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Copper Foils in the Printed Circuit Board Industry

From Rogers Corporation technical content — a brief overview of copper foil manufacturing and properties critical for circuit simulation.

Introduction

This article touches on copper foils used in the printed circuit board industry and many of the properties that are critical to understand for good circuit simulations. The topic is deep; for more detail, see the Rogers Technology Support Hub (a URL is provided at the end of this document).

Electrodeposited Copper (ED Copper)

ED copper is electrodeposited copper. The process is a simple electrolytic plating setup: cathode, anode, and a metal solution (here, copper). Copper is plated onto a slowly rotating titanium drum. The rotation speed controls the final foil thickness. The foil is then peeled off. A treatment is also applied (not shown in simplified diagrams); that treatment is important and is discussed later.

Rolled Copper

Rolled copper starts from a copper billet — a thick block of copper — which is passed through a series of reducing rolls until the desired thickness is reached. The resulting foil is wound at the end. That is the basic process for making rolled copper.

Copper Surface Roughness

Copper surface roughness can affect:

  • Insertion loss of a circuit
  • Phase response

The roughness of interest here is at the substrate–copper interface as the laminate is made.

There are many ways to measure copper surface roughness. One approach is a non-contact laser profilometer. Common metrics include:

  • Ra
  • Rt
  • Rz
  • Rq (also Sq, depending on measurement)

Rq (or Sq) is the root-mean-square (RMS) roughness of the copper foil. For some electromagnetic modeling tools, RMS, Rq, or Sq are the values to use. Other software may use different parameters.

Treated Surface and Roughness

The treated surface is applied after the copper is made. The treatment serves several purposes: thermal stability, bonding, and resistance to chemical attack, among others. It also adds nodulation (roughness). Roughness values are often reported in RMS (Rq).

  • Rolled copper is the smoothest (e.g. ~0.35 µm RMS) and can show directionality (e.g. a “grain” direction) due to how it is manufactured.
  • The other types shown are different electrodeposited (ED) coppers, with different roughness and surface morphology.

In a small area, ED copper can show large local differences between peaks (e.g. red in height maps) and valleys (e.g. dark blue). Rougher copper has more variation; rolled copper is much smoother but still has some local variation — that is normal.

Insertion Loss and Copper Type

Insertion loss has been compared on microstrip transmission lines (e.g. differential-length method), with loss in dB per inch on the y-axis. In one study, the same circuit design and the same material (e.g. 5 mil RO3003®, same lot) were used; the only variable was copper type.

The trend of insertion loss matched the trend of copper surface roughness (at the substrate–copper interface). Rougher copper gave higher loss.

Modeling Copper Roughness in EM Software

Several industry models are used in electromagnetic (EM) software to account for copper surface roughness and its impact on RF performance:

  • Increased roughness → increased insertion loss
  • Phase response is also affected: rougher copper can slow the phase velocity and increase phase angle

These conductor-loss models are based on different concepts. Some use RMS (Rq / Sq); others use SAI (Surface Area Index), also called Surface Area Ratio.

Surface Area Index (SAI) is the ratio of the measured surface area of the foil (in a given region) to the area of an ideal flat surface over the same region. So it compares the real rough surface to a perfectly flat one.

  • SAI = 1 → perfectly flat
  • SAI > 1 → rougher surface

Data at 77 GHz (insertion loss in dB per inch vs. SAI) show a good correlation: higher SAI corresponds to higher insertion loss. Models that use this copper-roughness relationship add appropriately to the conductor losses.

Other Effects and Topics

Besides insertion loss, copper roughness can affect:

  • Phase response and phase velocity
  • Group delay
  • Impedance
    among other things.

There are also other important aspects of copper not covered here, such as its effect on wire bonding and the wire bonding process. For more on these topics and on copper foils in general, the Rogers Technology Support Hub is the recommended resource.

Conclusion

This article focused on:

  1. How copper is made — electrodeposited (ED) vs. rolled.
  2. Copper surface roughness — how it is measured (e.g. RMS, SAI), where it matters (substrate–copper interface), and how it affects insertion loss and phase.
  3. Use in simulation — using RMS or SAI (or other parameters) in EM software to account for conductor loss and phase effects.

For additional information, technical tools, calculators, and technical papers, join the Rogers Technical Support Hub. Rogers technical information is also available via the Rogers mobile app (iPhone, iPad, and Android).

For the Rogers Technology Support Hub URL and latest resources, refer to the original video or Rogers Corporation’s official site.