HEAT DISSIPATION DESIGN GUIDE FOR HIGH POWER PCB

Whether using power electronic equipment, embedded system, industrial equipment, or designing new motherboard, we must deal with the problem of temperature rise in the system. Continuous high temperature operation will shorten the life of the circuit board, and even cause some key points of the system to fail. It is helpful to extend the service life of circuit boards and components by considering heat dissipation as early as possible in the design process.

Heat dissipation design starts with the estimation of operating temperature

Before starting a new design, it is necessary to consider the operating temperature of the circuit board, the working 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 a higher ambient temperature will keep it more heat, so it will operate at a higher temperature. Components that dissipate more power will require more efficient cooling methods to keep the temperature at the 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 temperatures of the components in the data sheet and the temperatures specified in important industry standards. Active and passive cooling needs to be combined with the correct board layout to prevent damage to the board.

Active cooling and passive cooling: which circuit board is suitable for you?

This is an important issue that any designer should consider. Generally, when the ambient temperature is much lower than the working temperature, the passive cooling effect is the best. The thermal gradient between the system and the environment can be large, forcing greater heat flow from your components and the board itself. With active cooling, even if the ambient temperature is higher, it can provide better cooling effect according to the active cooling system.

Passive cooling

Attempts should be made to minimize passive cooling of active components to allow heat to be distributed into the ground plane. Many of the active components include a heat sink at the bottom of the package, allowing heat to be dissipated through stitched vias to the nearby ground plane. These stitching through holes then extend all the way to the copper pad under the assembly. There are some PCB calculators that can be used to estimate the size of the copper pad required under the assembly.

Obviously, the copper pad under the assembly cannot extend beyond the edge of the actual assembly as this will interfere with the surface mount pad or through-hole pins. If a single pad does not reduce the temperature to the desired level, it may be necessary to add a radiator to the top of the device to dissipate more heat. Heat transfer pad or heat conduction paste can also be used to increase the heat flux into the radiator.

Evaporative cooling is another option. However, the evaporative cooling components are very bulky and therefore not suitable for many systems. If the system leaks or breaks, there will be liquid leakage across the board. At this time, the active cooling method can be used to provide the same or better cooling effect.

Active cooling

If you need to further reduce the temperature of active components such as FPGAs, CPU, or other active components with high switching speed, you may need to use fans for active cooling when passive cooling cannot solve the problem. The fan does not always run at full speed and may not even turn on. Higher temperature components and components that generate more heat require the fan to run faster.

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 the relevant components. A high-order harmonic switch is also generated at each of the AC drive switches. If a fan is used, the nearby cabling components will need to have adequate noise suppression / immunity.

Active cooling systems such as coolant or refrigerant can also be used to provide a large amount of cooling. This is an uncommon solution because it requires a pump or compressor to allow coolant or refrigerant to flow through the system. For example, a water cooling system is used to cool GPUs in high-performance game computers.

Some simple thermal design guidelines

The use of a ground plane below the signal path improves signal integrity and noise suppression, and can also act as a radiator. The assembly with a thermal pad extends the stitched vias down to the ground plane, which makes it easier for the formation to dissipate heat from the surface layer. Then, the heat generated in the trace on the surface is easily dissipated into the ground plane.

Wiring with high current, especially in DC circuits, will require a greater weight of copper in order to dissipate an appropriate amount of heat on the circuit board. This may require a wider routing than is normally used in high-speed or high-frequency equipment. Geometry affects the routing impedance of the AC signal, which means that you may need to change the stack so that the impedance matches the value defined in the signal standard or source / load component.

Be careful of thermal cycling in the circuit board, as repeated temperature cycling between high and low values can lead to stress accumulation in through holes and wiring. This can lead to tube rupture in through holes with high aspect ratio. Long time cycle can also cause trace delamination on the surface layer, thus damaging the circuit board.