Packing a cooler is a time-honored tradition for beach trips, camping, and cookouts. I remember the first time I was on cooler duty. Typical of my thought process, I put a bed of ice in the cooler and placed all perishables on top. As you might have guessed, my family was not impressed with my record-breaking cooler pack.
Cooling is also an important facet of PCB design, and the stakes are much higher than a few lukewarm sodas. Failing to account for PCB thermal conductivity is likely to result in a board with intermittent service disruptions and potential long-term damage, which could create a hazardous environment for anyone nearby. Like the cooler, PCB thermal conductivity relies on the location of features and the thermal path to lower temperature surroundings.
Enhancing PCB Thermal Conductivity with Common Features
In any electronics, heat is the enemy of product lifespan and reliability. Even materials and devices expected to operate in high-temperatureenvironments are susceptible to increased degradation rates over baseline operating conditions. Thermal management, therefore, becomes a paramount issue when designing boards for long-term or difficult-to-access installations. The design, beginning with layout and placement, must encourage a heat flow path from high-power components that balances the power needs of the circuit against the thermal load. High-power devices are likely to only reach optimal functionality with proper dissipation built into the design.
Whether discussing electronic or thermal circuits, conductivity represents the inverse of impedance; if the latter represents opposition to some flowing parameter of the circuit, the former is its acceptance. While the design may impose high or low impedance values on a particular subcircuit according to its overall function, there is rarely a reason not to optimize the heat flow for any device in the circuit. Excess heat modifies operating conditions, perhaps most notably with electrical resistance being a temperature-dependent measurement. The layout will be the earliest point when a design needs to contend with heat transfer, and there are many tools at a designer’s disposal to rectify its presence:
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- Thermal vias. Analogous to a copper trace for electrical routing in terms of baseline essentialness. Thermal vias are placed underneath components with high or variable operating temperatures (generally power components like regulators, transistors, etc.) to provide an additional passage for temperature. The location of the via underneath the device also helps move heat from the board-device interface, where convection cooling is less effective. Via structures can be resized to allow for an increased contact surface area (and therefore more effective cooling per via), but multiple vias can also accomplish this goal.
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- Package size. Depending on the density of the design, a larger package with the same power capabilities can be switched out for a smaller one. This step effectively reduces the heat source by spreading the absolute heat over a larger region. It can also be combined with an increase in thermal vias and copper pours for enhanced dissipation.
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- Ground plane. While grounding is essential to any design, some improved thermal performance can arise from additional grounding, and greater size pours (up to flooding the layer). Ground planes serve as the simplest (yet highly effective) heat sinks by drawing heat away from generating sources to much larger surface areas for a less concentrated dissipation. Ground planes in design can be coupled with increased copper plating on the appropriate layers for additional heat sinking. Designation of outer layers for ground planes further assists in cooling by allowing for increased convection cooling at an area ideally close to the dimensions of the board.
Thermal Impedance Path and Resistive Network
Thermal impedance functions opposite thermal conductivity and is analogous to its electrical counterpart. Just like the path of least impedance for current flow, so exists the most navigable path for heat. PCBs can be modeled as a dense yet easily reducible thermal resistive network with small assumptions that affect overall accuracy but are perfectly acceptable for a rough estimation. These resistances fall into one of four categories:
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- Resistance in the copper plane was measured laterally.
- Resistance of the thermal via.
- Resistance occurs at the air-copper surface interface of the board due to convection heating.
- Resistance of the substrate was measured vertically.
These four aspects are open to wide variability between material characteristics, feature size, and ambient effects. The most valuable takeaway is that these elements form an interconnected thermal impedance, and understanding this network helps further develop the thermal design.
The function of heat sinks follows naturally from a discussion of thermal impedance. Heat sinks are a large surface area of conductive material that can rapidly cool a thermally connected device or circuit. They come in many design configurations, but one style has the heat sink mounted on the bottom of the board (or, more generally, opposite the power circuitry side).
Would it be more efficient to mount the sink top side where the distance between the package and the component is minimized? It’s unlikely. A heat sink performs best when it follows the path of the least thermal impedance.
An extreme and non-practical example: a heat sink mounted directly to the top of the packaging would perform poorly because of the high impedance from passing through the exposed pad through the plastic case. Therefore, even though the path is longer and more thermally resistive components travel through the board, the overall conductivity is optimized.
Your Contract Manufacturer Understands How to Improve Thermal Performance
PCB thermal conductivity comprises a wide range of necessary design considerations that must be managed for a board to reach its intended level of performance. This rule is especially relevant in high-power circuits that quickly produce enough heat to overwhelm the physical properties of the board. Designers should be mindful of the best mitigating strategies to prevent heat buildup and the possible locations on the board where this may occur.
For any additional concerns, thermal or otherwise, VSE will help optimize your design for manufacturing and performance. We follow a simple motto: we’re a team of engineers that builds electronics for our customers. Coupled with our professional manufacturing partners, we’ll assist your design and production process in delivering an exceptional final product.
