Thermal and Thermal Relief Vias PCB Design Tutorial

 Thermal and Thermal Relief Vias

A via serves two primary functions:

  1. Facilitating the flow of electrical current between different layers.
  2. Facilitating the transfer of heat between different layers.

The primary purpose of a via is to establish electrical connectivity and is commonly referred to as just "vias." However, when a via is utilized for heat conduction, it is termed a "thermal via." It's essential not to confuse this with a "thermal relief via," which is intended for electrical connectivity with restricted heat conduction.

When a via is connected to a plane, it typically conducts heat effectively to the plane. However, if the via is also linked to a soldered component, such as a pin inserted into the via hole, the plane may draw out so much heat that soldering becomes challenging. For instance, when soldering a pin into a plated through-hole connected to a plane, the plane might extract excessive heat, making the soldering process nearly impossible. Therefore, there is a need to decrease the thermal conductivity of a via while preserving electrical conductivity.

The challenge lies in engineering a via to have low electrical resistance and high thermal resistance simultaneously. This is where a "thermal isolation moat" with a narrow drawbridge for electrical conduction becomes crucial. The concept is illustrated in the example of a thermal relief via below.

Thermal and Thermal Relief Vias PCB Technology

The narrow tabs in this design offer low electrical resistance, typically less than 0.5 milliohms, while maintaining high thermal resistance.

 A thermal relief via becomes necessary only when it connects to a soldered pin or resides within a solder pad designed for grounding connections to a decoupling capacitor. In cases where thermal isolation is required, such as connecting to a solder pad or a grounded capacitor, using a thermal relief via is advisable, although it occupies more space than a regular via.

It is a good practice to reserve the use of thermal relief vias only when essential for thermal isolation. If a via is situated between a signal trace and a plane without adjacent solder pads, or if it is connected to a pad through a 6-mil wide trace that is 20 mils long, a thermal relief via is unnecessary. In these scenarios, the trace itself acts as a sufficient thermal relief. Additionally, if the via is placed between two signal traces on different layers, a thermal relief via is not required.

It is crucial to differentiate between a thermal relief via and a thermal via. A thermal via serves the specific purpose of providing low thermal resistance between a component, such as a quad-flat-no-lead (QFN) part, and an underlying plane. These vias typically establish a connection between the attach pad beneath the component and the plane beneath it. An illustrative example of a QFN footprint, highlighting the thermal vias beneath the component, is depicted in the figure below.

Thermal and Thermal Relief Vias PCB Technology
 In the footprints designed for QFN (Quad Flat No-leads) components, the thermal vias are typically set at an 8 mil diameter or even narrower. However, it's crucial to be aware that utilizing this default footprint on your circuit board might lead to an additional cost imposed by the fabrication shop, particularly due to the presence of these narrow thermal vias. To avoid such premium charges, it is advisable to modify the thermal via size, ensuring that they are no narrower than a 13 mil drill diameter.

Thermal vias play a critical role in acting as heat spreaders for smaller components that generate significant power dissipation. It is essential to refrain from incorporating thermal relief structures in a via designated for thermal purposes, intended to conduct heat away from a component. Doing so would undermine the fundamental purpose of the thermal via.

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