RA Copper VS ED Copper: Copper Foil for Flex Circuit Boards

Introduction to Copper Foil

Copper foil is an essential component in the manufacturing of flexible printed circuit boards (PCBs). It serves as the conductive layer that allows electrical signals to travel through the circuit. The quality and properties of the copper foil used can significantly impact the performance and reliability of the final product.

In this article, we will compare two types of copper foil commonly used in flex PCBs: rolled annealed (RA) copper and electrodeposited (ED) copper. We will explore their properties, advantages, and disadvantages to help you make an informed decision when choosing the right copper foil for your flex circuit board project.

What is Rolled Annealed (RA) Copper Foil?

Rolled annealed (RA) copper foil is produced by rolling a copper ingot into a thin sheet and then annealing it to improve its ductility and conductivity. The annealing process involves heating the copper foil to a specific temperature and then cooling it slowly to relieve internal stresses and increase its softness.

Advantages of RA Copper Foil

1. High ductility: RA copper foil is highly ductile, making it ideal for applications that require bending or flexing of the circuit board.
2. Excellent conductivity: The annealing process improves the conductivity of RA copper foil, ensuring efficient signal transmission.
3. Smooth surface: RA copper foil has a smooth surface, which facilitates better adhesion to the substrate and reduces the risk of delamination.

Disadvantages of RA Copper Foil

1. Higher cost: The manufacturing process of RA copper foil is more complex and time-consuming, resulting in a higher cost compared to ED copper foil.
2. Limited thickness options: RA copper foil is typically available in thicknesses ranging from 18 to 70 microns, which may not be suitable for all applications.

What is Electrodeposited (ED) Copper Foil?

Electrodeposited (ED) copper foil is manufactured by electroplating copper onto a rotating drum or continuous stainless steel belt. The thickness of the ED copper foil is controlled by adjusting the current density and plating time during the electrodeposition process.

Advantages of ED Copper Foil

1. Lower cost: ED copper foil is generally less expensive than RA copper foil due to its simpler manufacturing process.
2. Wide range of thicknesses: ED copper foil is available in a broader range of thicknesses, from 5 to 150 microns, providing more options for various applications.
3. Good electrical properties: ED copper foil offers good electrical conductivity, making it suitable for most flex PCB applications.

Disadvantages of ED Copper Foil

1. Lower ductility: ED copper foil is less ductile compared to RA copper foil, which may limit its use in applications that require extensive bending or flexing.
2. Surface roughness: The surface of ED copper foil is relatively rough, which can affect the adhesion to the substrate and increase the risk of delamination.

Comparison Table: RA Copper vs ED Copper

Choosing Between RA Copper and ED Copper

When deciding between RA copper and ED copper foil for your flex PCB project, consider the following factors:

1. Application requirements: If your application demands high flexibility and repeated bending, RA copper foil may be the better choice due to its higher ductility. However, if your application does not require extensive flexing, ED copper foil can be a cost-effective option.

2. Budget constraints: If cost is a primary concern, ED copper foil may be more attractive due to its lower price point. However, keep in mind that the long-term reliability of your flex PCB should also be considered.

3. Thickness requirements: If your application requires a specific copper foil thickness that falls outside the range offered by RA copper foil, ED copper foil may be the only suitable option.

Frequently Asked Questions (FAQ)

1. Can I use a combination of RA and ED copper foil in my flex PCB design?

Yes, it is possible to use both RA and ED copper foil in the same flex PCB design. This approach can be beneficial when certain layers require higher flexibility (using RA copper) while others can use the more cost-effective ED copper.

2. How does the choice of copper foil affect the signal integrity of my flex PCB?

The choice of copper foil can impact signal integrity in several ways. RA copper foil’s smooth surface and higher conductivity can lead to better signal transmission and reduced signal loss. ED copper foil’s rough surface may cause more signal reflections and losses, particularly at higher frequencies.

3. Can I use RA or ED copper foil for high-frequency applications?

Both RA and ED copper foil can be used for high-frequency applications, but RA copper foil may be preferred due to its smoother surface and higher conductivity. The smooth surface minimizes signal reflections, while the higher conductivity reduces signal loss.

4. How does the annealing process affect the properties of RA copper foil?

The annealing process improves the ductility and conductivity of RA copper foil by relieving internal stresses and increasing its softness. This process involves heating the copper foil to a specific temperature and then cooling it slowly, allowing the copper crystals to reorganize and grow, resulting in better mechanical and electrical properties.

5. Can I specify the surface roughness of ED copper foil for my flex PCB?

Yes, you can specify the surface roughness of ED copper foil when ordering from a supplier. Different surface roughness levels are available, such as low profile (LP) and very low profile (VLP). Discuss your requirements with your supplier to ensure you receive the most suitable ED copper foil for your application.

Conclusion

Choosing the right copper foil for your flex PCB project is crucial for ensuring optimal performance, reliability, and cost-effectiveness. RA copper foil offers high ductility, excellent conductivity, and a smooth surface, making it ideal for applications that require repeated bending or flexing. On the other hand, ED copper foil provides a lower-cost alternative with a wider range of available thicknesses, making it suitable for applications that do not demand extensive flexing.

When making your decision, consider your application requirements, budget constraints, and thickness needs. By understanding the properties, advantages, and disadvantages of RA and ED copper foil, you can make an informed choice that best suits your flex PCB project.