PCB Blog - High-power PCB board heat dissipation design guide

Whether you are using power electronics on PCB board, embedded systems, industrial equipment, or designing a new motherboard, you have to deal with rising temperatures in your system. Continued high temperature operation can shorten the life of the circuit board and may even cause failure at some critical points in the system. Considering heat dissipation early in the design process can help extend the life of your boards and components. Thermal design starts with estimating operating temperature Before starting a new design, consider the temperature at which the board will be operating, the environment in which the board will operate, and the power dissipation of the components. These factors work together to determine the operating temperature of the board and components. This will also help with custom cooling strategies. Putting the board in an environment with a warmer ambient temperature will allow it to retain more heat, so it will run at a higher temperature. 

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 a thermal management strategy, be sure to check the allowable operating temperature of the component in the data sheet and the specified temperature in important industry standards. Active and passive cooling needs to be combined with proper 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. Typically, passive cooling works when the ambient temperature is much lower than the operating temperature. The thermal gradient between the system and the environment can be large, forcing a large heat flow away from your components and the board itself. With active cooling, even if the ambient temperature is higher, it can provide better cooling according to the active cooling system. passive cooling Attempts should be made to reduce passive cooling of active components to levels that allow heat to spread to the ground plane. Many active components include thermal pads on the bottom of the package that allow heat to dissipate through stitched vias to a nearby ground plane. These stitched vias then extend all the way to the copper pads below the components. There are some PCB board calculators that can be used to estimate the size of the copper pads needed under the components. Obviously, the copper pads under the component cannot extend beyond the edge of the actual component, as this would interfere with surface mount pads or through-hole pins. If a single pad doesn't reduce the temperature to the desired level, you may need to add a heat sink on top of the unit to dissipate more heat. Thermal pads or thermal paste can also be used 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 fluid leaks across the board. In this case, active cooling may be used to provide the same or better heat dissipation. Active cooling If you need to further reduce the temperature of active components such as FPGAs, CPUs or other high switching speeds, active cooling with fans may be necessary when passive cooling does not solve the problem. The fans don't run at full speed all the time, and sometimes they might not even turn on. Hotter components and components that generate more heat require fans to run at faster speeds. The fan is noisy because the PWM signal will make some noise from switching. The board will need a circuit to generate a PWM signal to control the fan speed, and a sensor to measure the temperature of the associated components. AC driven fans with electronic switching controllers also generate radiated EMI at the fundamental switching frequency and at each higher harmonic. If fans are used, nearby trace components will need to have adequate noise suppression/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 move the coolant or refrigerant through the system. For example, a water cooling system is used in high-performance gaming computers to cool the GPU. Some Simple Thermal Design Guidelines Using a ground plane under signal traces improves signal integrity and noise rejection, and it also acts as a heat sink. Components with thermal pads can extend stitched vias down to the ground plane, which will make it easier for the ground plane to dissipate heat from the surface layer. 

The heat generated in the traces on the surface layer is then easily dissipated into the ground plane. Traces that carry high currents, especially in DC circuits, will need to have more copper weight in order to dissipate the right amount of heat on the board. This may require wider traces than those typically used in high-speed or high-frequency devices. Geometry affects the trace impedance of an AC signal, which means you may need to change the stackup to match the impedance to the value defined in the signal standard or source/load components. Beware of thermal cycling in circuit boards, as repeated temperature cycling between high and low values can cause stress to build up in vias and traces. This can lead to tube breakage in high aspect ratio vias. Prolonged cycling can also cause trace delamination on the surface layer, which can damage the PCB board.