Rigid PCBs are built on FR4 substrates, which makes them stiff and less adaptable to tight spaces. Flexible PCBs, in contrast, use polyimide substrates that allow the board to bend and twist without damage. Conductive copper layers carry signals across the base material, while protective coverlays or conformal coatings ensure durability. Thanks to this flexibility, FPCs save space, withstand vibration better, and enable 3D packaging solutions that rigid boards simply cannot achieve.