Understanding Heat Dissipation Techniques in PCBs

All Printed Circuit Boards dissipate heat during operation. It is important for a PCB designer to factor in the level of heat that a PCB will generate during operation and figure out how to best manage the dissipation of the heat generated. Depending on the application, different PCB boards will generate different levels of heat which need to be dissipated or managed efficiently.

Operation under constant high temperatures can reduce the board life and even lead to board failure. This is why thermal issues should be considered in the initial stages of designing and manufacturing of PCBs to help the designer to extend the shelf-life of the board as well as the components.

Estimation of Operating Temperatures

When starting a new PCB design, the designer should consider the operating environment of the PCB, the power dissipation of the components, the heat tolerance of the board and components. Based on this they need to select the right materials and plan component placement and traces on the board for efficient thermal management.

There is nothing wrong with a board heating up, some high power applications do generate high levels of heat. This heat just needs to be expelled without significantly impacting the performance or life of the PCB.

Component and material selection is one of the main factors that decides the thermal resistance between the PCB and the ambient environment. By properly selecting the components and material and their placement will help the components to dissipate the heat evenly on the board. Inappropriate trace width and copper weight of the trace can also raise the temperature of the board.

What are the PCB cooling techniques?

PCB cooling is broadly classified into two types – Passive and Active.

When the ambient temperature is lower than the operating temperature of the PCB, passive cooling methods can be used to dissipate heat from the components and board. Under this condition, the thermal gradient between the PCB and the ambient will be large, thus forcing larger heat flux away from the components and board itself.

Passive cooling uses natural convection to release heat such as a thermal pad or thermal paste . A thermal pad is usually placed beneath an active component to allow heat to be dissipated to the nearby ground plane or conducted to the bottom of the PCB through stitching vias (thermal vias). A heat sink can be added to the ground place to dissipate more heat if a single thermal pad is insufficient to bring down the temperature to the required level. To increase the heat flux into heat sink, the PCB designer can also use the heat sink with a thermal pad/ thermal paste.

Evaporative cooling is another passive method used to cool active components that generate a significant amount of heat. However, if the evaporative cooling system accidentally leaks or ruptures, it will result in fluid leaking all over the board. Hence, this method is not ideal.

The active cooling method relies on external devices to enable heat removal. This method is used when the passive cooling does not cut it. Most often a fan is used in active cooling. The fan helps to provide better cooling for the active components like FPGAs, CPUs, or other devices with high switching speeds. Fans are noisy due to PWM signals, also AC-driven fans with an electronic switching control will become the cause for radiated EMI. Hence, if a fan is used, nearby traces components should have enough noise rejection/immunity or even shieling in extreme cases.

The active cooling system can also utilize a coolant liquid or refrigerant to provide cooling. For example, water-cooled systems are used in high-performance gaming computers to cool GPUs, and other systems are available for high-speed CPUs. But this is an uncommon solution because it needs a pump or a compressor to move the cooling liquid or refrigerant through the system.