Flexible Circuits
In many ways, the toughest environment for a flexible circuit is during prototyping and testing of the design. This is where the prototype is most likely to be mishandled, dropped, bent beyond its design limits and so on. As a result, the circuit board can be damaged and potentially fail to perform as it should in the end application. It is also the most common time where failure of a flex circuit happens. This is because the designer has not carefully considered how the circuit will be used.
For a flexible circuit to successfully operate and endure repeated bending it must be designed to be resilient. To do this, it is important that the circuit is designed with the minimum bend radius in mind as early as possible in the design process. This minimum bend radius is the smallest radius that the copper in the circuit can safely withstand without fracturing or causing delamination.
The conductive copper layers in the circuit are typically made of rolled annealed copper foil that can be 12 to 35 microns thick. These are bonded to the substrate using acrylic or epoxy adhesives that are optimized for flex circuit constructions. In dynamic flex applications the conductors can be covered with a solder mask layer to protect the copper from damage or oxidation. In addition, stiffeners made from materials like FR4, aluminum and ply can be added to the flex circuits in selected areas to reinforce them against bending.
Can Flexible Circuits Be Bent Repeatedly Without Damage?
When a flex circuit is subjected to repeated bending, the copper work hardens and becomes less able to withstand the pressure put on it. The flex circuit can then develop stress fractures or cracks at the points of high strain. This can lead to a loss of conductivity that could cause a system failure.
A common failure mode for a flex circuit is when it is bent to a tight angle such as a 90° that creates high stresses. Another failure mode occurs when the flex circuit is bent directly adjacent to an unsupported ENIG surface finished pad. These pads are commonly found on the edge of the flex circuit and are connected to a trace on a different layer. This unsupported pad section is exposed to bending forces that will cause the nickel in the ENIG coating to fracture/crack and break the copper trace underneath it.
A flex circuit designer can mitigate these issues by designing with the minimum bend radius in mind. They can do this by ensuring the minimum bend radius is calculated early in the design phase and that it meets industry standard guidelines. They can also ensure that their design has no plated through holes in the bend area and that the traces are staggered to avoid stress on the flex circuit at a transition point. Stiffeners should be placed where possible to prevent stress at the coverlay to substrate interface and at the copper to stiffener interface.