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Wide band gap (WBG) semiconductors are semiconductor materials that allow higher voltages, higher frequencies and operation at higher temperatures over conventional electronic materials, thereby allowing higher performance electronics to be constructed. The Defense Advanced Research Projects Agency (DARPA) understood the advantages of using WBG in aerospace and defense applications but also understood that thermal management was going to be a significant technical challenge to achieving the full potential of WBG technology. Accordingly, to overcome these and other challenges, DARPA sponsored the development of a new form of heat pipe, known as a Thermal Ground Plane (TGP), to effectively cool WBG devices.

Figure 1. TGP mounted in an Electronic Package.

The uniqueness of the TGP is that it integrates directly into the electronic package to effectively spread the concentrated heat to significantly reduce the internal thermal resistance (junction- to-case, θj-c), thereby enabling reliable module operation at higher currents. An example of how to apply the TGP is shown in Figure 1. TGPs are used in two regions inside the electronic package to improve heat removal. Region 1 involves inserting a TGP directly under the WBG devices. Region 2 involves inserting another larger TGP under the dielectric substrate to spread the heat evenly to the baseplate and ultimately to the coolant.

Conventional Heat Pipe Characteristics

Like all heat pipes, a TGP is a passive two-phase heat transfer device. Heat pipes typically employ three components: a vacuum-tight containment shell or vessel, a working fluid and a porous sintered-powder metal wick material that pumps the working fluid by capillary action.

State-of-the art wick structures are typically fabricated by sintering relatively large copper particles (10s to 100s of microns) into a structure inside the heat pipe. Particle size and sintering conditions determine wick characteristics, all of which determine thermal resistance and heat transport capacity.

Heat pipes transfer heat more efficiently and evenly than solid conductors such as aluminum or copper because of their lower total thermal resistance. Inside the heat pipe is a small quantity of working fluid (a variety of fluids can be used including water, acetone, or methanol) that functions as the cooling medium. Heat is absorbed by vaporizing the working fluid. The vapor transports heat to the condenser region where the vapor is condensed, releasing heat to a cooling medium. The condensed working fluid is returned to the evaporator region using the capillary action of the sintered wick structure. Since there are no moving parts, the TGP is reliable and operates continuously.

Figure 2. Thermal Ground Plane (TGP)

The physical and thermal characteristics of heat pipes offer several advantages for aerospace and defense applications. For example, heat pipes provide effective heat removal over long distances and withstand high-g conditions, shock and vibration, as well as freeze/thaw cycles.

Utilizing conventional heat pipes in military and aerospace systems is not new. The heat pipes used today are typically cylindrical in geometry and many inches long. In comparison, the TGP is planar and very small, typically 1" by 1" and .04" thick as shown in Figure 2. Furthermore, conventional heat pipes present challenges in harmonizing the coefficient of thermal expansion (CTE) between the heat source material and the heat sink material. If there are disparities in CTE between materials, then temperature changes will cause the devices to expand and contract, which create mechanical stresses on the interface bond that will eventually induce thermal failure. Using a solid conductor at the interface, like diamond, can improve thermal conductivity and CTE matching, but it is very expensive.

Developing the Optimum Heat-Pipe TGP Design

WBG devices generate a tremendous amount of heat in a small area, which makes keeping them cool a difficult problem. In many applications, solid copper is used under the WBG devices to spread the heat. This copper is typically attached to a dielectric substrate, which is placed on top of a copper or a ceramic such as an AlSiC baseplate. The heat must flow through the solid copper, the substrate and the baseplate before it enters the cooling medium. To interface with the electronic materials, the TGP was developed employing a planar geometry instead of a cylindrical shape.

Like any heat pipe, a TGP is a passive heat transfer device. It uses a two-phase cooling approach and the same reliable components and sintered powder metal wick structure of a regular heat pipe. But instead of a cylindrical shape, it uses a thin, planar structure known as a vapor chamber — sometimes referred to as a flat heat pipe. This form factor makes it an ideal substrate for mounting electronic devices.

Figure 3. Comparison of CTE of various electronic materials to the thermal conductivity. The TGP has an effective thermal conductivity that is 5 times higher.

Unlike a cylindrical heat pipe, the TGP planar geometry spreads heat laterally. Moreover, the TGP accomplishes longer heat transport distances and wider area of spreading, both of which increase the effective thermal conductivity.

DARPA-sponsored research determined the specific design characteristics required by thin (approximately 1 mm) vapor chambers. At these small dimensions, the vapor phase pressure drop can become a significant limitation for transport capacity. To overcome these limitations, novel fabrication approaches were developed that provide thinner wicks with greater ranges of microstructural control to enhance thermal performance.

Minimizing CTE Disparity

As a result of these developments, the heat-pipe TGP is designed to match the CTE of the device while offering significantly higher thermal conductivity.

In practical application, a TGP is tailored to closely match the CTE of various semiconductor materials, including silicon (Si), aluminum silicon carbide (AlSiC), gallium arsenide (GaAs) and gallium nitride (GaN). Eliminating CTE disparity increases product reliability by reducing junction temperature and thermal stress in the bonding material.

Figure 4. Module Temperature Comparison as TGP is Inserted (160oC down to 97.3oC).

As a result, the TGP can easily be combined with high performance, compact air-cooled (CAC) heat sink technologies to provide significant product application advantages. The TGP directly attaches to the electronic component, with the TGP substrate selected to match the CTE of the bond or interface material to minimize junction temperature and thermal bond stress. The CAC heat sink is then attached to the baseplate using a mounting method that supplies suitable clamping force.

Because the TGP and CAC heat sink material can be customized to have the same CTE, the entire assembly creates a circuit with the same thermal properties throughout. With closely-matched CTEs, heat-transfer effectiveness is dramatically improved. The heat flux in the small device area spreads from the TGP to the CAC heat sink surface area with minimal thermal resistance.