We all understand how important it is to regulate heat. In fact, PCBs or printed circuit boards, like all other things on this planet, like to exist in a tight temperature range. According to PCB Trace Technologies Inc, designers must be careful when designing PCBs to disallow accumulation of heat in specific regions, as the accumulated heat could cause catastrophic failures over time.
Excessive heat is detrimental to performance and causes deterioration of components, with the possibility of causing personal injury. Therefore, this is a topic concerning every designer, who must actively work towards controlling heat in their circuit design.
Excessive Heat Damages Electronics
It is common knowledge that electronics requiring more power to operate will generate more heat. For instance, components like power transistors, microcontrollers, and voltage regulators will heat up as their load increases.
Manufacturers build electronic components for operating with a specified temperature range. Therefore, component damage may result from the surrounding temperature exceeding the upper limit. Even when there is no breakdown, exposure to excessive heat can negatively affect the performance of a component.
Exposure to excessive heat over a prolonged period can reduce the lifespan of a component. As a rule of thumb, the lifespan of a component decreases by half for every increase of temperature by 10 °C it faces. Being negligent in the utilization of techniques for PCB heat dissipation can lead to a situation of permanent component failture.
To avoid circuit malfunction or failure, designers must design their PCB to operate and function within safe temperature limits. While some electronic components will operate without requiring additional cooling, others may require thermal management with the help of additional heat sinks, cooling fans, or a combination of several cooling mechanisms.
When to Cool?
As a good practice, designers prefer to manage the temperature at component and system level while considering dissipating excess heat to the operating environment. For a cooling mechanism, they consider several factors such as package properties and its heat dissipative properties. The manufacturer’s data sheet usually provides the information.
When the heat generated in the PCB is on the low side, cooling by natural convection is adequate. However, as the heat generation in the PCB tends to rise, the excess heat removal may require the use of thick copper, fans, heat pipes, heat sinks, or a combination of several cooling techniques.
Identifying Thermal Problems in PCBs
There are many ways to identify potential thermal problems in PCBs. Typical approaches include the use of visual inspection, infrared cameras, and thermal analysis tools.
Visual Inspection
An easy way of finding signs of overheating, visual inspection, without the board carrying power, can detect dry joints, partly or fully burnt components, discoloration, arcing, etc. Visible signs can include burnt components, bulging components, discolored spots on the board. In addition to the visual inspection, heating issues on the board typically generate a burning smell.
Infrared Cameras
Use of IR or infrared cameras can evaluate powered boards for heat dissipation issues that are invisible to the human eye. These cameras can show areas where there is excess heat generation. It is also possible to identify defective or counterfeit parts with thermal signatures differing from those of genuine components.
With thermal imaging cameras, it is also possible to detect locations where tracks have insufficient solder. As these locations typically have higher resistance, they generate more heat.
Thermal Management
Designers use several techniques for thermal management or removing heat from components and PCBs. Most common mechanisms include using heat sinks, cooling fans, heat pipes, and thick copper. Most often, they must use more than one method for cooling circuts that generate high heat. For instance, cooling a laptop requires the use of heat pipes, heat sinks, and a fan.
Heat Sink
A heat sink is typically a thermally conductive metallic part with a large surface area. During assembly, the operator attatches a heat sink to hot components like switching devices and power transistors. The heat sink transfers the heat from the hot component for dissipating it to the surrounding air in contact with its large surface area.
Cooling Fans
A cooling fan helps to displace the hot air surrounding the heated surface of a component or its heat sink. As the fan removes the hot air, cooler air from the surroundings rushes in to take its place. The cooling fan helps by constantly replacing hot air by cooler air, thereby improving the flow of heat.
Heat Pipes
Suitable for compact devices with limited space, heat pipes use convection to remove heat from hot devices. Typically, a heat pipe contains a small amount of fluid such as nitrogen, acetone, water, or ammonia. While the fluid absorbs the heat, it turns into a hot vapor that moves up the pipe. On the other end of the pipe is a condenser, which helps to cool the hot vapor, returning it to its liquid form. The liquid then flows back through the pipe to take the place of the hot vapor, and the cycle continues.
Thermal Via Arrays
There are two ways a designer can attach a heat sink or a heat pipe to a hot component. The component manufacturer usually provides a heat sink tab on the component for the purpose. For some components, the tab is on the top part of the component, so it is possible to attach the heat sink on the top.
For other components, manufacturers provide the tab at the bottom of the component. While it is not possible to mount the heat sink directly to the component, the designer must provide a copper pad under the component, over which it sits when mounted on the board.
The designer typically places an array of vias in the copper pad and they connect to another copper pad on the other side of the PCB. These are thermal vias and they transfer the heat from the component to the second pad. It is possible to attach the heat sink to this second pad for effectively removing the heat.
Thick Copper Traces
Designers also use heavy copper on traces that must carry high currents. Thicker copper traces have lower resistance and the heat generated in these traces are lower.
Conclusion
Thermal management techniques for PCBs depend on a number of factors. These include the amount of heat the components generate, the environment surrounding the board, the overall design, and the type of enclosure housing the board. For circuits producing low heat, additional cooling may not be necessary. However, for those generating substantial amounts of heat, providing a cooling mechanism is necessary to carry the excess heat away from the component. PCB Trace Technologies Inc advises designers to provide thermal management for PCBs running hot, and consider mechanisms that influence temperature right from the concept stage, and continue throughout the design and manufacturing stage.
