How Thermal Ground Plane and Compact
Air-Cooled Heat Sinks are Revolutionizing Thermal Management

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By Nelson J. Gernert


Electronic devices used in military and aerospace equipment are subject to temperatures ranging from -40 degrees to 100 degrees Celsius -- and in radar applications, even as high as 150 degrees C. These temperatures tend to drift higher as designers increase device density by putting more processing power inside a smaller package. As a result, device designers are looking for advanced, cost-effective thermal management and heat rejection technologies that can improve performance while reducing size and weight.

Two recently developed technologies -- a planar, two-phase heat pipe known as a Thermal Ground Plane (TGP) and compact, high performance air-cooled heat sink -- that can be combined in a compact assembly to overcome the constraints encountered with conventional thermal management methods.

 



Designing New Devices for More Power and Control

Increases in power and control have revolutionized the level of performance obtainable in semiconductor devices such as amplifiers, FPGAs, lasers, microprocessors, graphics processors, thyristors, insulated gate bipolar transistors (IGBTs), symmetric gate commutated turnoff thyristors (SCGTs), silicon controlled rectifiers (SCRs) and intelligent power modules (IPMs). But the most demanding arena for designers in modern electronics is the world of military applications, where electronics must not only operate flawlessly in harsh environments but also meet demanding Size, Weight and Power (SWaP) constraints in current and future generation equipment.

For example, naval onboard power conversion systems and aerospace power electronics offer a variety of harsh thermal and environmental challenges: rugged environments involving exposure to salt water, heavy weather, extreme temperatures and vibration; extremely high power loads (sometimes in excess of 3,000 W); high heat fluxes due to concentrations of electronic components. The technical challenge, however, is that increases in power and decreases in size also require designers to deal with tremendous increases in heat generation and heat fluxes, all within the context of smaller and smaller areas for thermal solutions. Thus, the combination of smaller footprint (thanks to increased miniaturization) and greater power makes life much more difficult for thermal engineers who need to develop solutions that protect the life and performance of these high-performance devices.

Of course, the challenge of increased heat loads doesn’t involve just the amount of space available for thermal solutions. The configuration and the location of powerful electronics systems also are factors that make solutions more difficult.

As a result, design engineers need to consider the following thermal solution constraints:

• Limited space availability due to packaging and other space competing electronic components nearby

• Operation in oddly shaped or situated spaces

• Near maintenance-free operation (in satellites, for example)

• Thermal solutions that can operate independently of gravity

• Resistance to shock and vibration and environmental corrosion

• Quiet operation

• Avoiding drawbacks associated with liquid-based cooling (pipes, storage of chemicals, short circuit potential, ongoing or continuous maintenance is required, high cost, etc.)

Conventional Materials Face Thermal Management Challenges

Cooling of electronic components has traditionally been performed using simple air-cooled (forced or natural convection) metallic conductors in the form of extruded heat sinks and heat spreaders. Typically, aluminum or copper extrusions are bonded to the electronics case and air is blown over the extrusion using a fan or blower.

Passive two-phase heat transfer devices such as heat pipes are another thermal management technology often used in military and aerospace applications because of their thermal efficiency, simplicity and reliability. Heat pipes employ three components: a vacuum-tight containment shell or vessel, a working fluid, and a capillary sintered-powder metal wick structure. Without moving parts, the wick generates a capillary action that circulates a heated liquid from the evaporator side to the condenser side of the heat pipe where heat is ejected. Although heat pipes provide effective heat removal over long distances and are bendable, flexible, and withstand high-g conditions, shock/vibration and freeze/thaw cycles, conventional heat pipes also have thermal transport and geometric limitations.

Another challenge for conventional heat pipes is harmonizing the 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.

Another cooling challenge for heat sinks is achieving adequate air side heat transfer directly at the electronic component level. Factors that limit the effectiveness of air as a cooling medium include: lack of available volume for the heat sink, required surface area is too high and the pressure drop is too large.

The alternative to air cooling is a pumped-liquid or refrigerant-based cooling system. But this solution creates its own set of challenges by adding complexity, weight and the potential for failure — a major issue where heat sink maintenance is difficult or impossible. Adding components such as pumps, compressors, nozzles and other liquid cooling components can also significantly increase cost.



TGP and Compact, High Performance Air-Cooled Heat Sinks: Extending the benefits of passive thermal technology

To overcome these challenges, a new form of heat pipe, known as a Thermal Ground Plane (TGP), was developed employing a planar geometry.

The heat-pipe TGP, uses the same reliable components and sintered powder metal wick structure of a regular heat pipe. It is a passive heat transfer device using a two-phase cooling approach. But instead of a cylindrical shape, the form is a thin, planar structure known as a vapor chamber — sometimes referred to as a flat heat pipe — that makes an ideal substrate for mounting electronic devices. Unlike a cylindrical heat pipe, the TGP spreads heat laterally due to its geometry. Moreover, the TGP accomplishes longer heat transport distances and wider area of spreading, both of which increase the effective thermal conductivity.


Figure 1: Thermal Ground Plane - A planar heat pipe in 30mm x 30mm x 3mm thick dimensions


Depending on the application, a heat-pipe TGP can be designed to match the CTE of the device while offering significantly higher thermal conductivity. Eliminating CTE disparity increases product reliability by reducing junction temperature and thermal stress in the bonding material. Ultimately, the value of a TGP is determined by its ability to provide higher performance when compared head-to-head with solid conductors of equivalent size and CTE. The TGP was tested against the standard CuMo substrate. The heat input area was 5mm by 5mm placed in the center of the TGP. The back side of the TGP unit shown in Figure 1 was cooled by a liquid cold plate. The comparative test results are shown in Figure 2. In the tested configuration, the TGP offers a peak effective isotropic thermal conductivity greater than 1200W/mK, which is superior to that of all known composites and comparable to mid-grade polycrystalline diamond (Figure 7), which also exhibits a significantly lower CTE. It achieves this performance using low cost materials and fabrication techniques, while also retaining key attributes required for use in precision electronics packaging applications.


Figure 2. Comparative performance of Radio Frequency Thermal Ground Plane and a homogeneous CuMo heat spreader of equivalent dimensions vs. one dimensional steady state (1DSS) heat input for varying coolant temperatures. Note that Resistance R2 includes the heat spreader and interface to the instrumentation block directly below the heat spreader and that all TGP tests used a gap pad at that interface.


As shown in Figure 3, the TGP can be tailored to closely match the CTE of various semiconductor materials, including silicon (Si), aluminum silicon carbide (SiC), gallium arsenide (GaAs), and gallium nitride (GaN) and so on. Depending on the size of the TGP, the increased thermal conductivity can be upwards of 3 to 10 times higher than the conventional approach. This increased conductivity means the electronics can be designed to handle increased power, a desirable differentiator for future product designs. The TGP allows the designer to effectively spread the heat but the next portion of the thermal circuit is to dissipate this increased heat. This is where the Compact, High Performance Air-Cooled Heat Sinks heat sink is applied.


Figure 3. Electronic Packaging Materials Coefficient of Thermal Expansion versus Thermal Conductivity.


Advanced Air-Side Cooling is a heat rejection method that replaces complex forced-air or liquid-cooling technologies with a very efficient, but less-expensive air-cooled heat pipe-vapor chamber combination based heat exchanger. Compact, High Performance Air-Cooled Heat Sinks is a passive heat sink design in a 3-dimensional configuration comprised of a thin (~2mm) vapor chamber heat-pipe base that enables vapor flow to spread heat laterally — plus planar, vertical heat-pipe blades or fins that are integral to the base. This is an isothermal solution…meaning that heat is transported from either a single or several discrete electronic elements equally to all heat sink surfaces. The heat sink base provides heat input, allowing the vapor to transport heat from the base to the fin tips. Several examples of these advanced heat exchangers are shown in Figure 3.


Figure 4: Compact, High Performance Air Cooled Heat Sinks are available for insertion into four chassis cooling formats, 3U through 6U, as well as customized configurations, providing thermal uniformity and low thermal resistance.



Compact, High Performance Air-Cooled Heat Sinks

This family of heat sinks are available in 3U, 4U, 5U and 6U configurations for cooling a variety of commercial and military applications. Table 1 provides dimensional and air flow rate information for each configuration. The fans selected are the EBM-Papst 8212 JH3, 80 mm, and the EBM-Papst 3212 JH3, 90mm; however, other fans can be used.



In most applications, these heat sinks would be configured as shown in Figure 4. Included with the design are: shroud enclosure, axial fan, mounting bracket, screws and springs that apply the proper mounting force to assure contact with the electronic components being cooled. They share a common Thermal-Base®, but have increasing heat pipe blade lengths, sized to maximize cooling capacity but fit into the 3U to 6U electronic enclosures.


Figure 5. Front and Back View of the Compact High Performance Heat Sink




Heat Sink Thermal Performance

Each of the Compact, High Performance Air-Side Heat Sinks were tested. Figure 5 is a photograph of the typical test setup. Because the size of the heat input footprint varies from customer to customer, the heat sinks were tested with two heat input sizes: Small and Large.


Figure 6. High Performance Heat Sink Test Setup


Small (30mm x 30mm) Heater Block Description: Figure 6 is a photograph of the small heater block. This configuration is typical for simulating heat coming from a high power electronic component such as a computer micro-processor unit (CPU) or a graphic-processor unit (GU). There are nine spring loaded thermocouples that penetrate through the heater block to measure the temperature of the surface of the heat sink directly in the heat flow path.


Figure 7. Small Heater Block (30mm x 30mm)


Large Heater Block Description: Figure 7 is a photograph of the large heater block. This heater block puts heat into the entire base of the heat sink. It simulates heat coming from a large bed of electronics. The heater block was constructed of aluminum fitted with ten 100 watt cartridge heaters. The heater block is equipped with 6 spring loaded T-type thermocouples that are in contact with the bottom surface of the heat sink surface, in the heat flow path. This provides a temperature measurement which is indicative of the probed surface. The six thermocouple readings are monitored, displayed, and recorded on a PC. Thermal grease is applied to heat input surface of the heat sink and clamped down with uniform pressure on the heater block.


Figure 8. Large Heater Block (2.5" x 3.65")


Small Heater Block Test Results: Figure 8 is plot of the thermal resistance, °C/Watt, of the 3U, 4U, 6U with 1 fan, and the 6U with 2 fans, using the 30 mm x 30 mm heat input area. Thermal resistance was calculated by:

Thermal Resistance = Tsurface Temperature – Tinlet Air Temperature/ Input Power

Using the 30mm x 30mm heater, Figure 8 shows the units were tested to a maximum of 300 Watts. The 300W heat input was a limit of the heater block and not a limit of the heat sinks. The thermal resistance was relatively constant for all size heat sinks up to the tested 300 Watts. The test results indicate that all the heat sinks were less than 0.05oC/W thermal resistance.

Large Heater Block Test Results (Vertical): Figure 9 shows thermal images of the heat sink during testing. Notice how isothermal the unit is when operating. Using the 5.5cm x 9.5cm heater, the heat exchangers were tested to a maximum of 1250 Watts, the maximum capacity of test bed heat source. The six thermal couples on the heat input surface we averaged to get the overall surface temperature. Heat sink thermal resistance was calculated as follows:

Thermal Resistance = TAverage surface Temperature – TAverage inlet Air Temperature/ Input Power

Figure 10 indicates that the thermal resistances were relatively constant for all size Heat Sink’s up to the 1250 Watts. Based on the data trends, it is expected these units can handle power as high as 2000 Watts. All the heat sinks registered thermal resistance values in the 0.015 to 0.032C/W range.


Figure 9. Thermal Resistance of the Heat Sink for the 30mm x 30mm heater block tested Vertical



Figure 10. Thermal images of the Heat Sink as it starts up. Bottom image shows isothermal conditions.



Figure 11. Thermal Resistance of the Heat Exchanger for the 95mm x 55mm heater block tested vertical


Heat Exchanger Design and Performance Summary


Table 2 summarizes the collected information on the heat exchangers.


Integrating TGP and Compact, High Performance Air-Cooled Heat Sinks together provides superior thermal management

As demonstrated above, TGP and Compact, High Performance Air-Cooled Heat Sinks technologies can be combined to provide significant product application advantages. The heat-pipe 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 Compact, High Performance Air-Cooled Heat Sinks heat sink is then attached onto the TGP using a mounting method that supplies suitable clamping force. Because the TGP and Compact, High Performance Air-Cooled Heat Sinks material can be customized to have the same CTE, the entire assembly creates a circuit with the same thermal properties throughout. Matching CTE improves heat transfer effectiveness dramatically by allowing the heat flux in the small device area to spread from the TGP to the Compact, High Performance Air-Cooled Heat Sinks surface area with minimal thermal resistance.

The TGP structure is effective up to 350 W/cm2 heat input. The Compact, High Performance Air-Cooled Heat Sinks heat exchanger provides ultra-low thermal resistance down to 0.02°C/W. Together, the TGP and Compact, High Performance Air-Cooled Heat Sinks combination can handle heat loads from 250 to 2,000 Watts, with custom solutions available.

In summary, TGP uses a sintered powdered wick structure and two-phase vapor chamber as an enhanced heat spreader. Compact, High Performance Air-Cooled Heat Sinks uses a vapor chamber with 3-dimensional active surfaces as an enhanced heat sink. Integrating both in an assembly with a CTE that matches the CTE of the device not only removes bond stress, but minimizes thermal resistance. The result is a thermal management solution that overcomes challenges in size, weight, cost, reliability, and thermal efficiency to meet the requirements of today's demanding avionics, vetronics, radar, medical electronics and computer/server microprocessors applications.

 

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