Thermal and Thermal Relief Vias
A via serves two primary functions:
- Facilitating the flow of electrical current between different layers.
- 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.
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 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.
See next PCB Layers.
