Flexible PCB manufacturer tells you what are the differences between Flex and Rigid boards

Flexible printed circuit boards are quite different than rigid boards:

• Material; Rigid boards are typically made out of FR4 (glass-epoxy compounds) while flexible circuits are made from polyimide. There are cases when rigid boards are built with polyimide, but it isn't as common.
• Coverlay; Flex pcbs either have a flexible mask or coverlay while rigid boards typically just use a solder mask. When a coverlay is used, the openings are routed, or laser cut. Then an adhesive (typically 1 or 2 mils thick) is used to adhere the coverlay to the flex.
• Stiffeners; Flexible printed circuit boards typically use FR4 or polyimide stiffeners to stiffen specific non-flexing regions. Stiffeners are either laminated to the flex or adhered using a PSA (pressure sensitive adhesive). Rigid boards do not need stiffeners.