flex PCBs accommodate for bending fatigue
For flex PCBs that must withstand repeated bending, the design and construction of traces and vias should be optimized to withstand fatigue. There are a few key principles to follow:
Minimize layers: The thickness of each layer adds to the overall flex pcb weight and can contribute to stress if it is not kept in check. If possible, use blind or buried vias instead of conventional plated ones, as these are more resilient.
Keep traces as short as possible: Long traces are more susceptible to damage and can cause circuit failures when bent. The length of critical signal lines should also be minimized to prevent failures. Use curved traces: Rather than straight angled traces, it is best to use curved traces in the flex area. This helps to ensure that bending doesn’t create stress on the trace and can prevent cracking or breaking of the flex circuit board.
How do flex PCBs accommodate for bending fatigue?
Use wide ends on the flex PCB: Wide end traces have less resistance to bending than narrower ones. Therefore, they are more likely to be able to endure bending and other types of physical abuse than narrower ones.
Make sure traces have a minimum of 20 mils clearance from the edge of the flex PCB. This will help to avoid a stress point where the copper annulus of a via meets the rigid-to-flex board interface. This is especially important with multi-layer flex areas. Using the CAD editor’s clearance rules will ensure this is done automatically for you.
Keep the minimum bend radius as large as possible: The smaller the flex PCB’s minimum bend radius, the more stress it will endure and the faster it will fail. This stress is caused by the compression of the inner flex layer and elongation of the outer flex layer. To avoid this, the flex PCB should have the minimum bend radius set as large as possible and be designed to be as flexible as possible.
Incorporate stiffeners: Some flex areas may require additional rigidity to support the circuit’s structural integrity. Selected flex areas can be reinforced with a layer of rigid dielectric material (like FR4 or aluminum) laminated to the flex layer stack. This option increases the flex PCB’s cost.
Choose the right substrate material: In order for a flex circuit to be reliable it needs to have a good tensile strength. It should also have a smooth and consistent grain structure. Rigid boards can get by with electro-deposited (ED) copper, but flex circuits need to have rolled annealed (RA) copper that can withstand more than 10,000 bend cycles.
Use proper component placement: This is particularly important for SMT components because they are more prone to damage from flexing and bending. The pad and trace arrangement should be designed with anchoring stubs, reduced coverlay access openings and a smaller solder mask outline than for rigid boards. This allows the SMT component pads to be anchored to the flex circuit and will reduce their vulnerability to bending fatigue.
