Five major difficulties and solutions for automotive SiC

In recent years, the proportion of third-generation semiconductor devices including SiC in automobiles has been increasing day by day. But in the eyes of professionals, this will not be a simple matter.

First, from the perspective of automotive lead frames, although Si, silicon carbide/gallium nitride lead frames are all made of copper, have the same manufacturing process, and require mold production, there is solder paste bonding between the chip and the lead frame in the traditional TO247 packaging method. This packaging method will have VOID problems and void problems. If used at high current and high voltage, it will be unstable and unreliable, which has become a difficulty in using the TO247 packaging method for SiC chips. With the development of new applications and new energy sources, semiconductor components must have the characteristics of high voltage and high current resistance. In order to avoid VOID problems, pressureless sintered silver AS9330 must be used for connection, and the lead frame must also be silver-plated. The connection tolerance is only 20um, and the precision requirements are very high, which also leads to a high technical threshold.

Second, the new SiC chip can be packaged using IPM and TPAK methods. Taking the SiC frame technology used in electric vehicle inverters as an example, the frame Copper Clip and SiC chip are connected using semi-sintered silver AS9330 connection technology, which can meet the needs of high-reliability and high-conductivity connections. Many Tier 1 controller companies and Tier 2 power module manufacturers use this sintered silver technology in automotive modules to a greater or lesser extent. Currently, sintered silver technology is mainly used in markets with high requirements for reliability and heat dissipation.

In the production of lead frames, in addition to providing high-reliability silver plating quality to meet the sintered silver lap technology, the film thickness of sintered silver is 50um-100um, which can be adjusted to correct the lap problem caused by poor coplanarity of the lap surface. The sintered silver lap technology can also require a tolerance of about 50UM for the coplanarity of the lap. The lead frame must be silver-plated, and it is partial silver plating. Compared with full plating, partial silver plating technology is difficult. A mold must be made, and partial silver plating is used where the chip is placed. If two chips are placed on one lead frame, the chip must be partially silver-plated, and the other lead frames must be made of nickel-palladium-gold. The material difference is a great technical challenge to the production of lead frames.

Third, due to the slow growth speed and high defect rate of SiC wafers, the wafers cannot meet the market demand. Obtaining SiC wafers is the entry ticket to the automotive power component market. Currently, SiC crystal growth mostly adopts the physical vapor method (PVT), which is slow and has a low yield. The main reason is that there are more than 200 types of crystal structures of SiC, among which the single-crystal SiC with a few crystal structures such as the hexagonal 4H type (4H-SiC) is the required semiconductor material. It is necessary to precisely control the silicon-carbon ratio, growth temperature gradient, crystal growth rate, and airflow pressure and other parameters. Otherwise, polycrystalline inclusions are likely to occur, resulting in unqualified output crystals.

Four. Full SiC Module is currently used in some high-end models, and most other electric vehicles use hybrid SiC modules. The most common one is to use SiC diodes with IGBTs, and most silicon carbide power modules are a combination of SiC SBD/MOSFET and Si IGBT/FRD/Diode.

SiC SBD packaging is very different from traditional SiC: Since SiC wafers are very hard and brittle, the cutting equipment and tools required for chip cutting of incoming wafers are different from those of general Si materials, and most of them use laser cutting machines; and generally high voltage/high current mostly use ceramic substrates, but because SiC is a hard and brittle material, it is more likely to warp due to stress during the packaging process. Some copper substrates or ceramic substrates even need to be bent first to achieve a tight fit; the purity and area of ​​the lead frame copper also change with the voltage/current, and low inductance and other materials for high-temperature packaging materials must be selected.

5. Since SiC itself is resistant to high temperatures, it is suitable for use under high voltage and high temperature conditions. However, because SiC generates high temperatures, the choice of heat dissipation substrate is relatively demanding. It is recommended to use the more expensive AMB substrate.