Thermal Vias in PCB Design for Heat Management

Thermal vias are specialized vias designed to transfer heat from one layer of a PCB to another. These vias are placed near heat-generating components to create a thermal pathway, helping to dissipate heat and reduce the risk of overheating. They are often used with heat sinks, thermal pads, or copper planes to enhance their effectiveness. They are simply plated through holes (PTHs) that are located under a surface-mounted heat source component in a printed circuit board (PCB) that allow heat transfer.

The vias establish a low thermal resistance path from the top copper to the bottom side of the PCB. While, only one via cannot dissipate heat well, and generally requires vias array. In electronics and PCB design, the need for another type of management can keep design teams up during the night. Good thermal management of a PCB design keeps a board from heating and design teams from exploding under pressure.

The Need for Thermal Management

Effective thermal management in PCB design can prevent over 50% of electronic system failures caused by excessive heat. Heat in PCBs is typically generated by high-power components, such as processors, LEDs, and power regulators. Key considerations include minimizing thermal resistance, influenced by factors like copper area, PCB thickness, and material. Early design-phase strategies, such as analyzing power components and leveraging manufacturer data, ensure better heat dissipation. Techniques like convection, radiation, and heat sinks help transfer heat from the board. Proactive planning for thermal management is essential to maintain functionality, reduce costs, and avoid failures in high-performance electronics.

How Thermal Vias Aid in Heat Management

Thermal vias create a path for this heat to move away from these components to areas with better heat dissipation capabilities, such as a ground plane or an external heat sink. This mechanism prevents localized overheating and ensures consistent operation. Adding thermal vias can take up valuable board space with little benefit.

Thermal vias are non-current-carrying vias between two layers provided for the purpose of permitting heat to conduct from one layer in a PCB to another. The goal is to reduce the temperature of a heated surface relative to a cooler surface. In typical scenarios, the heated surface could be a pad beneath a component, while the cooler surface might be a deeper copper plane in the PCB. Using thermal vias without a connected opposite copper surface is ineffective. Proper heat transfer requires an interconnected structure where heat flows efficiently from the heated surface to the cooler surface within the PCB design.

Effectiveness of Thermal Via?

What does it mean that a thermal via is more or less "effective?" One interpretation is that, as additional thermal vias are added, the temperature drops across the thermal vias lessens, meaning more heat is conducted away from the heated surface. We judge the effectiveness of thermal vias by the reduction of the absolute temperature on the pad, relative to the surrounding environment (or ambient temperature).

But even that poses a problem. As we have emphasized as often as possible, temperature is a "point" concept. That is, the temperature varies from point-to-point on a pad or along a trace because all the factors that impact temperature are also point concepts. From the reference image of heat profile, as a conclusion - If we look at the top of one of the thermal vias it is 50.32°C. A via nearby has a temperature of 46.99°C. At the approximate center of the pad the temperature is 50.57°C. Just a few mm away the temperature is 48.91°C. We believe the appropriate temperature is the maximum temperature of the pad.

Heat Profile of Thermal Vias

Thermal vias efficiently lower a PCB’s thermal resistance, enabling heat to spread more evenly across the board. However, their performance depends on factors like diameter, number, and plating thickness. For instance, multiple smaller thermal vias grouped under a heat source often perform better than fewer, larger ones.

Design Considerations for Thermal Vias

• Size and Number: Small, tightly packed vias are more effective for heat dissipation.
• Via Plating: Plated vias with thicker copper layers provide better heat conduction.
• Thermal Pads: Use thermal vias in conjunction with heat-dissipating pads or planes.
• Fill Material: Use filled or plugged vias to prevent solder wicking during assembly.
• Placement: Place thermal vias directly under heat sources for maximum efficiency.
• Cost Implications: Ensure cost efficiency by balancing via density with manufacturing constraints.

When to Use Thermal Vias:

Thermal vias are very simple holes located under a surface-mounted heat source in a circuit board that allows heat transfer. Simple vias or via-in-pad can provide a large reduction in thermal resistance. You can also place filled and capped vias directly under the thermal solder pad for circuit board applications that have a thickness greater than 0.70 millimeters. Filling a via with epoxy and capping it with copper prevents the solder flow from any uncontrolled solder flow. In addition, filled and capped vias ensure excellent soldering. Thermal vias are necessary when:

• High-power devices are used, such as LEDs or MOSFETs.
• PCB designs involve dense component placement.
• Heat sinks or thermal pads are incorporated into the design.
• Devices are used in high-temperature environments requiring advanced heat dissipation strategies.

The number and position of thermal vias has a direct impact on thermal resistance. Placing the vias as close to the heat source lowers thermal resistance by improving heat dissipation at a faster rate. Thermal vias work with double-sided boards with copper connecting the top and bottom surfaces of the PCB or can connect multiple layers of a PCB.

How to Place Thermal Vias Array?

While you can use different diameters for vias, the optimal final diameter for the best thermal conductivity is 0.30 millimeters. The optimal distance from via-to-via is 0.80 mm. Before vias array placement, it’s better to confirm the via-to-via distance first, and the gaps are dependent on different heat source components. Then consider where to place -the vias are placed directly below the heat-dissipating plate on the bottom surface of the package.

When heat dissipation from vias directly under the IC's heat-dissipating plate is insufficient, additional vias can be placed around the IC periphery. These vias should be positioned as close to the IC as possible. The setup—via diameter, quantity, shape, and related parameters—varies across companies and is often guided by specific design rules. It’s important to consider these factors and tailor the design to optimize heat dissipation effectively, keeping such principles in mind during research and implementation.

You can use simulation tools to define the size and shape of thermal pads as well as select the best substrate materials to handle larger thermal loads. In addition, you can use simulation tools to analyze the performance of components and to identify possible hotspots on the board. Simulations can show the heat flow from the top surface of the board and the underside copper plane and illustrate the relationship between thermal resistance and the number of vias. See our article on other different type of via used in PCB designs.

Comparison with Other Heat Management Methods

While thermal vias are efficient, they are often used in combination with other strategies, such as:

• Heat Sinks: External structures that radiate heat but require additional assembly.
• Thermal Pads: Highly effective but increase manufacturing costs.
• Copper Planes: Provide heat spreading but may not suffice for high-power applications alone.

See how the thermal management techniques employed in a PCB.

Conclusion

Thermal vias are a critical tool in PCB heat management, offering a cost-effective and efficient solution to dissipate heat in high-power applications. Their implementation enhances PCB reliability, improves performance, and supports compact designs. By carefully considering their placement, size, and quantity during design, engineers can ensure optimal thermal performance. In combination with other thermal management techniques, thermal vias are indispensable in creating robust PCBs for today’s high-performance electronics.

As you design your PCB and begin using thermal vias, follow a few quick rules. To improve heat dissipation, the through-hole contacts should have an increased copper layer thickness. While you can use different diameters for thermal vias, the optimal final diameter for the best thermal conductivity is 0.30 millimeters. The optimal distance from via-to-via is 0.80 mm.