Dos 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 dos.
Do keep flex flexible. This may seem obvious, but it’s worth saying. Decide just how much flex is needed up front. That is if your flex-circuit sections are only going to be folded during assembly and then left in a fixed position, then you have more freedom to choose the layer number and the copper type. On the other hand, if your flex-circuit sections are going to be continually moving, bending or rolling, then you should reduce layer number for each sub-stack of flex.
Do use curved traces. It’s best to avoid abrupt hard right-angle track work, and even better than to use 45° hard corners, route the tracks with arc corner modes. This reduces stresses in the copper during bending.