Ask the ASC Expert Ask the ASC Expert: Impact of copper roughness on RF Performance

Written by: John Bushie on June 14, 2019

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In this video video, ASC's Director of Technology John Bushie discusses the impact of copper roughness on RF Performance.


John Bushie with American Standard circuits here. In this segment, we're talking about copper roughness. Traditionally, when we talked about the treatments there on the back side of the copper that was adhered to the resin, or the dielectric surface of the material we were talking about electrodeposited copper

Generally it was a very pronounced, very rough surface finish with its main goal being adhesion to the laminate system or the resin system that it was adhered to. How it was traditionally formed or how it is formed these large tooth structures is basically building a series of nodules or dendrites on the backside of it, and basically what it ended up having is a very rough profile even in these areas that it looks like there's no tooth there's actually still a stack of these these little nodules so to speak.

It increased the surface area, gave you great adhesion, but one thing it did is it also degraded the RF performance particularly at higher frequencies. That created the advent of what were low profile very low profile and what is becoming even more common today ultra low-profile copper foils.

What they've done has been able to reduce the roughness on that backside using proprietary methods, while still maintaining the adhesion of the substrate in order to meet IPC specifications for peel strength. This is increasingly important when we start talking about outer layers, where there's going to be components or fairly heavy devices attached to these circuits

One of the latest advents is almost no profile copper. This starts to play in a realm that used to only be in the ultra high performance, ultra high cost rolled annealed copper foil territory. Rolled anneal had several advantages. It was formed in a rolling method which created a laminar grain structure, which is opposite or in line with that of the the electrical signal traveling through it. Electrical positive copper foils tend to have a very columnar structure, which is counter to that.

The ability to be able to reduce the roughness, increase the surface adhesion between these almost no-profile coppers has allowed us to use a material that's fairly low cost, roughly equivalent to these other forms of electrodeposited copper foil while achieving most, if not all the performance you can get from rolled annealed copper foil.

Needless to say, rolled anneal is still preferred in some of the ultra high frequency applications, but for the cost almost no profile is a huge benefit.

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