How to Select Copper Foil for High-Frequency PCB Design

Zachariah Peterson
|  Created: August 24, 2022  |  Updated: October 24, 2022
How to Select Copper Foil

The PCB materials industry has spent significant amounts of time developing materials that provide lowest possible signal loss for products with RF applications. For high speed and high frequency designs, losses will limit signal propagation distance and distort signals, and it will create an impedance deviation that can be seen in TDR measurements. As we design any printed circuit board and develop circuits that operate at higher frequencies, it may be tempting to opt for the smoothest possible copper in all designs you create.

While it is true that copper roughness creates additional impedance deviation and losses, how smooth does your copper foil really need to be? Are there other simple methods you can overcome some losses and still complete the routing you need in your PCB? In this article, we'll look at the balance between copper foil losses and other types of losses in a PCB, as well as some strategies that are commonly used to overcome roughness.

Types of Copper Foil for PCB Stackups

Before looking at which copper foil you should look for in your board, it's important to know something about the copper foil that actually available to be included in a PCB stackup. Copper foils are not something you always get to pick and choose to pair up with every laminate material. Some laminate manufacturers will provide multiple options that pair different types of copper foils with their material sets, but this is not the case with every laminate manufacturer or material. Unless you can procure materials separately and put them through a lamiantion process, you'll have to work within the material sets you can obtain from a lamiante manufacturer or distributor.

With this mind, here are the different types of copper foil you'll find in PCB materials:

Type of Copper

Description

Roughness

Electrodeposited

Has a rougher surface on one side of the copper foil

Highest (above 1 um)

Reverse treated

Uses a surface treatment to reduce roughness

Moderate (0.5 um to 1.5 um)

Rolled-annealed

Has a smoother, denser surface from a rolling process

Low (0.25 to 0.5 um)

Ultra-low profile

Additional treatments are used to reduce roughness

Lowest (comparable to rolled annealed, but can be less than 0.3 um)

 

The different roughness ranges can be the deciding factor on which type of copper to accept in your PCB layer stackup, so it is important to examine the requried board capabilities and compare them with the available copper and dielectric options. One thing you will find is that laminates marketed for high frequency PCBs will have a lower profile copper option, so you can get the benefits of low-loss dielectrics and smooth copper in the same package. However, some devices with operating at moderate (low GHz) frequencies will work just fine with standard FR4-grade epoxy-fiberglass materials and the type of copper will not create a noticeable difference in performance. Make sure you understand the types of copper that are called out in laminate datasheets if you have a performance target to hit.

High-Frequency Currents and Copper Foils

At higher frequencies, the skin effect will modify the impedance of a transmission line, and the magnitude of any impedance change due to the skin effect depends on the copper roughness. The interaction of high frequencies with copper traces produces losses through three primary mechanisms:

  • The impedance at mid-range frequencies will be larger and resistive lossses will be larger
  • Rougher copper confines field lines to a smaller volume, which increases the electric field flux and thus the dielectric losses
  • The increased impedance creates a slight impedance discontinuity, and if not matched it will increase reflections (S11)

We don't often look at skin effect at the front-end, but it's important to note that this will create impedance deviations and losses if not accounted for early. Typical frequency ranges where you will start to notice roughness are above 10 GHz.

Loss Budget

I would say the first important point in selecting the type of copper you'll use in your high-frequency PCB is to look at the loss budget for your highest frequency or highest bandwidth interconnects. For example, on an RF PCB, components that must source and receive an RF signal will have two specifications: transmitter output power and receiver sensitivity (or some similar names), both described in mW or in dBm. If you know an approximate board size or link length, you can get a pretty good estimate of the loss budget along one of your RF lines:

Loss Budget (dB) = [Tx Power (dBm)] - [Rx Sensitivity (dBm)]

This would be the total loss you can accept, although it's good to leave a few dB headroom above the Rx sensitivity value. Divide this by the length of the interconnect, and you now know the loss per length you can accept in your lines.

For high speed it's more complex because the signals do not have power and losses concentrated at a specific frequency. You can have high loss at high frequency, but as long as there is low loss in the receiver bandwidth range then the signal can be recovered at your receiver. Therefore, just like the case with input impedance, it's a good idea to select copper based by calculating losses at the bandwidth limit for your digital signals. This would be one of the following:

  • Nyquist frequency corresponding to the data rate
  • A value proportional to the inverse rise time (such as the knee frequency, or 0.35/(rise time))

Once you know the frequency that is important (either the carrier for RF boards or the bandwidth limit for digital boards), then you can move on to estimating losses and selecting copper.

A Process For Selecting Laminates and Copper

When matching up copper and dielectric materials, there is a simple process that can be followed to ensure you hit your operating targets.

  1. Plan out where you want to route the roughness-sensitive interconnects (surface layer vs internal layer), this will require a little bit of floorplanning with the major components
  2. Determine the loss tangent you can accept at your operating frequency
  3. After finding the material set that satisfies #2, look at your available copper options and choose the right copper roughness and surface plating for your design
  4. Make sure the copper option you select in #3 is available in the weight you need; heavier copper will have lower skin effect losses

I've listed Step #2 first because, at the operating frequencies where roughness is important, the dielectric will still dominate the losses and it will determine other aspects of the PCB stackup (layer count/thickness, etc.) that should be considered first. The plating and copper selection step should come next based on the laminate materials you have available.

PCB copper choice
This stackup example explicitly lists the laminate product name and the type of copper to be used in the design.

To ensure you fully specify your copper selection and stackup design for your manufacturer, use the design tools in Altium Designer®. You can determine impedance profiles and routing requirements inside the layer stack manager, as well as call out specific materials for use in the PCB stackup during fabrication. Once you’ve completed your PCB and you’re ready to share your designs with collaborators or your manufacturer, you can share your completed designs through the Altium 365 platform. Everything you need to design and produce advanced electronics can be found in one software package.

We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. Start your free trial of Altium Designer + Altium 365 today.

About Author

About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 2000+ technical articles on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, American Physical Society, and the Printed Circuit Engineering Association (PCEA). He previously served as a voting member on the INCITS Quantum Computing Technical Advisory Committee working on technical standards for quantum electronics, and he currently serves on the IEEE P3186 Working Group focused on Port Interface Representing Photonic Signals Using SPICE-class Circuit Simulators.

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