High performance heat exchangers (coldplates) are used to remove heat from electronic components so that they operate in a specified temperature range that provides both performance and reliability. Use of better thermal interface materials (TIMs), coolants, high performance coldplate designs and technologies have been investigated at the new Thermal Management Test Facility of the EMPF.

Coldplate designs can affect all four parameters (A, L, k and DT) of the heat transfer equation (see equation 1). By manufacturing coldplates using different technologies but designed to fit the specific application being tested, the cooling technologies can be directly compared and contrasted. In addition, by varying the different TIMs on the same coldplate, their relative effectiveness can be determined.

Fourier’s Law states that the flow of heat is proportional to the temperature gradient and the cross sectional area normal to the heat flow direction.

For a one dimensional heat flow at steady state, this can be expressed as:

The thermal conductivity (k) is an intrinsic property of how the bulk material internally conducts heat. It is not dependent on the size or shape of the material and more importantly, does not include any effects at the thermal interface.

Thermal resistance (R) is not an intrinsic material property and should be determined for each configuration according to this equation:

Thermal resistance more accurately predicts thermal performance than thermal conductivity since it does not ignore effects at the thermal interface.

When two surfaces are mated under pressure, the contact is not perfect even for highly polished flat surfaces. As shown in Figure 3-1, surface irregularities prevent intimate contact of large areas between the mating surfaces. Solid contacts only occur between the high points of the two mating surfaces, leaving a large number of voids between the low lying areas. Most of the heat transfer takes place via these solid contact points, but is restricted since the contact areas are very small. Heat transfer also occurs through the air entrapped in the irregular voids but is extremely low since the thermal conductivity of air is very low compared to the metals that are in direct contact. In order to eliminate the air gaps and improve thermal transfer, a thermally conductive material is used. This material conforms to the surface hills and valleys and displaces the air, providing more area for heat to flow and reducing the thermal resistance of the interface.

The thermal resistance at the interface (RI), can be significant when compared to the overall bulk resistance of the two mating bodies and therefore provides a barrier to increasing the heat transfer rates. This thermal resistance between two heat conducting surfaces depends on several factors such as:

• Geometry/flatness
  
• Surface finish of mating surfaces
  
• Hardness
  
• Modulus of elasticity
  
• Contact pressure
  
• Thermal conductivity
  
• Length of heat conducting path

As can be seen from the earlier heat flow equation (1), there are four primary parameters that need to be affected to enhance the conducted heat flow rate of any system. These are k, A, DT, and L.

Affecting the values of these four parameters provides an improvement in the heat transfer and subsequent removal process. Once a suitable material with high thermal conductivity is selected, the other three parameters can be varied to enhance the heat removal rate.

To increase the effective area of heat transfer (A), the voids created by the imperfect surfaces, as depicted in Figure 3-1, must be filled with suitable highly conductive materials known as thermal interface materials (TIM). Many different approaches have been adopted by the industry to fill in these voids. Thermal greases, soft metal films, phase change metal alloys (PCMA), soft metal plating, metal filled viscous compounds, and better machining and surface finishing techniques are some of the commonly adopted approaches.

Heat flow can also be enhanced by increasing the value of DT, the temperature difference between the heat source (T1) and the heat sink (T2). Since the temperature of the heat source is dictated by the electronic device performance, the only choice left for designers is to decrease the heat sink temperature. Several methods are used in the industry to lower the heat sink temperature. The suitability of any method is dictated by the application’s unique requirements. These could be cost, suitability of cooling material, and availability of cooling materials. The coldplate technology’s mechanical design, the heat exchange mechanism between coolant and the metal plate, and the type of liquid coolants are some of the many commonly used approaches to lower the heat sink temperature. Many suppliers have proprietary technologies of coldplates that use these parameters.

Another approach to enhance the overall heat flow rate is to reduce the overall heat transfer path length (L). This can be achieved by bringing the coldplate as close to the heat source as possible.

These three parameters of the heat transfer equation can be investigated using the EMPF’s Thermal Management Test Facility. Since it is cost prohibitive and time consuming to perform laboratory testing of all possible combinations and parameters for all configurations of TIMs, coolants and coldplate designs, thermal analysis modeling tools can also be used as a filtering tool to reduce the number of possible solutions to a selected few by performing “what if” analysis. Once the screening is completed, the prime thermal management candidates can be tested and evaluated in the EMPF facility.

The facility provides thermally controlled, 20 micron filtered coolant between 5°C and 40°C at flow rates from 0.5 gallon per minute (gpm) to 3 gpm into an insulated chamber. The chamber is lined with a 1" rigid foam to reduce the effect of outside temperature and to monitor any thermal change in the inside environment. The coolant passes through a flow meter, an external heatable zone, a pressure gauge, and dual thermocouple before entering the coldplate. The heatable zone allows a final adjustment of temperature using the water input temperature with a feedback controller. After exiting the coldplate, the coolant passes through another thermocouple and pressure gauge before returning to the chiller.

In our studies, three different coldplate designs, several TIMs, and two coolants were experimentally tested and performance analyzed. A copper tube coldplate with three heat spreaders, nine planar
resistors, fourteen thermocouples, and six electrical bus connectors is shown in Figure 3-2. The three zones can be connected to the two power supplies to provide even or uneven heating of up to 3600 watts across the coldplate.
As many as 20 thermocouples placed throughout the setup, provide detailed thermal information that is collected using a data acquisition system (see Figure 3-3).

The data obtained can be used to validate and adjust the thermal model (see Figure 3-4) to more accurately define the thermal performance of the system.

Different high performance coldplate designs and technologies have been investigated at the EMPF’s Thermal Management Test Facility, including some combination of thermal grease, soft metal film, PCMA, and innovative coldplate designs with foamed graphite, pin-fin, and copper tube liquid cooling technology. Figure 3-5 provides a graphical display of one test condition (3500w even, 0.5 gpm flow, 25°C input coolants) with a variety of coldplates, TIMs, and coolants.

From this data, we can clearly see performance differences. In design A, a large difference in temperature exists across the three zones (where zone #1 is closest to the input coolant and zone #3 is furthest away). This indicates an unequal flow of the coldest coolant across the plate. By redesigning the interior flow channels of the coldplate, this condition should be corrected. The design B coldplate showed similar performance temperatures without the wide differences between zones. With the same input temperature, flow, and TIM, the copper tube coldplate resulted in the highest temperatures at the coldplate surface.

By varying the TIM on the same coldplate and conditions, testing shows the thermal grease and the soft metal foil performed similarly. Final selection of TIM may include price and the cost/time for applying the TIM.

In addition, the coolant was varied between pure water and a potassium formate solution using the same coldplate and TIM. The pure water easily performed better at 25°C. However, the potassium formate solution has a wider range of temperature use and would certainly be required if sub-zero coolant was needed.

By using this thermal test facility at the EMPF, a wide array of cooling technology can be investigated and performance evaluated before committing to a final design. Modeling can be performed and the operating parameters specific to an application can be tested. Contact the Helpline at 610-362-1320 with any electronics manufacturing-related questions.