To perform at the higher levels expected of each succeeding generation of electronic devices, the more those electronics have to function at greater operational levels. To do this, complex component packages with larger pin counts are used in increasingly denser circuitry requiring greater power. And while this PCB design technology produces the required results, it also generates undesired heat. To combat the problem of excessive heat, design engineers rely on different layout techniques, including thermal vias.
Heat Flux in PCB Design
Heat flow density, or heat flux, is the transfer of heat on a circuit board (per unit area per unit time) from the source to a cooler location. The heat transfer method falls into three key areas: radiation, convection, and conduction.
Heat Transfer Mechanisms
|Radiation||The exchange of thermal energy without a physical medium through electromagnetic waves is called radiant energy|
|Convection||The transfer of heat through the movement of liquid and gasses|
|Conduction||The direct transfer of heat through matter, such as the metals of a circuit board|
Circuit boards are a compilation of electronic components and interconnecting metal traces. All electronic components generate heat to a certain degree, and high-powered active devices can create significant amounts of thermal energy. This heat radiates from multiple sources and moves across the board through airflow and direct connection between device pins soldered to traces.
Heat is the enemy of peak electronics performance and can slow circuit speeds down, lead to thermal fractures, or cause outright failures of components or the entire board. Fortunately, there are many ways to control the heat, including part placement on the board and active cooling systems such as fans. Thermal vias are another effective way to facilitate heat flux, which we will look at next.
What is a Thermal Via?
There are many ways to manage thermal energy on a circuit board. Placing “hot” components so they are separated from each other and laying out the board to optimize the airflow are two examples. Another method is to leverage the natural heat sink of the ground plane in a multi-layer board. Large areas of metal on a board provide a natural way to collect and disperse the heat of hot components, and this heat can be efficiently conducted from the component to the plane through a thermal via.
Thermal vias are typically drilled through the board at a larger size than the other signal vias on the PCB, with the drill size determined by the amount of conducted power and heat. Thermal vias are usually embedded in a large landing pad under the hot component and connect to an internal ground plane. However, designers must be careful when choosing the location of thermal vias to avoid soldering problems during assembly. Just as the thermal via conducts heat away from the component during operation, it can also wick heat away during the soldering process, resulting in a poor or “cold” solder joint.
Thermal vias are created along with other thru-holes by drilling through the composited board layers during fabrication. Once drilled, the entire circuit board undergoes electroplating with a layer of copper. Electroplating begins with preparing the exposed metal of the traces and vias chemically, then connecting the circuit board to an electrical charge and repeatedly dipping it in chemical baths. These baths alternate between copper plating and cleaning, and the process features rigorous control to ensure an even distribution of copper thickness. Next, we’ll look at how you can make the best choices for thermal vias in your circuit board design.
How to Plan for the Most Effective Use of Thermal Vias
There are many considerations to keep in mind when you are adding vias to your circuit board design:
- Choosing the correct via drill size aspect ratio for your design
- Using a via pad size to produce the correct annular ring
- Proper copper balancing of the circuit board
- Choosing the optimum drill size for thermal vias to conduct heat
- Ensuring the most optimum thermal relief connection for the vias
Failure to properly consider any of these points could result in problems during manufacturing, degraded circuit board performance, or premature failure of the assembled board. At VSE, we have many years of experience helping our customers make the best via choices for their design. We aim to build you a circuit board that will give you the highest level of performance. To that end, our staff regularly helps our customers understand potential thermal problems with their designs and how to manage PCB heat flux.