Semi flex PCB is a flex-rigid PCB, typically 4 layers, made entirely of FR4. Unlike conventional flex-rigid PCBs, the flexing element is not polyimide but a thin FR4 core with two layers of copper specially treated to flex without cracking. A less robust solution uses a depth-milled layer rather than a core.

While this type of flexible circuit may be more expensive to design and produce, it does offer a number of important advantages. For instance, the reduced size makes it easier to fit more components into a smaller space. This can actually help to lower overall system costs. Additionally, because they require the need for fewer interconnects and related parts and components, these semi flexible pcb can also prove to be more reliable and require less maintenance in the long run.

As with all types of flexible circuit boards, the semi-flex PCB will perform well in even the harshest environments, especially those featuring extreme heat. A semi-flex  board is also easy to test, making it well-suited for prototyping.