The performance and longevity of a flexibly circuit board can be directly attributed to the layout of the circuitry. Signals should never be routed at sharp angles. BEST TECH recommends the largest radius possible for your design. 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.
When design flex with 2 layers+ stagger traces on the front and back
• 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
Traces should be perpendicular to the bend.
1)The conductors shown in the “incorrect” column will have stress risers at the their angles. Premature failures (conductor fractures) can occur at these points. The conductors shown in the “correct” column have their angled corners “radiused” to reduce the stress points. However it is still preferred to not have any traversing conductors in the bend area as shown in the “PREFERRED” column especially in dynamic flexing applications.