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.
Recently, our team ran into a painful issue: cracked traces on an FPC. The problem quickly escalated, tying up engineering resources and causing significant losses for the supplier as well. As often happens after a failure, an old debate resurfaced—should FPCs use Rolled & Annealed (RA) copper or Electro-Deposited (ED) copper?
From a product standpoint, the answer is actually quite clear for us. Most of the FPCs in our designs are required to bend, flex, or move repeatedly during normal operation. Because of that, we explicitly require RA copper for FPC fabrication.
However, not every engineer is deeply familiar with copper foil materials, and RA copper is not always clearly specified on the drawings. This creates room for suppliers to substitute materials quietly. If nothing goes wrong, no one notices—but once a failure occurs, it often turns into a long and messy dispute.
That’s why Workingbear strongly recommends clearly specifying RA copper on FPC drawings. In manufacturing, material choice matters, and you generally get what you pay for.
I’ve written before about the differences between ED copper and RA copper in FPCs, but it’s worth revisiting the fundamentals. If you look at microscopic cross-sections of copper foil, the difference is obvious. ED copper has a vertical, column-like grain structure, while RA copper has a horizontal, layered grain structure.
When an FPC needs to bend or undergo repeated flexing, this structural difference becomes critical. ED copper’s pillar-like grains are more prone to cracking under mechanical stress. RA copper, with its layered structure, distributes stress much more effectively.
ED copper has a vertical, column-like grain structure
RA copper has a horizontal, layered grain structure
ED copper (Electrodeposited copper)
RA copper (Rolled & Annealed copper)
An easy way to visualize this is with a deck of cards. Stack the cards vertically, and it’s easy to pull them apart—that’s ED copper. Lay the cards flat in layers, and they resist separation—that’s RA copper.
In short, RA copper’s horizontal grain structure gives it significantly better flex-life performance than ED copper, making it the preferred choice for dynamic FPC applications.
ED copper: Vertical pillar grain structure (electrodeposited)
RA copper: Horizontal layered grain structure (rolled & annealed)
That said, real-world manufacturing is rarely that simple. Even when RA copper is specified as the base material, we still frequently see cracking issues in double-layer and multi-layer FPCs.
The reason is process-related. For FPCs with two or more layers, interlayer connections require electroplating, which means ED copper is deposited on top of the original RA copper. In many cases, the plated copper thickness is equal to—or even greater than—the original RA copper thickness. This plating process can also disrupt the original grain structure of the RA copper.
This likely explains why multi-layer FPCs have much poorer bending durability compared to single-layer FPCs, even when RA copper is used.
At this point, Workingbear hasn’t found a truly effective method to significantly improve the flex durability of double-layer or multi-layer FPCs. If you have experience or insights in this area, I’d genuinely love to hear your thoughts.
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