Thermal Vias for Circuit Board Heat Management: Techniques and Tips

August 6, 2019 Cadence PCB Solutions

Fumarole letting off steam in a park

 

One often-repeated phrase is “let off some steam.” When practicing change in management, for instance, we encourage project teams to “blow off steam” as part of managing—not eliminating—resistance. After all, resistance is not futile. But…where did the idiom “blow off steam” originate? A little detective work shows that “blow off steam” traces back to 1857 and the need to release steam from boilers on steam engines. Blowing off steam kept the boilers from overheating and exploding under pressure.

In electronics and PCB design, the need for another type of management can keep design teams up during the night. Good thermal management of a PCB design keeps a board from heating and design teams from exploding under pressure.

The Need for Thermal Management 

Studies show that runaway heat causes more than 50% of all electronic system failures. Let that statistic sink in for a moment and then give it some logical thought. If we can implement good thermal management, we can dramatically drive down the number of electronic system failures. Hmmm…seems worth some time and effort!

Thermal resistance is the difference in temperature between two closed surfaces divided by the total heat flow between the surfaces. The amount of thermal resistance often depends on PCB design factors. While using surface mount components has a positive impact on reducing thermal resistance, the area and thickness of the copper foil on the PCB and the thickness and material used for the PCB has a greater effect. Very simply, broader and thicker materials dissipate more heat. Yet, limitations exist because of the standard use of materials and because of product specifications.

You and your design team should consider how to implement good thermal management practices during the design phase and as you develop your PCB. If you wait later in the PCB design process, thermal management becomes much more difficult and costly. First, perform a quick inventory of the number of power semiconductors, motors, and other components that attempt to mimic an 1857 boiler.

Second, use the information provided in component libraries and by manufacturer data sheets to determine the thermal characteristics of major components. For example, the design of a Gallium Nitride (GaN) power device features a thermal pad on the bottom of the package that directly attaches to the die substrate. The largest amount of heat generated in the die moves down to the thermal pad and then transfers to the PCB.

Then, consider the different methods for transferring heat away from your board. Convection seems like a “cool” technique in that it transfers heat through gases and liquids and uses some type of medium. Unfortunately, applying gases and liquids to an operating PCB may not seem effective. Radiating could provide another method for chillin’ the board but that method really doesn’t work well for many PCB applications. You could choose to use aluminum heatsinks for hot components but limitations also exist in terms of the entire PCB.

 

Circuit board under thermal imaging

Thermal analysis of a circuit board

 

For most applications, though, combinations of horizontal thermal conduction through copper surfaces, vertical thermal conduction through an array of thermal vias and strategically placed heat sinks provide the best options. Going back to the GaN power device example, copper planes in the PCB work as heat spreaders and establish the horizontal thermal conduction. Thermal vias establish a low thermal resistance path from the top copper to the bottom side of the PCB. At this point, the design uses a heatsink attached to the bottom copper plane to dissipate heat into the ambient air.

Thermal Vias are More Than a Bunch of Hot Air

Thermal vias are—very simply—holes located under a surface-mounted heat source in a circuit board that allow heat transfer. Simple vias or via-in-pad can provide a large reduction in thermal resistance. You can also place filled and capped vias directly under the thermal solder pad for circuit board applications that have a thickness greater than 0.70 millimeters. Filling a via with epoxy and capping it with copper prevents the solder flow from any uncontrolled solder flow. In addition, filled and capped vias ensure excellent soldering.

The number and position of thermal vias has a direct impact on thermal resistance. Placing the vias as close to the heat source lowers thermal resistance by improving heat dissipation at a faster rate. Thermal vias work with double-sided boards with copper connecting the top and bottom surfaces of the PCB or can connect multiple layers of a PCB.

As you design your PCB and begin using thermal vias, follow a few quick rules. To improve heat dissipation, the through-hole contacts should have an increased copper layer thickness. While you can use different diameters for thermal vias, the optimal final diameter for the best thermal conductivity is 0.30 millimeters. The optimal distance from via-to-via is 0.80 mm.

 

Vias on a circuit board

Vias and proper via management can increase heat dissipation of a circuit board

 

You can use simulation tools to define the size and shape of thermal pads as well as select the best substrate materials to handle larger thermal loads. In addition, you can use simulation tools to analyze the performance of components and to identify possible hotspots on the board. 

Simulations can show the heat flow from the top surface of the board and the underside copper plane and illustrate the relationship between thermal resistance and the number of vias. Your use of ECAD/MCAD tools within your PCB design software also allows you to select the correct number of thermal vias, the best geometrical pattern for delivering the highest thermal dissipation while maintaining the physical stability of the PCB. 

Cadence’s suite of design and analysis tools for circuit layout and moving designs toward manufacturing make layer management easy. Consider Cadence Allegro PCB Designer when you’re looking for your next software to work through tough via placement challenges. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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