PCB News - High power PCB heat dissipation design guide

Whether you're using power electronics, embedded systems, industrial equipment, or designing a new motherboard, you have to deal with rising temperatures in your system. Continuous high temperature operation will shorten the printed circuit board life, and even lead to the failure of some key points of the system. Consider heat dissipation early in the design process to help extend the life of the circuit board and components.

The heat dissipation design begins with estimating the operating temperature

Before starting a new design, you need to consider the temperature at which the board is running, the operating environment of the 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 customize cooling strategies.

Placing the circuit board in a higher ambient temperature will allow it to retain more heat, so it will operate at higher temperatures. Components that dissipate more power will require more efficient cooling methods to keep temperatures at set levels. Important industry standards may dictate the temperature of components and substrates during operation.

Before designing heat dissipation management policies, check the allowable operating temperatures of components in the data table and those specified in important industry standards. Active and passive cooling needs to be combined with the correct board layout in order to prevent damage to the board.

Active cooling vs. Passive Cooling: Which is right for your board?

This is an important question for any designer to consider. Usually, when the ambient temperature is much lower than the operating temperature, passive cooling effect. The thermal gradient between the system and the environment can be large, forcing greater heat flow away from your components and the board itself. Use of active cooling, even if the ambient temperature is higher, according to the active cooling system can provide better cooling effect.

Passive cooling

An attempt should be made to level the passive cooling of the active component to allow heat to be distributed to the ground layer. Many active components include heat pads located at the bottom of the package, allowing heat to be dissipated to the nearby ground formation through the sutured hole. These suture holes then extend to the copper pad below the assembly. There are PCB calculators that can be used to estimate the size of the copper pad required under the component.

Obviously, the copper pad below the component should not extend beyond the edge of the actual component, as this would interfere with the surface mount pad or through hole pins. If a single pad cannot reduce the temperature to the desired level, a radiator may need to be added to the top of the device to dissipate more heat. A heat pad or paste can also be used to increase the heat flux into the radiator.

Evaporative cooling is another option. However, evaporative cooling components are bulky and therefore n

ot suitable for many systems. If the system leaks or breaks, there will be fluid leakage throughout the plate. At this point, active cooling method may be used to provide the same or better cooling effect.

Active heat dissipation

If you need to further reduce the temperature of active components such as FPgas, CPUS, or other active components with high switching speeds, active cooling using a fan may be required when passive cooling does not solve the problem. Fans don't run at full speed all the time, and sometimes they may not even turn on. Hotter components and components that generate more heat require fans to run faster.

The fan is noisy, because the PWM signal will produce some noise due to the switch. The development board will need a circuit to generate A PWM signal to control fan speed, and a sensor to measure the temperature of the relevant components. An AC driven fan with an electronic switching controller also generates radiated EMI at the base switching frequency and at each of the higher harmonics. If a fan is used, the nearby wiring assembly will need to have adequate noise suppression/disturbance rejection.

Active cooling systems such as coolant or refrigerant can also be used to provide substantial cooling. This is an unusual solution because it requires a pump or compressor to flow coolant or refrigerant through the system. For example, water cooling systems are used to cool gpus in high-performance gaming computers.

Some simple thermal design guidelines

Using the grounding layer below the signal path improves signal integrity and noise suppression. It also acts as a heat sink. Assemblies with thermal pads extend the suture hole down into the ground layer, which makes it easier for the ground layer to dissipate surface heat. The heat generated in the surface trace is then readily dissipated into the subsurface.

Wires carrying large current, especially in dc circuits, will need to have a larger copper weight to dissipate the right amount of heat on the board. This may require a wider wire than is typically used in high speed or high frequency equipment. The geometry affects the wiring impedance of the AC signal, which means you may need to change the stack to keep the impedance matching the values defined in the signal standard or the source/load component.

Beware of thermal cycling in the circuit board, as repeated temperature cycling between high and low values can cause stress to build up in the through-holes and wiring. This can cause pipe breakage in through-holes with high aspect ratios. Prolonged cycling can also create trace layering on the surface layer, which can damage the board.