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Introduce improved semiconductor devices in response to the packaging trend of power discrete components and modules for the most demanding power switch applications. Wide bandgap types such as silicon carbide (SiC) and gallium nitride (GaN) will significantly improve the performance of power switch applications, especially in applications such as automotive traction inverters.
In power applications that require low loss, high-frequency switching, or high-temperature environments, silicon carbide ceramic substrate power semiconductor technology has significant advantages over traditional silicon-based devices. For example, the dielectric strength voltage of Sic is about 10 times that of silicon, and low loss is crucial for performance ratio, while SiC technology can reduce power loss by up to one-fifth.
Some advantages of these technologies have been proven and deployed, especially the application of silicon carbide in independent charging stations (high voltage), and recently in traction inverter components (high temperature, high switching frequency), becoming truly applicable in the field of automotive electrification. With the potential to significantly improve performance, the industry still faces challenges.
What new packaging innovations can be deployed to achieve the full performance advantages of these promising semiconductor devices?
The first step in improving the packaging design of power modules, even before the emergence of silicon carbide, involved using directly bonded copper on ceramic substrates, such as aluminum oxide and aluminum nitride, to replace substrates made of pure copper. These ceramic substrates exhibit significantly lower coefficient of thermal expansion (CTE) characteristics while still providing reasonable thermal conductivity.
CTE can be modified by adjusting the thickness of copper relative to the thickness of the aluminum oxide core. For example, reaching 7-9 ppm/Celsius provides a better match for low CTE semiconductor molds during installation. By doing so, the total CTE mismatch (chip to substrate) is now 3-7ppm, instead of the 13-15ppm in the case of semiconductor chips mounted on copper lead frames. Directly bonding copper DBC ceramic substrates is very common in today's multi chip power module systems, but copper lead frames are also selectively used, especially for single-chip devices.
Another recent development is the use of copper as a metallization on ceramics, which has improved their thermal cycling performance compared to copper metallization. The top metallization of the ceramic substrate is etched to form a physical circuit that can accept chip connections, followed by top wire bonding. And substrate surface treatment is also common, which can provide strong surface protection before the chip mounting process that usually includes reflow soldering. Typical solder materials include high lead for chip connections and lower melting point solder for connecting the substrate bottom to the module heat sink.
In a similar manner, single or double tube chip packaging (such as IGBT diodes) is used with heavy-duty copper lead frames and top solder wires for power connection and control. Lead wire can be replaced with copper clips to improve chip cooling. This configuration also provides improved thermal cycling performance. Like single-chip packaging, the module that replaces aluminum solder wires with more robust top connections achieves additional chip cooling, higher current density, and improved power cycling. The double-sided cooling of IGBT diode modules was proposed more than 15 years ago and has been deployed in many traction inverter components used in hybrid electric vehicles.
In modules based on DBC alumina and aluminum nitride, the top connection is also implemented using the same material. Based on the contact area of the top side chip, a typical implementation can reduce the thermal resistance of the module by 30%. Double sided cooling modules may require the ability to provide gaps for non power oriented solder wires, such as small gates and current sensing solder pads. In these cases, spacers are used when it is necessary to ensure the minimum air gap between substrates in higher voltage applications. Gaskets can be made of thermally and electrically conductive materials such as copper, but due to the fact that the chip size of traditional silicon may be very large, 12mm x 12mm, when using only a soldering adhesive layer between relatively thin copper and the surface of the die. Feasible alternative solutions for spacers here include composite materials such as copper molybdenum and laminates such as copper Invar copper or copper molybdenum copper.
To ensure sufficient power cycling performance and welding connection lifespan, the current load is distributed across multiple dies, thereby reducing the current density of each die. Although this approach requires more equipment to achieve the given function, it requires derating to ensure a robust installation product lifespan.
As the industry transitions towards wide bandgap devices such as silicon carbide, the packaging of these devices will become a key factor affecting the reliability, performance, and cost of new modules. SiC has higher efficiency at higher operating temperatures, and ideal packaging designs should support this fact to improve chip efficiency. One of the most promising attachment materials deployed with silicon and SiC is sintered silver. Silver is an almost ideal accessory material, but its melting point makes it unsuitable for use as a reflow metal. However, it has very high thermal conductivity and exhibits an attractive low resistivity. All these characteristics are superior to welding, including power cycling capability, which will be discussed later.
ceramic substrates
With the increasing mass production and application of different types of sintered silver, the types of materials are also increasing. Although the initial application of sintered silver relied on silver nanosheets, thin films and preforms have become feasible product types, enabling new manufacturing processes. Wafer level lamination can now be achieved using nano silver films. Once the wafer is laminated, standard equipment can be used to cut the wafer. An alternative process has also been developed, which can laminate individual chips on a wafer and immediately sinter them onto the target substrate. This is called the mold transfer film process. The advantage is that only laminating and sintering known good chips together with SiC can provide significant advantages.