Thermal design starts from estimating operating temperature
Before starting a new design, you need to consider the operating temperature of the circuit board, the operating environment of the circuit board, and the power consumption of the components. These factors work together to determine the operating temperature of the circuit board and components. This will also help to customize the cooling strategy. Placing the circuit board in an environment with a higher ambient temperature will keep it more heat, so it will run at a higher temperature. Components that dissipate more power will require more efficient cooling methods to keep the temperature at a set level. Important industry standards may specify the maximum temperature of components and substrates during operation. Before designing a thermal management strategy, be sure to check the allowable operating temperature of the components in the data sheet and the specified temperature in important industry standards. It is necessary to combine active and passive cooling with the correct circuit board layout to prevent damage to the circuit board.
Active cooling and passive cooling: Which one is right for your circuit board?
This is an important issue that any designer should consider. Generally, when the ambient temperature is much lower than the operating temperature, the passive cooling effect is the best. The thermal gradient between the system and the environment will be large, forcing a large heat flow from your components and the circuit board itself. Using active cooling, even if the ambient temperature is higher, it can provide a better cooling effect according to the active cooling system.
Attempts should be made to minimize passive cooling of active components so that the heat can be distributed to the ground plane. Many active components include thermal pads on the bottom of the package, allowing heat to be dissipated through stitched vias to nearby ground planes. These stitched vias then extend all the way to the copper pad under the component. There are some PCB calculators that can be used to estimate the size of the copper pad required under the component. Obviously, the copper pads under the components cannot extend beyond the edges of the actual components, as this will interfere with surface mount pads or through-hole pins. If a single pad cannot reduce the temperature to the desired level, you may need to add a heat sink to the top of the device to dissipate more heat. You can also use a thermal pad or thermal paste to increase the heat flux into the heat sink. Evaporative cooling is another option. However, evaporative cooling components are very bulky and therefore not suitable for many systems. If the system leaks or ruptures, there will be liquid leakage across the board. In this case, it is better to adopt active cooling method to provide the same or better heat dissipation effect.
Active cooling If you need to further reduce the temperature of active components such as FPGA, CPU or other high switching speeds, when passive cooling cannot solve the problem, you may need to use a fan for active cooling. The fan does not run at full speed all the time, and sometimes it may not even turn on. Components with higher temperatures and components that generate more heat require fans to run at faster speeds. The fan is noisy because the PWM signal will generate some noise due to switching. The development board will need a circuit to generate a PWM signal to control the fan speed, and a sensor to measure the temperature of related components. AC-driven fans with electronic switch controllers also generate radiated EMI at the basic switching frequency and each higher harmonic. If a fan is used, nearby wiring components will need to have sufficient noise suppression/interference immunity. Active cooling systems such as coolant or refrigerant can also be used to provide substantial cooling. This is an uncommon solution because it requires a pump or compressor to flow the coolant or refrigerant through the system. For example, water cooling systems are used in high-performance gaming computers to cool GPUs.
Some simple thermal design guidelines Using a ground plane under the signal traces can improve signal integrity and noise suppression, and it can also act as a heat sink. Components with thermal pads can extend the stitched vias down to the ground plane, which will make it easier for the ground plane to dissipate heat from the surface layer. Then, the heat generated in the traces on the surface layer is easily dissipated into the ground plane. Traces carrying large currents, especially traces in DC circuits, will need to have a larger copper weight in order to dissipate the appropriate amount of heat on the circuit board. This may require traces that are wider than those typically used in high-speed or high-frequency equipment. The geometry affects the trace impedance of the AC signal, which means you may need to change the stacking to match the impedance to the value defined in the signal standard or source/load component. Beware of thermal cycling in the circuit board, because repeated temperature cycling between high and low values can cause stress to accumulate in vias and traces. This can cause the tube in the high aspect ratio through hole to break. Long-term cycling will also cause delamination of traces on the surface layer, thereby destroying the circuit board.
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