Signals travel along copper pathways on a PCB, and several PCB design factors play a role when propagating these signals. Overlooking any PCB design element can result in signal distortions.
One of the pivoting attributes is the type and quality of the Copper and its adhesion to the PCB’s dielectric layer. More attention to PCB copper is probably justified since this material is not entirely as simple as many may think.
Two types of copper foils are typically used in manufacturing PCBs: Electrodeposited (ED) and Rolled-Annealed (RA) Copper.
The RA Copper fabrication process starts by passing pure copper ingots through a series of rolling processes, resulting in the desired thickness of copper foil.
During the ED Copper fabrication process, Copper is plated onto a drum. The rotation speed of the drum determines the copper foil thickness. So thicker Copper is going to take a longer time. The side of the Copper next to the drum will be very smooth, and the other side of the Copper will be rougher. The copper grain structure will parallel to the plane. The grain structure of RA Copper is different from ED copper, and this impacts the etching process.
RA Copper, on average, has a surface roughness of 0.35 microns RMS (root mean square). The smooth side of ED copper is a little rougher than that and usually averages around point .35 to .40. The rough side of the ED Copper can vary a lot depending on how the Copper is made. Some of that roughness is desirable because it improves adhesion with the laminate: the rougher, the Copper, the more bond area and better peel strength. But rougher Copper surface increases insertion loss, and the resistance slows down the signals.
On very thin circuits, such as those for high-speed digital or millimeter wave ones, the thinner the substrate is, the more sensitive the circuit will be to the surface roughness of the Copper. When the copper planes are really close together, in the case of a really thin laminate, the copper planes have a pretty significant influence. And the roughness does impact the performance a lot.
For very high-frequency applications, the circuit geometry usually needs to be etched very accurately, and this can be done better if the Copper is thin.
So if you have a really sensitive design using thin materials, it’s good to know what the average roughness is and try to understand what to expect for the variation of the roughness.
| Characteristic | Electrodeposited (ED) Copper | Rolled Annealed (RA) Copper |
|---|---|---|
| Appearance | Red | Yellowish |
| Surface roughness | The surface facing the solution is much rougher. | About the same surface roughness on both sides of the copper. Smoother than ED Copper. |
| Properties | Mechanically very robust. Rapid thermal cycling can cause thermal stress cracks in narrow conductors formed with ED copper. Has greater tensile strength and elongation before breaking. As copper roughness increases, the conductor loss will increase, and so will the insertion loss. | Superior resistance to conductor cracking from thermal stress where thermal shock might be a concern. Better elastic elongation before reaching permanent deformation. Enables improved etch definition for fine-featured, high-frequency circuits. |
| Applications | Suitable for applications where mechanical stress may be critical factor. | Ideal for high-frequency applications. |
Selecting the right PCB materials for a circuit’s optimum performance and reliability is an essential step in the PCB design process. The type of copper foil, its surface roughness, and how well it adheres to the dielectric material are consequential factors for the optimal functionality of a PCB, especially for high frequency application.