PCB Cooling Strategies, Part 2

Estimated reading time: 8 minutes

A cold plate radiator is a better solution to the heating issue, because the heated area is relatively large. It is also designed to strengthen the entire PCB, preventing external distortion. For designers who utilize CPCI or ATCA cards, it has a very good strengthening effect. Therefore, this cooling method is also known as conduction mode cooling, and is considered the military and industrial grade. In this layout design, you must consider the left and right screw holes and two bright copper areas. These are fixed areas of​ the cold plate. Since the cold plate will be close to the PCB board, the components need to be placed far away from this area.

Water cooling involves pumping the medium through a pipe to cool the board. During this process, the heat is transferred from the liquid cooling radiator to the environment. The number of fans can be reduced to minimize vibration and noise. Due to the high specific heat capacity of water, the effect of water cooling is much greater than air cooling. However, compared to air-cooled heat sinks, water-cooled heat sinks are more complicated and have a certain risk of leakage, and the overall cost is higher than other heat sinks. Water cooling is currently used in high-end applications in the thermal industry.

Fan cooling generally uses a turbofan, where the kinetic energy is converted into pressure through the scroll casing. When the impeller gas is discharged, the impeller pressure is lower than the inlet pipe pressure, causing the new gas from the pressure difference to be taken in by the impeller. The gas is continuously discharged from the fan, to better dissipate the heat from the chip.

With the rise of increasingly thin and light electronic devices, the power consumption and heat dissipation of electronic devices have increased correspondingly. Some lightweight cooling modules have been produced, and are commonly seen in laptop computers. Since laptops are constantly becoming more lightweight, their internal space has become greatly compressed. Now that the chip cooling environment is very limited, the use of exhaust fans and heat pipes are combined.

In PCB design, we must consider the temperature limits beneath heat sink and heat pipes, and ensure that we do not place temperature-sensitive components there. If placed to closely to high-temperature devices, it will cause damage to components.

There are three methods of heat transfer: conduction, convection, and radiation. The transmission between the CPU and radiator are primarily conduction and convection. Heat transfer from radiation is also present, but its role in the fan radiator is limited. These three cooling methods are not isolated, but work together.

We will now discuss how to use copper in the PCB structure to maximize the cooling as shown in Figure 11.

In Figure 11, we can see that the heat source of the components is dissipated using different methods. To maximize heat dissipation, it is most effective to connect the copper downward using through-holes to the inner plane, a common method used for vias and copper design. In summary:

• With high thermal components under the cooling pad, we recommend evenly placed through-holes. The heat dissipates through these vias to the inner and the bottom layers of the copper. 
• Try to connect components to the bottom of the cooling pad GND or other large copper area. Make use of the surface copper’s good heat dissipation.

We try to ensure that the through-hole and inner plane layer of copper foil are connected. We can use a separate piece of copper foil to enhance heat dissipation, as shown in Figure 12:

In Figure 13, we have used open spaces on the copper brazing area below the high-heat components to dissipate heat. This involves the use of the bottom of the steel net.

Conclusion

Thermal design should be studied throughout the PCB design process. We are constantly innovating to develop the best cooling capacity for our products. These processes often require the intervention of thermal simulation staff. The design will not only need validation and optimization, but also require information feedback. We must constantly adjust the design. Thermal management research still has a long way to go.

Bin Zhou is a senior PCB design engineer for EDADOC. He has 10 years of experience in high-speed design. His responsibilities include high-speed PCB design solutions, HDI, R&D and training.

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