Oct 19, 2020
Direct Bonded Copper (DBC) and Active Metal Brazed (AMB) ceramic substrates are used in multi-chip modules for electric power conversion in order to convert:
Each application comes with its own specific requirements for the multi-chip power modules, hence the variety of substrates inside the modules. Among other measures, voltage, current and mission profile are critical parameters to consider in the selection of the substrate for a given application. In this blog, we look at common applications for multi-chip power modules to understand the rationale behind each technology.
Motor drives are indispensable in industrial applications and automation systems which require motion in precise control, a swift response to command and an exact fidelity to position requirements (such as robots, machine tools, forklifts, elevators). By varying the frequency and voltage of the power supply to an electrical motor the drive can control speed and torque.
For more than 30 years IGBT power modules with alumina (Al2O3) DBC substrates were developed to address the needs of motor drives. The primary function of Al2O3 DBC substrates are to carry multiple chips, interconnect the chips to realize an electrical circuit and provide electrical isolation. Their high ampacity, owed to their thick copper metallization, make them the best choice for power conversion. In addition, they provide a low thermal resistance to dissipate semiconductor losses. Finally, reliable materials such as Al2O3 DBC substrates improve the lifetime of modules because their coefficient of thermal expansion (CTE) matches the CTE of the semiconductor devices.
For high power modules, multiple DBC substrates are usually attached on a baseplate. This baseplate is typically made of copper and provides an additional heat capacity for applications where surge currents are expected. However, the stress resulting from the different CTE of the baseplate and substrates may lead to solder fatigue and delamination. Especially in cases when modules are exposed to thermal cycles. Depending on their duration and repetition, such cycles can significantly affect the lifetime of modules. Therefore, modules without a baseplate may be the better choice if heat capacity is not an issue. At any rate, large-scale production of Al2O3 DBC substrates enables an attractive price-performance ratio for most industrial applications.
Inverters can be found in home appliances to control compressor motors, fan motors and drum motors in air conditioners, refrigerators and washing machines. For many years the power conversion job has been done with discrete devices because this solution was cheap and sufficient. However, increasing requirements regarding efficiency, controllability and miniaturization have led system manufacturers to adopt small size multi-chip modules. They are produced from multiple manufacturers in standard footprints and are more reliable than existing solutions with discrete devices. This market segment remains very cost sensitive. Therefore, these modules are produced with either insulated metal substrates (IMS) or alumina (Al2O3) DBC substrates depending on the rated current and input voltage since these substrates deliver the best price-performance ratio.
Power modules are widely used in applications that convert clean energy, such as photovoltaic and wind energy, into usable commercial power. While both renewable energy sources require a very high conversion efficiency, other requirements differ.
On one hand, photovoltaic inverters require a high robustness regarding humidity and low system cost but have low demand on power cycling since they are typically always on. The output power starts at few kilowatts for residential and commercial applications and extends into the megawatt range for utility-scale plants. Discrete devices are preferred for applications below 10kW. Systems with higher output power are using power modules with and without a baseplate depending on the power range. Therefore, Al2O3 DBC substrates are again the best technology for these applications.
On the other hand, wind turbines are rated in the megawatt power range. Limited space leads to strong requirements regarding power density. In addition, power modules are operating under variable wind speed and experience large power cycling stresses but need to operate for up to 25 years without any failures. Therefore, high power modules with a baseplate have been developed with robust joining technologies and reliable materials such as zirconia doped Al2O3 DBC substrates to achieve the required lifetime.
All trains require power inverters to drive the electric motor, but the number of inverters per train, voltage and current vary depending on the type of trains (tramways, subways, regional or high-speed trains). Rail traction needs very high-power modules with voltage > 1.7kV and power ranging from 100kW to 1-2MW. A thick ceramic substrate is necessary to provide enough electrical isolation though this increases the thermal resistance. This can be offset by materials with better thermal conductivity. Therefore, thick (0.63mm and more) aluminum nitride (AlN) is usually the preferred material.
Automotive is probably the most exciting opportunity for power electronics because hybrid and electric vehicle market is expected to grow rapidly. Many new power module platforms are currently under development to address the needs of this market segment. However, there is no ‘one size fits all’ solution and almost no standard package to cover the range of possible automotive applications.
Al2O3 and zirconia doped Al2O3 DBC substrates are enough for low voltage applications below 60V, such as belt starter generators in mild hybrid vehicles. Although, with increasing levels of electrification, not only the output power but also the requirements regarding power density, power cycling and robustness are increasing. Consequently, new materials and joining technologies are necessary to improve the heat dissipation from the power semiconductor devices. Silicon nitride (Si3N4) AMB substrates are the perfect fit for such applications. Nonetheless, the thermal conductivity of Si3N4 is much higher than for Al2O3. That said, Si3N4 AMB substrates have great mechanical properties and are available with up to 0.8mm thick copper metallization to achieve the highest ampacity and heat spreading.