Don'ts of Flex Circuit
It’s easy to look at the problems of layer stack design, parts placement, and cutouts and think we can solve these issues easily. But remember that flex circuits have some gnarly material quirks which range from relatively high z-axis expansion coefficients of adhesives, to the lower adhesion of copper to substrate and coverlay, to copper’s work hardening and fatigue. These can be compensated for largely by following some don’ts.
Don’t bend at corners. It is generally best to keep copper traces at right-angles to a flex-circuit bend. However there are some design situations where it’s unavoidable. In those cases you should keep the track work as gently curving as possible, and as the mechanical product design dictates, conical radius bends is a better way.
Don’t change widths unexpectedly. Whenever you have a track entering a pad, particularly when there is an aligned row of them as in a flex-circuit terminator, this will form a weak spot where the copper will be fatigued over time. Unless there is going to be stiffener applied or a one-time crease, it’s advisable to taper down from the pads.