The Pure FR4 rigid-flex is a variant where the flexible section is replaced with thin FR4. It remains flexible enough for installation but offers greater robustness and bend resistance than semi-flex.
is the "pure-FR4-rigid-flex". Here we ahve a standard rigid-flex with the same manufacturing process, except that the flexible part is replaced by a very thin FR4 core. This PCB is then still flexible enough for installations and performs better than semi-flex in this regard. This "pure-FR4-rigid-flex" allows smaller bending radius than semi-flex PCBs.
The big advantage compared to standard rigid-flex PCBs is the absence of Polyimide. This means that the whole board has a far lesser hygroscopic (absorbs less humidity) tendency, which is critical for some applications. Although the manufacturing process is almost identical to standard rigid-flexible boards, they are also slighty cheaper. This is due to the fact that pure FR4 stack-ups are easier to laminate than hybrid-stack-ups.
Roughly the following characteristics apply:
