Power electronic substrate

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The substrate role in power electronics is to provide interconnects to form electrical circuits (such as printed circuit boards), and to cool components. Compared to the materials and techniques used in low power microelectronics, this substrate must carry a higher current and provide higher voltage isolation (up to several thousand volts). They should also operate over a wide temperature range (up to 150 or 200 ° C).

Video Power electronic substrate

Direct bound copper substrate

Direct bonded copper substrate (DBC) is commonly used in power modules, due to excellent thermal conductivity. They consist of ceramic tiles (generally alumina) with copper sheets bonded to one or both sides with high temperature oxidation processes (copper and substrate heated to carefully controlled temperature in a nitrogen atmosphere containing about 30 ppm oxygen under conditions this, the successful copper-oxygen eutectic form binding both to copper and oxide is used as a substrate). The upper copper layer may be formed before shooting or chemically etching using a printed circuit board technology to form electrical circuits, while the lower copper layers are usually kept plain. The substrate attaches to the heat spreader by soldering the bottom copper layer onto it.

Ceramic materials used in DBC include:

• Alumina (Al ₂ O ₃ ), which are widely used because of their low cost. However this is not a really good thermal conductor (24-28 W/mK) and fragile.
• Aluminum nitride (AlN), which is more expensive, but has a much better thermal performance (& gt; 150 W/mK).
• Beryllium oxide (BeO), which has good thermal performance, but is often avoided because of its toxicity when the powder is digested or inhaled.

One of the main advantages of DBC substrates is its low heat expansion coefficient, which is close to silicon (compared to pure copper). This ensures good thermal cycling performance (up to 50,000 cycles). The DBC substrate also has good electrical insulation and good heat dissipation characteristics.

Related techniques use seed layers, photoimaging, and then additional copper coating to allow fine lines (as small as 50 micrometers) and through-vias to connect the front and rear sides. These can be combined with polymer-based circuits to create high density substrates that eliminate the need for direct connection of power devices to the heat sink.

Maps Power electronic substrate

Substrate brazed metal active

Another technology to attach thick metal coatings to ceramic plates is the AMB (active metal) technology. With this process the metal foil is soldered to the ceramic using ales solder paste and high temperature (800 Â ° C - 1000 Â ° C). The process itself requires a vacuum. Therefore, although AMB is electrically very similar to DBC, it is only suitable for many small productions.

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Insulated metal substrate

Insulated metal substrates (STIs) comprise a metal base plate (aluminum is generally used because of its low cost and density) covered by a thin dielectric layer (usually epoxy-based layer) and a copper layer (35 μm to more than 200 μm thick). The FR-4-based dielectric is usually thin (about 100 m) because it has poor thermal conductivity compared to the ceramics used in the DBC substrate.

Due to its structure, the IMS is a one-sided substrate, which can only accommodate components on the copper side. In most applications, the base plate is attached to the heatsink to provide cooling, typically using thermal grease and screws. Some IMS substrates are available with copper plates for better thermal performance.

Compared to classic printed circuit boards, IMS provides better heat dissipation. This is one of the simplest ways to provide efficient cooling to surface mount components.

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Other substrate

• When an electrical device is connected to the correct heatsink, no thermally efficient substrate is needed. Classic printed circuit board (PCB) material can be used (this method is usually used with hole technology components). This also applies to low-power applications (from milliwatts to several watts), since PCBs can be upgraded thermally by using thermal vias or track widths to increase convection. The advantage of this method is that multilayer PCBs allow the design of complex circuits, whereas DBC and IMS are mostly one-sided technologies.

• Flexible substrate can be used for low power applications. When they are built using Kapton as a dielectric, they can withstand high temperatures and high voltages. Their intrinsic flexibility makes them resistant to thermal cycling damage.
• Ceramic substrate (thick film technology) can also be used in some applications (such as automotive) where reliability is of the utmost importance. Compared to DCB, thick film technology offers higher levels of design freedom but may be more cost-effective.

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References

• The thermal performance of STi, DBC and thick film substrate is evaluated in Thermal analysis of high power module Van Godbold, C., Sankaran, V.A. and Hudgins, J.L., IEEE Transactions in Power Electronics, Vol. 12, N Â ° 1, Jan 1997, pages 3-11, ISSN 0885-8993 [4] (limited access)

Source of the article : Wikipedia