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In PCB design, effective thermal management is essential for achieving reliable performance. Thermal reliefs are specialized structures used in PCB design to address heat dissipation issues when component leads connect to large copper areas, typically ground or power planes.
This guide provides a detailed explanation of the function and design techniques of thermal reliefs to help you further improve your skills in PCB heat dissipation. ELE PCB also offers high-performance PCB design, manufacturing and assembly services. So, if you’ve got any questions, just give us a shout!
What Is PCB Thermal Relief?
Thermal relief refers to a series of copper connections that connect a PCB pad to the rest of the copper pour/copper plane. Instead of an exposed pad soldered onto a large copper-area-connected pad, their thin traces (typically in the shape of a cross or star, as shown) allow for controlled heat transfer at a smaller area.
Why Thermal Relief is Important
Essentially, thermal relief regulates heat dissipation. If one connected their pad directly to a copper plane, the heat would’ve spread throughout the entire plane too quickly. This will cause the heat to dissipate rapidly, meaning that the solder will not reach the required melting temperature. However, with thermal relief, there is sufficient and localized heat generation at the pad, creating even and reliable solder connections.
Furthermore, it also prevents manufacturing errors by exposing PCB pads to the proper amount of temperature over time or having different parts of the board at too different temperatures (cold joints, delamination).
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Control of heat: Thermal relief adds a level of resistance that controls how quickly it travels and allows for hot enough pads and even soldering.
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Save mass production time: The more consistent the soldering behavior across pads, the more repeatable the process will be, allowing for reduced defects and time and ongoing stable machine profiling for wave or reflow systems.
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Solder joint reliability: Thermal cycling fatigue decreases upon equal distribution of soldering, which means that the mechanical strength of solder is integrated over time, making this a reliable solution for consistently harsh environments.
Thermal Relif Pads vs Solid Connection Pads
Thermal relief pads refer to a connection where the pad is connected to the copper pour through narrow copper foil channels in four directions. Even heating during soldering makes it easy to solder and disassemble manually. This reduces stress and the risk of pad detachment.
Solid connection pads refer to a connection where the pad is fully covered by copper foil, forming a complete 360° surround connection. Their high current-carrying capacity makes them right for high-current and high-power applications.
| Scenario | Recommended Design | Reason |
| Hand soldering | Thermal relief | Reduces heat dissipation, allowing the solder joint to reach melting temperature more quickly. |
| High-current or high heat dissipation applications | Solid connection | Increases current-carrying capacity, but soldering difficulty and cold joint risk should be considered. |
| High-frequency signal circuits | Prefer thermal relief | Reduces the impact of parasitic capacitance on signal integrity. |
| Wave soldering | Strongly recommend thermal relief | Prevents PCB substrate warping caused by localized high temperatures. |
When to Use Thermal Reliefs
Thermal reliefs are used where copper interconnections may cause excessive thermal dissipation during soldering. As mentioned earlier, this may cause even reflow, consistent solder fills, and maintain stability throughout the process. Yet they are not always used exclusively and depend on electrical and thermal needs.
Through-Hole Components
Through-hole pads that connect to ground or power planes need thermal relief to avoid heat sinking during wave or manual soldering. Spoke connections enable even solder fill and ensure intermetallic connections.
Large Copper Areas and Planes
Copper pours and planes require thermal relief since it minimizes the copper mass attempting to pull heat away from subsequent solder joints. This will ensure those connections do not become cold solder joints after working with a large mass of copper.
Multilayer and HDI Boards
Since the increase in thermal mass adds soldering difficulty for outer-layer connections, these boards will always benefit from thermal reliefs. Using thermal relief, one can effectively heat through various layers of copper to ensure appropriate melting and workability.
High Current Paths
Power traces will use solid connection vias under large heat-dissipating components. Additionally, high-current nets will favor solid copper connectivity as these connections dissipate heat and need strong conductivity.
When is it inappropriate to use thermal reliefs?
- High-power components: Devices that require effective heat dissipation, such as power modules and power MOSFETs, often use full connections to improve thermal conductivity. In the diagram below, using thermal reliefs would reduce the overall current-carrying capacity.
- High-frequency signal ground plane: Certain RF circuits require low-impedance grounding, necessitating direct, full connections of copper foil.
- Test pads/mounting holes: Mechanical mounting holes or test pads are usually fully connected to ensure stability.
PCB Thermal Relief Design Considerations
The following points should be considered when designing thermal relief pads on PCBs:
Width of Spokes and Number of Spokes
The shape, width, and quantity of spokes impact how much heat is transmitted during soldering and how much current the spokes can carry in operation.
In general, a pad is relieved 2 to 4 spokes from the pad to the plane; smaller spokes (0.2–0.3 mm) for thermal/electrical disassociation are recommended for small signal pads, while larger spokes (0.4–0.6 mm) are recommended for higher current flow needs for power nets.
Clearance to Copper Plane
In order to avoid solder bridging, there’s a clearance that’s necessary for copper above as well. For typical FR-4 panels, the clearance is 0.25–0.4 mm for pads, with no copper planes surrounding it, as expected in typical copper weights. This number may increase slightly with higher weights, as there needs to be more clearance to control consistent resistance to heat.
Pad Surface Area and Thickness of Copper
The thicker the copper, the faster it absorbs heat. Boards using 2 oz or greater copper, this is where patterned types should be wider reliefs or substituted in the same relief quality as thinner copper. This goes for larger pads as well because bigger masses will not absorb temperatures the same way.
Nets with High Current
Power/ground connections needing a lot of current require a thermal relief; exposed seams would work best or partially filled, so proper solderability generates successful current-carrying components.
Check With DFM and Simulation
It’s always good to check with DFM clearances and thermal simulation tools when in doubt. These help form an easy path of soldering from the pad to see its successful probability. The success depends on how copper is used and how many pads are on the board.
Thermal Management Methods Comparison
Different thermal management methods are used for different functions. Below are the most commonly used methods for thermal management purposes and when to use.
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Method
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Purpose
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When to Use
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Thermal Relief
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Avoids soldering heat conduction between the pad hole and the copper plane.
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Requirement for soldering pads connected to components that should retain temperature but need acceptable connections.
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Thermal Vias
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Dissipates heat vertically through PCB layers.
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Where localized heat is generated. Under power switches, regulators, or under BGA components.
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Copper Pours and Planes
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Allows heat to dissipate from one spot on the surface of the PCB.
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Where power is distributed or where high current areas exceed maximum delta temperature rise tolerances.
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Heat Sinks
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Transfers heat away with an aluminum or copper substrate for better conduction.
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With high power dissipation ICs, such as linear regulators or switching devices.
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Metal Core PCB (MCPCB)
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Board stack-up changed from FR4 core boards, the core is changed to aluminum or conductive material to dissipate excessive heat by conduction.
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For LED lighting and automotive applications, which need a good thermal spread.
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Conclusion
The primary function of PCB thermal reliefs is to strike a balance between welding reliability, mechanical strength and electrical performance. By making targeted design adjustments, such as modifying the width and quantity of connection lines, these PCB pads can meet the requirements of a variety of applications, serving as a crucial optimisation tool for PCB engineers.
ELEPCB knows the difference between good and bad thermal reliefs and employs DFM factors to make sure that even the most complex of stack-up or copper percentages allow boards to possess the proper solderability and stability.
For professional PCB manufacturing and assembly, contact us. Your PCB will be created based on your thermal relief needs and performance specifications.
FAQs
A1: No. Only pads connecting to large areas of copper, like ground/power planes, need it. There are solid connections for signal pads or high-current vias.
A2: 2 to 4 spokes in a symmetrical arrangement are sufficient to not overly change the thermal path while still providing suitable mechanical stability.
A3: Yes. If the spokes are too small/narrow or if there aren’t enough, the resistance becomes too high and causes a voltage drop. There must be a compromise between creating thermal relief and maintaining electrical connections.
A4: Copper thickness means faster conduction, and therefore wider spokes or more spokes might help in solderability/circulation needs and current capabilities.
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