Working for manufacturer engineering couple years. Sharing the manufacturing experience, skill, DFx, design, and information. Be care of this blog is personal share and content may not be 100% correctly.
Modern electronic devices keep getting smaller, and that trend has driven the widespread use of Flexible Printed Circuit Boards (FPCs). One interesting thing about FPCs is that in many designs you can actually see the internal traces from the outside, which makes their structure relatively easy to understand.
If we take a single-layer FPC as an example, its structure is quite simple. It usually consists of a copper layer, with insulating cover films on the top and bottom. In some designs, a stiffener is also added to provide extra mechanical support. Because the structure looks simple, many engineers tend to overlook how important the copper foil itself really is.
In reality, the quality and type of copper foil directly affect the reliability, flexibility, and lifetime of the FPC.
Did you know that copper foil used in FPCs generally comes in two main types?
Electrodeposited copper (ED copper)
Rolled and annealed copper (RA copper)
Understanding the difference between these two materials can help engineers choose the right copper for their specific applications.
ED copper (Electro Deposited copper)
RA copper (Rolled & Annealed copper)
Crystal structure
Photos
How ED Copper and RA Copper Are Made
ED copper is formed by a plating process, where copper is deposited layer by layer until the required thickness is reached. Because of this process, the crystal grains grow in a vertical, column-like structure. This structure tends to be less dense, and electrodeposited copper may also contain pinholes or co-deposited impurities, especially when the copper foil becomes very thin.
RA copper is produced by heating and mechanically rolling copper until it reaches the desired thickness. This process creates a flattened, layered crystal structure, somewhat like stacked fish scales. Because of this structure, RA copper tends to be denser and mechanically stronger, especially when the material needs to withstand repeated bending.
Flexibility Performance
Not very resistant to repeated bending. Easier to etch during PCB/FPC manufacturing.
Much better bending durability. Harder to etch due to its denser structure.
Typical Applications
Suitable for static applications. Used when the FPC does not need frequent movement or bending
Ideal for dynamic applications. Recommended when the FPC must flex repeatedly during operation.
Crystal lattice structure
Pillar (vertical column structure)
Slice (horizontal layered structure)
Surface roughness
Higher (easier for etching and lamination)
Lower (may require additional surface treatment)
Deflection Degree
Lower
Higher
Price
Medium (usually thicker foil costs more)
Higher overall (However, when the thickness is below 0.5 oz, the thinner it is, the more expensive it becomes because it requires more rolling passes.)
Final Thoughts from Workingbear
When designing an FPC, engineers should not focus only on the circuit layout or mechanical shape. The choice of copper foil material can have a huge impact on product reliability.
If the flex circuit will remain mostly stationary, ED copper may be sufficient and more economical.
But if the design involves frequent bending or movement, RA copper is usually the better choice because of its superior flex durability.
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