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Heat cannot be efficiently exchanged with stagnant air surrounding a hot device; however, it can be transferred away from the electronic component to the PCB using thermal vias. A thermal via is a good conductor of heat that runs between the top layer and bottom layer of the PCB, dissipating heat through simple conduction. In simple terms, thermal vias are plated holes located under, or electrically connected to, a surface-mounted heat source on a PCB that allows heat transfer through the hole (Figure 1-1).
The efficient vertical heat transfer through thermal vias in the Z-axis is especially important to allow heat distribution over large areas of the PCB in the X-axis. Thermal vias run the design continuum from simple through-hole vias connecting the two external layers to complicated buried and blind microvias in stacked or staggered structures.
It should be noted that thermal vias, while the least expensive method of thermal management, are not always effective. It depends on the distribution of heat sources, the layout of the planes, and the cooling conditions on the bottom side. Also, if the air on the other side is stagnant, thermal vias are almost only useful with a heat sink on the bottom side.
This chapter will address three common thermal via designs: thermal via arrays, copper planes, and via fill.
Thermal Via Arrays
The premise that the thermal loss of a component is primarily transferred to the base of the package allows for the design integration of a path for heat dissipation into the physical PCB. A very common and cost-effective approach is to place an array of PCB thermal vias directly underneath the component (Figure 1-2). After the component is soldered to the PCB, the base of the component is connected to the thermal vias on the top side of the PCB.
Heat is then dissipated through these thermal vias down to the bottom side of the PCB. The efficiency of heat transfer through thermal vias is directly related, but non-proportional, to the amount of copper available to dissipate the heat. That being said, in many cases, the use of standard open vias (unfilled) may not provide the required thermal transfer on their own as the amount of actual copper is limited to the circumferential sides of the via. The most common method to improve this thermal performance is to combine the thermal vias with other thermal management techniques, such as copper planes, via fill, or heat sinks. A thermal simulation should be done.
A design consideration is that adding unfilled (open) plated through vias to the SMT pad (via-in-pad) creates some challenges because it may result in a solder wicking issue during assembly. This means that solder tends to flow (wick) down into the vias during the assembly reflow process, which may create solder voids on the pad. If open thermal vias need to be used, there are some things that can be done to minimize this problem: For example, use a small via diameter. The surface tension of solder limits the amount of solder wicking on smaller vias. With 0.3 mm or smaller via diameter, the solder wicking can be reduced. You can also fill the vias with thermally conductive materials. It eliminates the solder wicking but adds cost and an extra manufacturing step (see the via fill section).
Further advice includes the following:
- Eight mils (0.2 mm) is the typical minimum mechanical drilling size. Twelve mils (0.3 mm) is more common and lower cost if the design will permit
- IPC-6012 specifies a minimum 20-µm (0.8-mil) copper plating thickness for a Class 2 PCB, but manufacturers target 25 µm (1 mil). As discussed earlier, the more copper thickness plated in the via, the more heat can be transferred
- PCB thickness may have an impact on the thermal via performance
To download The Printed Circuit Designer’s Guide to…Thermal Management: A Fabricator’s Perspective, click here. You can also view other titles in our full library. Check out other books from American Standard Circuits, including The Printed Circuit Designer’s Guide to… Fundamentals of RF/Microwave PCBs and Flex and Rigid-Flex Fundamentals.