials such as SiC and GaN, will pave the way for new developments.