IGBT Thermal Management

Figure 1 IGBT Baseplates for Traction and Power Conversion.

Insulated Gate Bipolar Transistors (IGBTs) are used in power conversion applications where AC is converted to DC and DC to AC.  Applications include traction (Trains, Subway Systems),  Power Generation (Wind Energy, Hydroelectric, Power Conditioning) and also in hybrid electric vehicles where power is generated by conventional motors, and power is recovered from breaking.  Hybrid Electric applications will be discussed elsewhere.  

Figure 2 Schematic of IGBT Module Assembly.

These power IGBT applications generate a lot of heat, so heat and thermal dissipation are critical to the reliability of the the electronics.  AlSiC provides this thermal management for these applications with a CTE value that is compatible with the assembled electronics and substrate materials.  Examples of AlSiC IGBT bases are shown in Figure 1.

Figure 2 shows a schematic of the Power IGBT assembly.  The electronics (the actual IGBT) is attached to a metallize ceramic substrate which provides the electrical isolation needed and electrical connections.  The IGBT and ceramic substrate assembly is attached the the baseplate by soldering.  The baseplate provides the functional attachment of the IGBT assembly and electronics to a cold plate that will ultimately provide the heat sink for the thermal dissipation.   

Power IGBTs use either Aluminia (Aluminum Oxide) or AlN (Aluminum Nitride) that is metallized with a thin layer of copper (~300 µm).  The Copper is thermally applied by direct bonding process (often times this is referred to as direct bonding - DBC for direct bond copper).  

Alumina and AlN have a very low thermal expansion values of 6.7 and 4.5 ppm/C which are very compatible with the electronic IGBT silicon die (see material properties).  These materials are however compatible with materials like Copper and Aluminum that have CTE values of 17 and 23 ppm/C over the same temperature range.  AlSiC is compatible with DBC substrates with a CTE value of 8.0 ppm/C over the same temperature range.

Figure 3 Ultrasonic Image of Cu Baseplate @ 400 thermal cycles

To illustrate the need for CTE compatibility in power electronics systems the next set of pictures show ultrasonic images interrogating the solder interface between the DBC and the baseplate material contrasting Copper and AlSiC from a paper entitled "The new 6.5kV IGBT module: a reliable device for medium voltage applications" by Thomas Schuetze, Herman Berg, Oliver Schilling Aug 1, 2001.


In the case of the Copper IGBT baseplate the solder layer fails by delamination after about 400 thermal cycles (Figure 3).  The thermal dissipation path continues to degrade as the delamination continues.  As a consequence the electronic system is subjected to higher and higher thermal load that will ultimately cause the device to fail.

Figure 4 Ultrasonic Image of AlSiC  Baseplate @ 30,000 thermal cycles

For AlSiC IGBT baseplate there is no delamination even after 30,000 thermal cycles.  In power electronic systems this equates to long term reliability. 

AlSiC (200 W/mK) has a lower thermal conductivity than Cu (400 W/mK).  But as illustrated above in the Cu system the designer must consider stress compensation for these systems to manage the CTE difference between the Cu and the attached substrates.  Stress compensation layers increase the length of the thermal dissipation path, so the benefit of Cu high thermal conductivity may never be realized.  Often times the AlSiC system can be optimized in design to have very thin solder layers for a shorter thermal dissipation path.  As a result AlSiC baseplate solution can have equal or better thermal dissipation as compared to Cu baseplate solutions.

Long term reliability is very important in systems where failure is not an option or where replacement of the IGBT can be expensive. 

Windfarm in Iowa - changing out IGBTs is costly - there is a lot of corn in this picture.

please visit the CPS website www.alsic.com for more information.