As seen in the January 2019 issue of Flex007 Magazine:
Co-written with Anaya Vardya, American Standard Circuits.
One of the primary advantages of moving to a flexible circuit design from a rigid board is the ability to package the flex in three dimensions, bending or folding into imaginative configurations and saving precious space in the final package. While flexible materials are robust and can withstand many flex cycles, nearly everyone can share a war story about the flex that didn’t originally perform as expected with copper cracking after installation.
I [Tara] remember an example from my early days in flex fabrication. We had built a fairly simple, double-sided flex with FR-4 stiffeners on both ends. After installation, the customer contacted us because the copper was cracking while it was being bent. In that case – and most cases even today – fabricators often have only a 2D view of the design. After some investigation of how the flex was being used, we made several recommendations to improve the performance of the circuit. Materials were adjusted, traces were re-routed to keep all of the traces on one layer in the critical bend area, polyimide stiffeners were added to guide the bend exactly where it needed to be in the unit. Rather than plating electrodeposited (ED) copper onto the flexible rolled-annealed (RA) copper, we button-plated and only plated ED copper on the pads and plated through-holes. The end result? Success! No more cracking.
Stories like this are not uncommon. Fabricators have quite an arsenal of tips and tricks to help improve flex life and avoid damage to flex materials during installation and use, yet are often building the circuits without knowledge of how it is going to be used in the final application. While our intention is to share some of the common methods of improving flexibility, we also want to strongly encourage everyone to communicate the flex and bend areas in the fabrication drawings or have discussions with your fabricator prior to release to take advantage of the knowledge they have to draw from and improve the performance of your new design.
Electrodeposited (ED) Copper Foil
ED copper is formed by electrolytic deposition onto a slowly rotating polished drum from a copper-sulfate solution. When an electric field is applied, copper is deposited on the drum as it rotates at a very slow pace; the slower the pace, the thicker the copper. The side against the drum provides the smoother finish.
Rolled-Annealed (RA) Copper Foil
RA copper foils are created by successively passing an ingot of solid copper through a rolling mill and then applying high temperature to anneal the copper. RA copper foil has higher ductility and elongation than ED copper, which is why it is best for bending applications.
Grain Direction Matters
RA copper is primarily used in flexing and bending applications. Circuit direction and orientation on a manufacturing panel are critical to maximizing the flex life in dynamic flexing applications. Grain direction is positioned perpendicular to the bend axis because bending across the grain direction will inherently negate the advantage of the elongated grain structure.
Routing in Bend Areas
In addition to the conductors being perpendicular to the bend axis, it is also critical that the conductors do not traverse in the bend area.
Button Plating
When dealing with multilayer flex applications that require a lot of bending, it is important to specify that button plating or via-only plating is required. Typically, when plating copper in the plated through-holes, the fabricator will be plating the full panel. This plating process uses ED copper, which is not as flexible as the RA copper foil. This can impact the flexibility of the circuit in applications with tight bend radius or dynamic flexing requirements. When button plating is specified, the fabricator will plate only the copper pads and through-holes – not the full panel. Many cracking failures have been resolved once the process to manufacture the flex circuit has been changed from plating copper on the circuit layers and vias to a via-only plating operation.
Balanced Construction
The construction should be balanced from its centerline including copper, dielectric, and adhesive thicknesses. An unbalanced construction will cause stress to occur in one direction, decreasing flex life.
Minimum Bend Radius
Even though flex circuits are very pliable and flexible, there are limits to their flexibility. If the bend radius is too tight, the result can be delamination and conductor fracture.
There is a rather complex formula to determine an acceptable bend radius but as a rough rule of thumb:
- Single-sided and double-sided flex applications: The minimum bend radius should be six times the overall thickness; as an example, if the overall thickness of the flex circuit is 0.012″, the minimum bend radius should be 0.072″.
- Multilayer flex and rigid-flex with bonded inner layers: The minimum bend radius should be 12 times the overall thickness; as an example, if the overall thickness of the flex circuit is 0.030″, the minimum bend radius should be 0.360″.
Multiple Flex Layers on a Rigid-flex
For improved bending, inner layers should not be bonded together in the flexing area; this is typically referred to as “loose leaf” construction. Figure 5 in the article illustrates a 14-layer rigid-flex where the inner layers were not bonded together to improve bend radius.
Crosshatched Copper Shielding
Using a crosshatched pattern rather than a solid copper shield can greatly increase the flexibility of a flexible circuit. This can be customized by removing more or removing less material based on design needs. Crosshatching can also be applied in just the critical bend areas leaving sections that are not exposed to bending and flexing as full copper layers.
Conclusion
As mentioned earlier, this is certainly not an exhaustive list of all the options available to improve the flexibility of a design but a good list to review when working on a design and a place to start discussions with your fabricator. Most everyone can tell their tale of the “flex that didn’t flex”. This short list will help decrease the odds of that occurring. We strongly encourage early and clear communication about flexing and bending requirements with your fabricator who can provide suggestions for your specific application.