# PCB Design Guide to Via and Trace Currents and Temperatures Douglas Brooks and Johannes Adam

We have assembled 20-plus years of knowledge into these pages. Starting with a historical background, this book covers: (a) PCB materials (copper and dielectrics) and the role they play in the heating and cooling of traces; (b) The IPC curves found in IPC 2152 along with equations and simulations that fit those curves; (c) Sensitivity analyses showing what happens when we vary the environment (adjacent traces and planes, changing trace lengths, thermal gradients, etc.); (d) Via temperatures and what determines them; (e) Thermal vias and via current densities; (f) Fusing issues, what happens when traces are overloaded, and (g) the relationship between DC and AC heating of traces. There are supplemental chapters or appendices on measuring the thermal conductivity of dielectrics and measuring the resistivity of copper traces (and why many prior attempts to do so have been doomed to failure.)

Every topic is explained from a theoretical standpoint, simulated, and in many cases supplemented with experimental results. Never before has such a thorough compendium of trace and via heating relationships been available, especially so conveniently.

Virtually every electrical device manufactured in the world has a printed circuit board in it. Printed circuit board designers have lots of things to think about. Typically, the first is just connecting the dots that (inter)connect all the components. Then, how to do so on the minimum number of trace layers. Then, there may be all the signal integrity issues to deal with. And, finally, how do we size the few power traces there are for carrying the required current?

This latter question constitutes only a very small fraction of the total job. But it can be the most mysterious. In part, because there aren’t many reliable guidelines to work from. Then, many of the guidelines that exist are incomplete, contradictory, or, even worse, flat wrong. This book is intended to cover the entire topic of the relationship between PCB trace and via current/temperature relationships. It covers:

• the theory behind the various aspects of the topic,
• the results of simulations based on those theories,
• and, in many cases, summaries of experimental results based on, and confirming, the simulations.

What are some problems your book can help solve?

• How to determine how hot a PCB trace will be carrying a certain amount of current and, equally as important, how will the temperature vary with material, design, and environmental modifications.
• Understanding why is there no relationship between the current through a via and the temperature of the via? The most important and surprising result in the book!
• Understanding why current is relevant in thermal design considerations but current density is not.
• Calculating whether an overloaded trace will melt, and if so, how long will it take (and why)?
• Learning if AC currents heat a trace the same way DC currents do, and does the relationship vary with frequency.

What are important Features of your book and the Specific Benefits a buyer can expect to derive from those Features?

Feature: Understand precisely what material and design decisions effect trace temperatures, how, and why.

Benefit: Optimize designs with regard to board area (geography) and layer count.

Feature: Understand that via temperatures are unrelated to current, and that current density is irrelevant.

Benefit: Designs become much more efficient and designers can significantly reduce the number of vias required, thereby leaving many more routing channels open for traces.

This is THE most important and surprising feature in the book!

Feature: A thorough discussion of Onderdonk”s Equation and a rigorous derivation of his/her equation (Appendix D7).

Benefit: It is our belief that this derivation does not exist anywhere else. There is no original work of Onderdonk known to exist. Therefore, senior engineers interested in the subject can see what is effectively a source document.

What audiences will use this book?

Anyone doing PCB design work. PCB designer, engineer, technician, etc.; Circuit design engineer; System design engineer; Project manager; PC board manufacturer