What is PCB Thermal Conductivity?

PCB (printed circuit board) thermal conductivity refers to the ability of a PCB material to conduct heat. It is a measure of how well heat transfers through the PCB substrate material from high temperature areas to cooler areas. Thermal conductivity is expressed in units of watts per meter-kelvin (W/mK).

Different PCB materials have different thermal conductivity properties:

As you can see, metals like aluminum and copper have much higher thermal conductivity than standard PCB base materials like FR-4 glass epoxy and polyimide. This is why copper layers and thermal vias are used to help transfer heat through the PCB.

Factors that Affect PCB Thermal Conductivity

Several factors can influence the thermal conductivity of a PCB:

1. Material properties – As shown above, the inherent thermal conductivity of the PCB substrate material has the biggest effect.
2. Thickness – In general, a thicker PCB will have lower thermal conductivity than a thinner one of the same material, as heat has a longer distance to travel.
3. Copper layers – More and thicker copper layers will increase the overall thermal conductivity of the PCB, as copper conducts heat much better than the base material.
4. Thermal vias – Adding vias (vertical interconnect access) filled with copper or other conductive material provides additional heat transfer paths through the PCB.
5. Solder mask – Solder mask coatings can slightly reduce PCB thermal conductivity.

The Importance of PCB Thermal Conductivity

Thermal conductivity is a critical parameter in PCB design, especially for boards with high power components that generate significant heat. If this heat is not dissipated efficiently, it can lead to a host of problems:

Overheating and Component Failure

Excess heat buildup can raise component temperatures beyond their rated limits, leading to suboptimal performance, malfunction, or complete failure. Overheating is a major source of reliability issues in electronic devices.

High temperatures have various negative effects on components:

Proper thermal management through optimized PCB thermal conductivity is essential to keeping component temperatures within acceptable ranges.

Thermal Expansion and Physical Stress

When PCBs heat up, the substrate material expands. If the PCB cools down and heats up repeatedly, this thermal cycling causes the board to continuously expand and contract.

Different materials have different coefficients of thermal expansion (CTE). This means they expand by different amounts with the same temperature change. If materials with mismatched CTEs are used in a PCB Assembly, the uneven expansion can put physical stress on the board and components.

Common CTE values for PCB materials:

Thermal cycling stresses can lead to:

• Warping or cracking of the PCB
• Solder joint fatigue and failure
• Damage to components or their leads

Using materials with similar CTEs and managing PCB temperatures through good thermal conductivity helps minimize thermally induced stresses.

Impact on Signal Integrity

While not as obvious as overheating or physical damage, poor thermal conductivity can also negatively affect PCB signal integrity. Temperature fluctuations across a circuit board cause copper traces to expand and contract at different rates.

These uneven physical changes can alter the impedance and propagation delay of high-speed signals. This leads to issues like:

• Increased jitter
• Reduced eye width
• Compromised timing margins

Maintaining steady, even temperatures across the PCB through appropriate thermal conductivity helps preserve consistent signal integrity.

Improving PCB Thermal Conductivity

There are several ways to optimize the thermal conductivity of a PCB during the design process:

Use High Thermal Conductivity Materials

One straightforward approach is to choose PCB substrate materials with higher inherent thermal conductivity. While standard FR-4 is cheap and popular, it has relatively poor heat transfer properties.

Some high thermal conductivity PCB materials include:

These specialized materials can significantly improve heat dissipation compared to standard FR-4, but often come at increased cost.

Optimize Copper Usage

Since copper has excellent thermal conductivity, strategic use of copper layers and features can greatly enhance a PCB’s overall heat transfer capability:

• Use thicker copper layers, especially on outer layers. Increasing from 0.5 oz to 2 oz copper can improve thermal conductivity by 20-30%.
• Add more copper layers. This provides more heat conduction paths through the board.
• Pour copper planes on unused PCB area. These copper floods help spread heat laterally.
• Place copper pours on outer layers over high-heat components to aid heat dissipation to the environment.

Utilize Thermal Vias

Thermal vias are copper-filled holes that provide low-resistance heat conduction paths vertically through a PCB. They allow heat to transfer efficiently from inner to outer layers where it can dissipate into the surroundings.

Tips for effective thermal via usage:

• Place arrays of thermal vias under or around high-heat components like power transistors.
• Use more smaller vias rather than fewer larger ones. This maximizes heat transfer from the component into the via.
• Connect thermal vias to copper pours or planes on inner and outer layers to aid lateral heat spreading.
• Use thermally conductive via fill material like solder rather than non-conductive epoxy.

Use Heatsinks

For components that generate large amounts of heat, using heatsinks mounted to the PCB can greatly improve thermal conductivity to the ambient environment. Heatsinks are metal structures with high surface area that efficiently radiate heat.

Considerations for heatsink usage:

• Choose heatsinks with the appropriate thermal resistance for the power dissipated by the component.
• Use thermally conductive pads, paste, or adhesive to minimize thermal resistance between the component and heatsink.
• Place the heatsink on the PCB such that it does not physically interfere with other components or features.
• Ensure there is adequate airflow around the heatsink for optimal convective cooling.

Measuring and Simulating PCB Thermal Conductivity

To ensure a PCB design has appropriate thermal conductivity, it is important to analyze and validate its thermal performance. This can be done through measurements on prototypes or by simulating the design with thermal modeling software.

Thermal Measurements

Directly measuring PCB temperatures can give valuable insight into its thermal conductivity and overall thermal performance. Common methods include:

• Thermocouples – These sensors use dissimilar metal junctions to generate a temperature-dependent voltage. They can be attached to various PCB locations to map temperatures.
• IR cameras – Infrared cameras provide a visual thermal map of the PCB, showing how heat is distributed across the board surface.
• Resistance temperature detectors (RTDs) – RTDs are sensors whose resistance changes with temperature. They can be mounted on the PCB for precise temperature measurements.

Thermal measurements should be performed under realistic operating conditions, such as maximum expected ambient temperature and component power dissipation.

Thermal Simulation

Thermal simulation software uses mathematical models to predict PCB temperatures and heat flow based on the design and operating conditions. These programs can evaluate a PCB’s thermal conductivity and identify potential hot spots before the board is even manufactured.

Common thermal simulation techniques:

• Finite element analysis (FEA) – FEA models break the PCB into small elements and solve heat equations for each element. This provides detailed 3D temperature distributions.
• Computational fluid dynamics (CFD) – CFD simulates fluid flow and heat transfer, taking into account convection effects from PCB airflow.
• SPICE modeling – SPICE circuit simulators can model electrical and thermal effects concurrently, giving insight into electrothermal interactions.

Simulations allow designers to explore the thermal impacts of different stackups, materials, copper features, and component placements. This aids in optimizing PCB thermal conductivity and overall thermal performance.

Frequently Asked Questions (FAQ)

1. What is a good PCB thermal conductivity value?

The ideal thermal conductivity for a PCB depends on its specific application and thermal requirements. In general, a PCB should have sufficient thermal conductivity to keep all components below their maximum rated operating temperatures under worst-case conditions.

For most applications, a thermal conductivity in the range of 1-10 W/mK is adequate. High-power applications may require values of 50 W/mK or more.

2. How much does PCB thermal conductivity affect reliability?

PCB thermal conductivity has a significant impact on reliability. Insufficient thermal conductivity leads to high component temperatures, which accelerates various failure mechanisms. In general, the failure rate of electronic components doubles for every 10°C increase above their rated temperature.

Improving PCB thermal conductivity enhances reliability by keeping components cooler. A design with good thermal management can have a significantly longer lifespan than one with poor heat dissipation.

3. What is the most cost-effective way to improve PCB thermal conductivity?

The most economical ways to enhance PCB thermal conductivity are through optimizing the use of existing materials, rather than using exotic high-conductivity substrates.

Techniques like using thicker or additional copper layers, adding thermal vias, and placing copper pours are relatively low-cost ways to improve heat transfer. Designing an efficient PCB Stackup and layout to minimize heat buildup also helps without adding significant cost.

4. How do you choose between thermal vias and heatsinks?

Thermal vias and heatsinks serve different purposes and are often used together. Thermal vias are best for transferring heat vertically through the PCB, while heatsinks are used to dissipate heat from the PCB surface to the ambient environment.

In general, thermal vias should be used first to get heat away from sources and spread it through the board. Heatsinks are then used where there is still a large amount of heat to dissipate, typically on high-power components or areas with concentrated heat.

The choice between vias and heatsinks ultimately depends on the specific thermal requirements and physical constraints of the PCB design.

5. Can you have too much PCB thermal conductivity?

While high PCB thermal conductivity is generally beneficial, there can be some drawbacks to excessive thermal conductivity:

• Increased cost – Materials with very high thermal conductivity, like ceramics or metal core PCBs, are significantly more expensive than standard FR-4.
• Mechanical stress – If heat spreads too quickly through a PCB, it can lead to uneven thermal expansion and contraction, causing physical stress.
• Overcooling – In some cases, components may actually rely on self-heating to maintain their optimal operating temperature. Cooling them below this point can degrade performance.

In most cases, these issues can be avoided by designing the PCB stackup and thermal conductivity to be appropriate for the specific application requirements. Thermal simulations can help identify and mitigate potential problems.