Thermal PCB design is an important consideration as temperature can have a profound influence on the life of electronics. Perhaps you already know the “rule of 10” that suggests the life of a product is reduced by a factor of two for every ten-degree Celsius raise in temperature. Surface Mount technology (SMT) cooling presents a unique challenge as the devices tend to concentrate heat into smaller areas. Also, conventional heatsinks are not as readily available.
In this engineering brief, we will explore the use of the thermal via. Specifically, we will explore the solution used by the designers who developed the Printed Circuit Board (PCB) artwork for the Arduino LEONARDO as pictured in Figures 1 and 2. Here we see a close up image of the SOT-223 packaged NCP1117ST50T3G Low-Dropout (LDO) positive 5 VDC regulator and the 12 thermal vias.
Figure 1: Thermal image of the voltage regulator on the Arduino LEONARDO (2017-REV3d). A total of 12 thermal vias flank the SOT-223 packaged LDO regulator.
Tech Tip: PCB design is an art that requires time and persistence to develop. The designer / artist must know the properties of the components including aspects of electrical, physical, and thermal. Board revision is common as the board may be improved for reduced cost and to easy manufacturability. For example, the Arduino LEONARDO featured in this article is an older 2017-REV3d. The latest edition of the board has improved thermal handling for the voltage regulator. The simple vias of the old board have been swapped out for larger solder filled vias.
Figure 2: Image of the heatsink pad on the back side of the Arduino. Note that this side of the heatsink is cooler than the side with the LDO regulator.
Description of the experiment
Visualizing the operation of the thermal via is very easy, provided you have a thermal imager at your disposal. Power is applied to the barrel connector of the Arduino LEONARDO. Series connected resistors are then connected to the Arduino’s 5 VDC rail. The result is a significant thermal event captured in Figures 1 and 2 where the regulator is significantly hotter than the surrounding PCB. Note that the resistors are visible on the thermal image. In Figure 1 they are in the lower left corner.
The image in Figure 1 was captured using an older Fluke Ti32 thermal imager. The images were then processed using Fluke SmartView Classic. Fluke suggests the TiS55+ as the modern replacement. Additional information about the thermal images may be found in these articles:
What is a thermal via?
All surface mount components utilize their PCB pads as a form of heatsink. For our purposes, the term pad is a section of copper clad PCB material dedicated to heat dissipation. The pad is often significantly larger than the component footprint. Figure 1 shows a typical example with a large pad for the SOT-223 packaged LDO regulator.
A thermal via is used to transfer heat from a pad on one side to the PCB to a pad on the other side of the PCB. The result is a larger heatsink with a lower thermal resistance. For a given condition, the component will operate cooler thereby the useful life.
The thermal via is a relatively low-cost measure used for surface mount devices. To better understand the technique, let’s first explore the datasheet for the LDO featured on the LEONARDO.
In Figure 3 we see curves for thermal resistance and maximum power dissipation as a function of pad size for the SOT-223 packaged device. For simplicity the datasheet assumes a square pad. The downward sloping curve shows the thermal resistance while the upward sloping curve shows the maximum power dissipation. As an example, consider a pad with 15 mm sides. The thermal resistance (to air) is approximately 85 °C/W and the maximum power dissipation for the device is approximately 1.2 W with the understanding that maximum ambient temperature is 50°C.
Figure 3: These curves show that the maximum power dissipation for the SOT-223 LDO is a function of pad size.
Tech Tip: Note that there is a point of diminishing return as we increase the pad size. In Figure 3 the knee in the 15 to 20 mm range. Further increasing the pad size does not appreciably increase the device power handling.
How does a thermal via work?
The thermal vias are implemented as a group of thermally conductive pathways to transfer heat from the pad on one side of a PCB to an adjacent pad on the other side of the PCB. The result is a heatsink with a lower thermal resistance. These thermal channels are necessary as most PCB material is a poor thermal conductor. If both pads are the same physical size and if the thermal via are perfect, we can see that the physical size of the heatsink material has been doubled.
It’s tempting to assume that the cooling capacity has been doubled. However, that is not the case as the PCB pads are physically located back-to-back with the PCB material sandwiched between. This shared semi-insulated PCB material interface lowers the effectiveness of each side. Also, as implied in Figure 1, heat spreads from the source to the periphery. Like walking around the circumference of a hill, heat distribution is on a gradient with equal temperature rings around the source. The further away we are from the source, the less effective the cooling. This is the primary reason for the point of diminishing returns associated with the curves in Figure 3. Because of the gradient heat distribution, the periphery is less effective at dissipating heat. Finally, as seen in Figure 2, the heatsink opposite of the component will be less effective as the thermal vias are not perfect – it’s cooler and consequently not as effective.
Tech Tip: Do not place thermal via underneath the soldered portion of the pad. We know that solder will wick into the via. This is an uncontrolled process that can cause voids between the surface mount component and the pad. The result is a weak solder joint with higher thermal resistance – exactly what we were trying to avoid.
What are the attributes of an ideal thermal via?
The ideal thermal via has a zero thermal resistance channel that passes heat from one side of the PCB to the other. In practice, this is difficult to achieve as it requires milling a hole in the PCB and then filling it with a thermal conductive material.
In practice, the thermal via is constructed using the same techniques used for the electrical via or the through hole component. The thermal via is nothing more than a plated drill hole. It is simply a via that connects one side of the PCB to the other. The copper plate provides an electrical as well as a thermal channel.
The thermal conductivity of the thermal via may be improved by filling the plated hole with solder. This technique is shown in Figure 4 with the latest edition of the Arduino LEONARDO. Here we see eight large vias that have been filled with solder. We can confidently say that this is an improved thermal management technique when compared to the dozen of unsoldered via shown in Figure 1. We recognize that filling the thermal via requires an additional manufacturing step not applicable to the 2017-REV3d pictured in Figure 1. The technique would not be used unless it was necessary.
Figure 4: Latest revision of the Arduino LEONARDO. The SOT-233 LDO regulator pad is flanked by eight solder-filled thermal vias.
Tech Tip: The thermal via is no different than a traditional via used to electrically connect one side of the PCB to the other. Instead, the term “thermal” differentiates how the via is used. For example, the Arduino LEONARDO pads shown in Figure 1 are connected via 12 thermal vias. It’s important to recognize that the solder side of the PCB has no electrical connections. Consequently, the purpose of the vias is thermal and not electrical. The naming convention is complicated when the PCB artist uses a group of thermal vias to provide a low-resistance through hole electrical contact.
Parting thoughts
The thermal via is an essential design tool for all PCB artists. It provides a relatively low-cost method to improve the cooling characteristics of our surface mount power devices. As shown in this article, the technique is effective yet there are limitations. From the datasheet we see that there are diminishing points of return for pad size. This gradient nature of heat transfer limits the cooling effectiveness as material far from the heat source does not appreciably contribute to device cooling.
Be sure to test your knowledge by answering the question at the end of this note. Also, please leave your comments, questions, and concerns in the space below.
Best Wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.
Highlighted Experience
Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 146 unique posts and provided an additional 480 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls. A collection of Dahlen’s Arduino educational articles can be found at Arduino education content.
Connect with Aaron Dahlen on LinkedIn.
Questions
The following questions will help reinforce the content of the article.
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What is the purpose of a thermal via?
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Define thermal resistance. For full credit, use the term to describe the attributes of the ideal heatsink.
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What is the difference between a thermal and a conventional via?
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What is the advantages and disadvantages of filling a thermal via with solder?
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With respect to Figure 3, what determines the minimum pad size?
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What is meant by point of diminishing returns with respect to pad size?
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Research and then describe at least three additional methods to cool surface mount devices.
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What problems are encountered when thermal vias are placed directly underneath the solder tab of an electrical component?
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Research and then describe the operation of an LDO. For full credit, describe how to calculate the LDO’s dissipated power.
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Describe the relationship between junction resistance and heatsink thermal resistance.
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Describe situations where the dual pad heatsink with thermal vias is undesirable.
Critical thinking questions
These critical thinking question expand the article’s content to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.
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Suppose three surface mount devices are installed in row, describe the relative temperature of the devices.
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We are tempted to believe that more thermal via are better. Describe the limitations.
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Describe the analogy between electrical and thermal resistance.
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Consider the cooling for the Arduino LEONARDO. Could the cooling for the chosen LDO be significantly improved by increasing pad size? Also, what recommendations would you have for the Arduino design team to improve the device cooling. Be sure to balance your recommendations with recognition to cost and size.
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Research the concept of thermal emissivity as it applies to a thermal imager. Use the concept to explain why the metal tab of the SOT-233 device appears cooler than the black epoxy package.
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All things being equal, from a thermal perspective, why is a circular heatsink pad with the device at its center the ideal model?
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Describe the work of Pierre-Simon Laplace as it relates to heat transfer.