Managing Thermal Effects in PCB Design

The behavior and characteristics of electronic components depend to a large extent on their operating temperature. Not only components that are running hot, temperature also affects other components indirectly. This may be due to their proximity to a hot component, or the thermal properties of the PCB material. Therefore, to ensure a reliable, safe, and error-free operation throughout the device’s lifetime, a designer must account for the PCB and its component’s thermal properties. This includes the right choice of materials for the PCB, adequate dimensions of the heat sinks, proper trace width and weight, and the required number and positioning of thermal vias, as these govern proper thermal management, safety, and reliability of the product.

Thermal Resistance

Just as current flow through a material encounters electrical resistance, heat flow faces thermal resistance, which is a property of the material through which the heat is flowing. This is a vital concept in the thermal management of PCBs. Primarily, it quantifies how effectively a material conducts heat. Selecting a material with lower thermal resistance properties will ensure it can dissipate heat in a better manner.

There are two main modes of heat transfer—conduction and convection—both contributing to thermal resistance. Just as Ohm’s law describes electrical charge transfer, heat transfer follows a similar concept. Just as a voltage differential drives a current flow, a temperature differential drives the heat flow.

In conduction, heat transfer is mainly through molecular interaction within the material. In electronic equipment, the heat transfer is primarily through the PCB substrate, its conductive traces, and any heat sinks present. The material surrounding a hot component tends to conduct the heat away due to the difference in temperature between the component and the contacting material. On the other hand, convection is primarily heat transfer through a fluid medium, such as liquid or air. For instance, as components heat up, they transfer heat to the colder air surrounding them.

Both conduction and convection modes of heat transfer occur simultaneously, while the ambient temperature is the main driving force. Oftentimes, PCB designers overlook the thermal mass of components, unless they happen to be working on high-pulse or high-power applications. Thermal mass is another important factor as every material can store heat, and materials with higher thermal mass absorb and retain more heat energy before allowing their temperature to rise. Conversely, materials with lower thermal mass can change their temperature due to the addition or removal of relatively lower amounts of heat energy.

Effect of Heat on PCBs and Electronic Components

A designer must ensure the safety, reliability, and optimal performance of an electronic circuit. They do this primarily by managing the heat generated by the onboard components. This is important, as, with any temperature rise, traces on the board, and the components themselves undergo a change in their electrical properties. For instance, rising temperatures increase the resistance value of traces and resistors, while capacitors exhibit increased leakages. Electronic components degrade with prolonged thermal stress, eventually leading to failure. The design of microcontrollers and microprocessors allows them to throttle their performance with rising temperatures, resulting in suboptimal functioning.

Excessive heat build-up in the PCB can give rise to many effects like signal distortion, noise generation, and deterioration of signal integrity. Therefore, PCB Trace Technologies Inc. recommends paying close attention not only to component placement, but also to the positioning, width, and thickness of traces and the size of vias carrying high currents, as a change in temperature can alter their resistance, and hence, the amount of heat they generate.

Not only components and traces, but prolonged thermal stresses due to thermal cycling also affect solder joints as well, with changes in the board temperature leading to their expansion and contraction. Inadequate cooling can have drastic effects on solder joints, eventually leading to a mechanical failure of the joint.

Managing Thermal Effects Through Design

Depending on the application, PCB Trace Technologies Inc. manages thermal effects during PCB design in various ways. These include:

Heat Dissipation

We consider the environment and the target use case of the PCB, as the temperature difference between the two is the primary driver of heat dissipation. Therefore, an application for high-temperature environments merits a different design specification compared to the one that must operate in a cooler environment.

For instance, PCBs for the military, aviation, or manufacturing industry will undergo different thermal stresses as compared to those for consumer devices. Therefore, we prefer to adhere to industry standards like IPC-2221 and IPC-2152 in addition to environmental requirements and additional governmental regulations.

Component Placement

We identify potential thermal hot spots early on in the design, thereby ensuring adequate heat dissipation. These often include large integrated circuits, high-power devices, and high current-carrying traces that can generate copious amounts of heat.

Typically, we space such traces and components far apart on the board, thereby avoiding the formation of potential hot spots, while spreading the heat evenly across the board surface. Placing the hottest components in the center of the board helps to maximize the area for spreading the heat.

If different package sizes are available for the same component, selecting a larger size typically increases the thermal contact area of the component with the board. In the same way, wider traces with greater weight can dissipate more heat, especially in applications that must handle high power.

Pad Size Considerations

Just like thicker and wider traces, thicker and larger mounting pads also help to dissipate more heat from the hot component to the surrounding board material. We often use heavy copper on the top layer of a board, resulting in thicker pads, thereby tweaking its thermal properties. However, as the pad thickness also affects its impedance, we consider the signal integrity requirements of the trace as well.

Thermal Vias

Frequently, we use thermal vias to transfer heat from the side on which the hot component is mounted, to the opposite side of the board. Thermal vias, being heat conductive, help to spread the heat from the hot side of the board, through the entire stack, to the other side of the board, generally culminating in an exposed copper plane. While most often, the copper plane alone acts as a heat-dissipating surface, an external heat sink can help augment its function.

Customarily, we place thermal vias in a regular pattern within the thermal solder pad under the hot component. Unlike capped vias, thermal vias remain exposed, facilitating good thermal contact with the underside of the hot component, thereby reducing the thermal resistance to a large extent. Incidentally, the vias are filled with conductive epoxy to prevent solder wicking during assembly.

Where possible, we connect the copper plane to the ground plane. This helps in providing an excellent low-impedance return path. To help with soldering the components, we recommend providing thermal relief around their solder pads.

Metal Core

To facilitate even better heat dissipation, PCB Trace Technologies Inc. recommends using metal core PCBs. Unlike regular boards, metal core PCBs use a very thin substrate separating a metal core and the copper circuitry. The substrate acts as an electrical insulator but with very high thermal conductivity. The heat generated from any component is thus effectively transferred to the metal core, which helps to further dissipate it to the environment or to an external heat sink.

Conclusion

To obtain a reliable and optimal performance from a PCB assembly, there is no alternative to managing its heat generation. PCB Trace Technologies Inc. recommends avoiding thermal hot spots, whether from a high-current device or an integrated circuit.

Spreading the heat evenly across the PCB is possible simply by adequately positioning and spacing the components, carefully balancing the trace width and thickness, and adjusting the mounting pad size. While providing adequate cooling, this also helps with maintaining the signal integrity requirements.

In extreme cases, thermal vias and metal core PCBs can help with transferring heat from one side of the PCB to the other. Thereafter, adding an external heat sink can further augment thermal management.