AI servers, switches, and optical modules are driving PCBs toward higher frequencies.
From 56G to 112G, and now to 224G, copper foil has been the medium for signal transmission throughout, which places higher demands on it.
What Exactly Does Copper Foil Do in a PCB?
Copper foil serves three main functions in a PCB.
First, the PCB forms signal traces. These copper traces transmit high-speed signals between chips, between chips and connectors, and between switch chips and optical modules.
Second, it forms power and ground planes. High-speed signals do not travel alone on a single line; they must rely on a reference plane.
The ground plane (zero potential) and power planes provide return paths and also determine impedance, crosstalk, electromagnetic field distribution, and power integrity.
Third, it contributes to heat dissipation and interlayer connectivity.
Copper has good thermal conductivity; large copper areas, power planes, and ground planes help dissipate some of the heat, while electroplated copper and vias connect different layers.
Therefore, copper foil is not merely an ordinary metal sheet in a PCB.
It serves multiple functions simultaneously: signals, power, ground, thermal management, and interlayer connectivity.
In the era of high-speed electronics, the most critical aspect is the interface between signal lines and the reference plane.
Skin Effect and Proximity Effect: Why Smooth and Rough Are Contradictory
With low-frequency currents, one can roughly understand that the current flows relatively uniformly throughout the cross-section of a copper conductor.
But this is not the case with high-speed signals. High-frequency signals encounter two effects.
One is the skin effect: as the signal frequency increases, the current concentrates more near the surface of the conductor.
High-frequency current penetrates deeper and deeper into the copper, but at this point, the signal does not perceive the entire block of copper—only its surface.
The closer the surface roughness is to the skin depth, the more pronounced the effect becomes.
In this case, the high-frequency current must travel along the undulating surface, resulting in a longer path and making control more difficult.
The other phenomenon is the proximity effect.
High-speed signal lines sit next to a reference ground plane, and another trace flanks the differential pairs; electromagnetic fields cause the current to concentrate in the surface regions closest to each other.
Consequently, the current on the signal line tends to concentrate toward the reference plane.
Therefore, the point of highest current concentration for high-speed signals is often the interface between the copper foil and the dielectric.
This necessitates placing the smoother side against the resin to reduce conductor loss.
However, while a smooth copper surface reduces signal loss, it does not bond firmly to the resin. This is the core dilemma of high-speed copper foil.
The goal for high-end copper foil is to strike a balance between low surface roughness and sufficient peel strength, with the surface roughness kept below the skin depth of the signal transmission.
Grading and Manufacturing of Copper Foil
Standard electrolytic copper foil has a higher surface profile and is more suitable for traditional circuit boards.
LP stands for Low Profile, referring to copper foil with a low profile. VLP stands for Very Low Profile, indicating an even lower profile. H
VLP stands for Hyper Very Low Profile. This technology further reduces surface roughness and targets high-speed, high-frequency, low-loss circuit boards.
High-end applications also use terms such as HVLP2, HVLP3, and HVLP4.
While definitions may vary slightly among manufacturers, the goal is consistently to achieve a lower-profile copper foil surface better suited for high-speed signals.
Electrolytic copper foil is the mainstream choice for PCBs.
Manufacturers produce it through an electrochemical process: copper ions deposit on a rotating cathode roller to form a thin copper layer, and operators then peel the layer off the roller surface; manufacturers call this side the “smooth side.”
The other side faces the electrolyte, where copper grains grow freely; manufacturers call this side the “rough side.”
To achieve a low-roughness surface on both sides of electrolytic copper foil, one cannot rely on mechanical polishing after the fact.
The key lies in the front-end deposition and back-end surface treatment processes.
Manufacturers must control grain growth during the electroplating stage.
Current density, electrolyte formulation, additives, temperature, flow patterns, and the condition of the cathode roller all influence the morphology of the copper grains.
For high-end, low-profile copper foil, where grain refinement and uniformity are essential, the coarse side must not grow too high from the outset.
Further Double-Sided Surface Treatment
Surface Engineering: Enhancing Copper Foil Reliability for High-Speed PCBs
Copper foil requires further double-sided treatment, with surface engineering applied to both sides.
In high-end multilayer boards, HDI boards, and IC substrates, both sides of the copper foil may come into contact with resin, dielectric materials, or subsequent processing environments; treating only one side is insufficient.
First, a micro-roughening layer is applied. Low-profile micro-roughening is used to replace traditional high-profile roughening.
Traditional roughening can cause the growth of relatively tall copper nodules or dendrites on the copper surface.
Micro-roughening, on the other hand, involves electrochemical deposition on the copper surface to form short, dense, and uniform copper particles.
Next, manufacturers perform a fixation treatment after micro-roughening.
If the process does not securely bond the fine copper particles, they may flake off during subsequent lamination, etching, and cleaning processes.
The process involves depositing an additional cover layer to secure these particles, reduce flaking, and improve reliability.
Multi-Layer Surface Treatments for Better Interface Performance
In addition to the micro-texturing layer and the fixation layer, the process typically includes a barrier layer, an anti-oxidation layer, and a coupling layer.
The barrier layer reduces copper migration and high-temperature interface failure.
The anti-oxidation layer protects the copper surface. The coupling layer enhances the chemical bond between the copper surface and the resin.
Among these, silane coupling agents are an important class of materials.
One end of the silane molecule interacts with the copper surface treatment layer or the oxide surface, while the other end is compatible with or reacts with the resin system.
It acts as an interfacial bridge, enabling a more stable bond between the metal surface and the organic resin.
Previously, chromium salts were also used in copper foil surface treatment.
Chromium improves oxidation resistance and thermal reliability, helping the copper foil remain stable during storage, lamination, and in high-temperature environments.
However, due to the toxicity of hexavalent chromium and environmental concerns, these processes have begun to be phased out.
From Conductivity to Surface Properties: The Future of High-End Copper Foil
In summary, the key challenge in high-speed PCBs has shifted from “conductivity” to “surface properties.”
The value of high-end copper foil lies not only in the purity and thickness of the copper but also in its ability to achieve sufficiently low surface loss while adhering firmly to the resin.
Upgrades to high-end copper foil involve not only making it thinner and smoother but also making surface treatments more environmentally friendly and reliable.
Customers demand not only low loss but also long-term reliability, mass-producibility, and compliance with environmental requirements.
Conclusion
As high-speed electronics continue to evolve from 56G to 112G and 224G transmission rates, the role of copper foil in PCB manufacturing has become increasingly critical.
Copper foil is no longer simply a conductive layer; it directly affects signal integrity, power distribution, thermal management, and interlayer reliability.
For high-frequency PCBs, the key challenge has shifted from basic conductivity to controlling surface characteristics.
Surface roughness, skin effect, proximity effect, and resin adhesion all influence signal loss and long-term reliability.
Advanced copper foil technologies, such as LP, VLP, HVLP, and HVLP2–HVLP4, aim to achieve an optimal balance between low transmission loss and strong bonding performance.
Through precise manufacturing control and advanced surface treatments, high-end copper foil can support the demanding requirements of AI servers, high-speed switches, optical modules, and other next-generation electronic systems.
In the future, copper foil development will continue to focus on lower loss, improved reliability, environmental compatibility, and stable mass production to meet the growing demands of high-frequency PCB applications.
