Direct Cooling of Power Modules Using Microchannel Structures

As integration and switching rates increase rapidly, the concentration of heat losses is similarly changing. The continuous trend to shrink die size steadily increases the amount of heat to be dissipated. Raising chip operating temperatures is limited by reliability. High-power modules require excellent thermal performance and dependability, therefore adequate cooling is critical to reliable operation. One solution is to use a microchannel copper structure.

High-power electronics generally comprise a number of silicon power dies soldered onto one or more substrates, which are usually direct bonded copper (DBC) metalized ceramics soldered onto a base plate ^[1]. The most common of these is a copper plate. The general buildup of a power module is shown in Fig. 1 ^[2]. Silicon-based power chips (e.g. IGBTs, MOSFETs, and diodes) are soldered to a ceramic-based substrate which is then soldered to a copper base plate. The other side of the copper plate is mounted to an air-cooled aluminum heat sink ^[3].

When space is limited, the module is mounted onto a liquid-cooled plate. Thermal grease is applied between the module's base plate and the heat sink/water-cooled plate. Thermal resistance of a power module system is the sum of each layer's resistance ^[4], which depends on the thermal conductivity and thickness of its material. Fig. 2 shows the thermal conductivity (left) and thermal resistance (right) of typical materials arranged in the order used for power modules ^[2]. As shown in the Fig. 2 thermal resistance chart, the air-cooled heat sink and thermal grease are the main thermal barriers, contributing the highest amount to the thermal resistance.

Much of the overall thermal resistance occurs between the back of the substrate and the heat sink. During operation, thermal cycling leads to thermal mechanical strain of solder interfaces, especially between the base plate and the substrate. Delaminating of the substrate from the base plate will reduce the reliability of the module. One way to overcome mechanical and thermo-mechanical problems while increasing thermal performance is by directly cooling the substrate backside using a microchannel cooling structure ^[1,3].

REDUCE/ELIMINATE THERMAL BARRIERS

In recent years, the thermal resistance of power modules has focused on ceramic isolation substrates. Materials such as aluminum nitride (AlN) and thin alumina were introduced, which decreased the substrate's thermal resistance to values similar to those of solder layers in the base plates. Further reduction, using much higher thermally conductive materials such as diamond, is typically not cost effective. Increasing the copper thickness in the substrate provides additional advantages from heat spreading, but this method reduces the disposable substrate area ^[2].

Further advantages can be achieved by directly attaching the substrate to the heat sink. This type of module is becoming more popular as performance needs grow. The base plate is eliminated along with the solder layer between it and the substrate. This only improves thermal performance when the devices are very densely packed.

Otherwise, the heat-spreading effect of the copper base plate compensates for the thermal resistance of the added solder layer. As shown in Fig. 2, the air-cooled heat sink is responsible for most of the thermal resistance, followed by the thermal interface material between the module and the heat sink. The best opportunity for improved thermal performance is eliminating these two items ^[2].

COLD PLATES

Assembling power modules on liquid-cooled cold plates is a well known method for increased cooling (Fig. 3) Many types of cold plates are available from various manufacturers.

The air-cooled heat sink is replaced by a cold plate to improve thermal resistance. In this way, the interface material has the largest thermal resistance. And, due to this high thermal resistance, there is no need to develop high-performance cold plates with high inner heat-exchange properties. Any improvement in the cold plate is negligible. Typical cold plates are simple, large channels in aluminum plates, or copper tubes pressed into aluminum slabs.

The simplest way to eliminate the thermal resistance of the grease between the module and the heat sink is to flow liquid directly to the backside of the module (Fig. 4). Heat transfer is limited by the area of the exchange surface and uncontrolled flow conditions ^[2].

Better heat-exchange rates can be achieved by jet impingement cooling. The heat-exchange area is still limited to the footprint of the module. To increase the heat-exchange area, the base plate can be produced with fins (Fig. 5). All of these methods are challenged with sealing the liquid cooling enclosure to the backside of the module.

LIQUID-FLOW COOLING IN THE BASE PLATE

To overcome the sealing problem, the cooling liquid flows inside the base plate (Fig. 6) ^[2]. The DBC substrate is directly joined to the liquid-cooled base plate, eliminating the thermal barrier of the solder layer between substrate and base plate. Inside the base plate are a number of posts running from top to bottom and connected by parallel wings.

Fig. 7 shows a 3D microchannel isolated (MCI) cooler ^[1]. Fig. 7a is a cooler section with a directly integrated DBC substrate. A number of copper layers are placed between two DBC substrates that are bonded to each other and to the substrates. The sealed inlet and outlet are placed on the substrate, which is opposite the power-circuit substrate ^[3]. Fig. 7b shows the MCI cooler with a solder-joint-integrated DBC substrate, which is soldered to the cooler ^[3].

Copper-to-ceramic and copper-to-copper direct bonding technology opens the possibility to manufacture complex microchannel coolers with high inner surfaces for excellent heat transfer ^[4]. The coolers are built up by stacking eight to 10 inner layers and top and bottom sealing layers — with inlet and outlet water openings. The stacking procedure of the inner layer consists of eight to 10 hexagonal-shaped copper layers bonded together ^[1]. The inner layer forms pillars from top to bottom and free wings into the liquid.

Fig. 8 shows thermal resistance as a function of water flow for four substrates. Substrates B and D are directly integrated and A and C are soldered on a copper cooler. For low flow rates, a strong dependency on water flow can be observed. The decrease at the start of the flow is followed only by a slight decrease for higher-volume flows, caused by the change of flow characteristics from laminar to turbulence. As expected, the lowest thermal resistances, (R_th), are achievable with integrated AlN substrates. But the difference between soldered substrates and integrated substrates is not very large ^[3].

Fig. 9 compares the thermal resistance of a standard module on a liquid-cooled metal plate with a thermal grease interlayer (Column 1), a module with a liquid-cooled microchannel base plate and alumina substrate (Column 2), and a module with an integrated AlN substrate (Column 3) ^[4]. The graph shows increased thermal performance using liquid-cooled base plates compared with standard modules mounted on a standard liquid cold plate with thermal grease.

The thermal resistance of power modules can be reduced using several measures. Using isolation substrates with higher thermal conductivity, lower-thickness copper is currently being introduced as standard in power modules.

The most substantial progress can be achieved by using liquid-cooled microchannel systems. Direct liquid cooling of power electronics can reduce the thermal resistance between a semiconductor chip junction and ambient. Liquid-cooled base plates have very low thermal resistances and allow stacking of power modules.

REFERENCES

1. Credle, K., Exel, K.., Meyer, A. and Shulz-Harder, J., New Generation of DBC Substrates for High Efficient Cooling of Power Devices, Proceedings of International Conference and Exhibition on Power Conversion and Intelligent Motion, PCIM ‘98, Nurenberg, Germany, 1998.

2. Schulz-Harder, J., Exel, K. and Meyer, A., Direct Electronic Cooling of Power Electronics Devices, CIPS 2006.

3. Schulz-Harder, J., Exel, K., Meyer, A., Licht, T. and Loddenkotter, M., Micro Channel Water Cooled Power Modules, PCIM 2000.

4. Schulz-Harder, J., Efficient Cooling of Power Electronics, Proceedings of the Power Conversion and Intelligent Motion Conference — China (PCIM - China), Shanghai, China, 2006.