A flexible PCB consists of:
PI substrate - a PI substrate is the core of an FPC. The PI material is usually yellow, but it can also be black, white, and transparent.
Rolled copper foils - the etched copper foils are the circuit layers of the flexible PCB. Rolled copper foils dramatically increase the ductility and repeatable bending capability of the FPC. The maximum FPC copper thickness is 3oz.
PI layers - PI layers insulate the copper circuit layers, and the materials are the same as the substrate but only the thickness is smaller. The two external PI layers of the flexible PCB are coverlay, which is equal to the solder mask in normal PCBs.
Adhesive layers - adhesives layers are optional for flexible PCBs. They are between the copper layers and PI layers to bond them. Flex PCBs with adhesive look darker than the PI materials, and those without adhesive look bright and have a close color to the PI materials.
Flexible PCB vias - PCB vias are laser drilled on FPC layers and interconnect different circuit layers. The vias that penetrate all FPC layers are plated-through holes (PTH); vias that penetrate from the external layer to an internal layer are blind vias; vias that penetrate the internal layers are buried vias; vias with an aperture of 0.25mm and below that penetrate only one layer are microvias.
Flexible PCB stiffeners - stiffeners are the rigid pieces laminated at the end of flexible PCBs for connection with the system/device interfaces. Stiffeners are optional for flexible PCBs. There are FR4 stiffeners, PI stiffeners, stainless steel stiffeners.
A PI substrate is laminated with two copper layers on its two sides. A two-layer flexible PCB consists of a PI substrate, two rolled copper layers, two PI layers (coverlay). Multilayer flexible PCBs are manufactured by laminating alternating layers of copper and PI on the flexible PCB core.