Rigid Flex Circuits Be Bent Repeately Without Damage
Rigid flex circuits offer the benefit of greater interconnect density within a much smaller package size and lower overall system weight than is possible with traditional rigid PCBs. They are also highly aesthetically pleasing. But repeated bending of rigid-flex circuits beyond their minimum bend radius can cause fatigue that reduces their reliability over time. Fortunately, careful handling and assembly practices can prevent this damage.
The process for producing a rigid-flex PCB starts with stacking various layers of materials together to form the board’s core. Depending on the final product, the rigid-flex circuit can have one or two, three, or four layers. Once the layer stack-up has been finalized, it can be laminated into a rigid-flex board.
The rigid flex circuits board can then be cut to the required length and soldered into place using surface-mount components. This will require the use of solder paste that is compatible with the copper-based materials of the flex circuit. Rigid-flex circuits must also be designed to accommodate the flex area’s unique bending characteristics, which are typically more pronounced than those of rigid PCBs.
Can Rigid Flex Circuits Be Bent Repeately Without Damage?
When a flex circuit is bent repeatedly, it can stretch and compress the copper lines in the PCB, resulting in the loss of contact between the copper and the underlying dielectric. This can lead to failure over time, such as broken copper traces, cracked vias, and delamination. To ensure that the rigid-flex circuit withstands repetitive bending, designers must follow best practices for design for manufacturability (DFM) and work with experienced rigid-flex fabrication partners.
It is important to design a rigid-flex circuit with a bend ratio that is no more than 3:1. This will help to minimize stress and strain on the copper layers and reduce the likelihood of fractures and delamination. It is also helpful to avoid sharp or abrupt changes between wide and narrow traces. Instead, a gradual transition is recommended. This helps to reduce stress concentrations and increases the longevity of the flexible circuit.
Depending on the frequency of bending, it is also advisable to include stiffeners in areas where the flex circuit meets components that are attached to the board. This will limit the number of times that a flex circuit can be bent, and it will also increase the strength of the bending area.
In addition to stiffeners, it is a good practice to use anchoring to support the copper-based parts of a flex circuit. This method of connecting a trace to a pad or a through-hole reduces stress by concentrating the force of bending on the points where the anchor is connected, rather than at the trace itself. Advanced tools can auto-generate anchor geometries, making this an easy way to improve the longevity of a flex circuit design.