A Flex Printed Circuit or Flex PCB is a basically a flexible Base material, with a patterned arrangement of printed circuitry on Copper and components, with or without flexible overlay. These flexible electronic assemblies may be fabricated using the same components used for rigid printed circuit boards, but allowing the board to conform to a desired shape (flex) during its application. 

Flexible PCB / FPCB Applications 

FPCBs are used widely in everyday technology and electronics in addition to high-end, complex completed components. Two of the most prominent examples of flexible circuit usage is in modern portable electronics, devices, hard disk drives and desktop printers. 
Flexible circuits are also used extensively in other applications and industries including: 
Communications 
Consumer Electronics 
Automotive 
Medical 
Industrial 
Aerospace 
Military 
Transportation
In Automotive, Transportation they are extensively used due to their improved resistance to vibrations and movement.

Flex PCB Capacity

Material type:

PI, PET, PEN, FR-4.

Surface finishing: Lead-free,

ENIG Gold, OSP, Immersion silver, Immersion Tin. HASL. Polyimide, PET.

Max layer: Flex board: 1-6layer, Flex-Rigid Board:2-8 Layers. 

Min. hole size: 

punching hole: 0.50mm

Drill Hole:0.20mm

Max Panel size: 250MM*400MM.

Min. Board thickness:

Copper Thickness:1/3OZ-2OZ

Min slot: 24mil*35mil(0.6X0.9MM)

Size Tolerance

Conductor width: +/-10%

Hole size: +/-0.05mm.

Single:0.1mm

2layers: 0.13mm

4layers: 0.30mm

6layers:0.50mm