Design Guidance

2022-11-08 BEST

Routing Flex Traces

The layout design of a flex circuit will be an important element to affect the working performance and life cycles of a flexible printed circuit board (FPCB). As it will be always bending, or folding, the signals trace should be designed with a round angle, instead of a sharp angle as the latter one be broken easily in the application. BEST TECH recommends designing the largest radius of flexible circuits as possible as you can.

In addition, I-beaming will not only reduce flexibility of your circuit, it will increase stress contributing to the thinning of copper circuits at the bend radius.

2 layers flex circuits, top and bottom side of trace, staging together.

Correct (not on the same location through top and bottom)
Correct (not on the same location through top and bottom)

Incorrect (on the same location through top and bottom)
Incorrect (on the same location through top and bottom)

In an “I-Beam” condition, during flexing the conductors on the inside radius, will be stressed (see sketch below) and will be prematurely failed. By staggering the conductors as shown in the “YES” column, the stress will be evenly displaced during the bend optimizing flexible life.

 

Use curved traces instead of traces with corners

Curved  Traces

The bending area should be within the straight trace area

traces-perpendicular-to-the-bend

1)The traces shown in the “incorrect” will have stress risers at when bending. The failures of trace broken can be happened at these areas. The traces shown in the “correct” column have suitable radius or corners to share and reduce the bending stress. However, it is still preferred to not have any traces in the bend area as shown in the “Preferred” column, especially for flexible circuit applications in which many times of dynamic moving is needed.

2)There shall be NO “vias” in the bend areas, as the via will be became broken very easy during the assembly, bending process.