Electronics vs. Physics: Why Vias Don’t Get Hot

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Most of are aware that when we pass an electrical current through a trace (conductor), the trace will heat up. This temperature increase is caused by the I²R power loss dissipated in the resistance of the trace. The resistance of a copper trace is mostly determined by its geometry (cross-sectional area), and there are lots of studies trying to look at the relationship between the current down a trace (of known size) and the resulting temperature of the trace.

But the situation is much more complicated than this. There are physical properties that exist that result in helping to cool the trace. These properties are usually a combination of conduction of the heat away from the trace through the material, convection of the heat away from the trace through the air, and radiation of the heat away from the trace. A stable temperature is reached when the I²R heating equals the cooling—i.e., when the electronic and physical properties are balanced.

We have spent several (enjoyable) years collaborating on these effects. Douglas is an electrical engineer and understands electronics. Johannes is a thermodynamics physicist, and understands heat transfer. Between us we learned a lot about the thermal characteristics of vias.

Vias Don’t Get Very Hot
Traces heat because of the current through their resistance, resulting in I²R power losses. This temperature increase is caused by what we call “Joule heating”:

Joule heating is the physical effect by which the pass(age) of current through an electrical conductor produces thermal energy. This thermal energy is then evidenced through a rise in the conductor material temperature, thus the term “heating.” One can see Joule heating as a transformation between “electrical energy” and “thermal energy,” following the energy conservation principle.

We are all aware that, in general, for a given trace an increase in current through the trace will correspond to an increase in temperature of the trace. When it comes to vias, the industry guidelines have generally been to size the cross-sectional area of a via to equal that of its parent trace. Then the via will be the same temperature as the trace. 

To read this entire article, which appeared in the November 2022 issue of Design007 Magazine, click here.

10/07/2020 | I-Connect007 Editorial Team

Johannes Adam is the creator of a simulation tool called Thermal Risk Management (TRM) used to help PCB designers and design engineers predict hot spots on the board before during layout. He and Douglas Brooks, founder of UltraCAD Design, have used the tool to produce several technical articles and a book on the subject. In this interview, they tackle the biggest misconceptions they see from designers and engineers who deal with thermal management issues.

08/06/2019 | Pete Starkey, I-Connect007

The Microvia Summit on May 16 was a special feature of the 2019 event in Baltimore, since microvia challenges and reliability issues have become of great concern to the PCB manufacturing industry. It provided updates on the work of members of the IPC V-TSL-MVIA Weak Interface Microvia Failures Technology Solutions Subcommittee and opportunities to learn about latest developments in methods to reveal and explain the presence of latent defects, identify causes and cures, and be able to consistently and confidently supply reliable products.

07/03/2018 | Alistair Little, Electrolube

Think very carefully about the sort of environment your PCB is likely to encounter. It is easy to over-engineer a product so that it will survive the very worst of conditions, but worst conditions may only be fleeting or transient. Therefore, a resin solution with a lower temperature performance specification will often cope. Take temperature extremes, for example. Your application may experience occasional temperature spikes of up to 180°C, which you might feel deserves treatment with a special resin.