What is Rigid-Flex printed circuit
A Rigid-Flex Printed Circuit could be defined as"a printed circuit device which has areas of rigid and flexible dielectrics with coextensive conductors throughout and interconnected by plated-through-hole(PTH)". The most common type of Rigid-Flex consists of a rigid printed circuit board(PCB) with flexible appendages extending and terminating at connectors. The rigid sections serve as strain reliefs, stiffeners and provide selective support for components; While the flex areas serve as an interconnection assembly.
Rigid-flex is different from a flexible circuit with an added stiffener. Rigid-flex incorporates plated through-holes that extend through the entire rigid section, there are challenges to create a robust and reliable plated through-hole interconnect which like a rigid board. As with rigid boards, we can also incorporate microvias using HDI process technologies to take advantage of today's most advanced packaging components.
The flexible sections typically do not have plated through-holes(PTHs) to avoid reducing flexibility. However, some designs may require "sub laminations" with plated through-holes or microvias. Another design may have inner layers that terminate with plated through-holes for receiving soldered connectors. Obviously, these requirements will add complexity to the design.
Copper Foil Type
The preferred copper foil for flex circuits is a wrought rolled annealed . This copper has excellent ductility and elongation properties and offers:
High resistance to cracking during flexing
High resistance to foil fractures in PTHs during thermal exposure.
Covercoat
A preferred choice with a proven history would be DuPont Pyralux LF. Acrylic adhesive is coated onto a polyimide dielectric film (DuPont Kapton) in a B-stage form. For rigid-flex manufacturing and long-term service life, the material has the following desirable properties:
High bond strength to treated copper surfaces
9-10 lbs./in. (maintains value after solder)
Dielectric strength >3000 volts/mil
Dielectric constant = 3.5
Passes 10-second solder float at 288oC
Passes MIL-P-50884 qualification of 100,000 cycles
Most PCB chemicals have negligible effect on material
Good electrical properties
Good thermal properties
Good flex endurance capability
Good chemical resistance
Rigid Metal-Clad Laminates
Generally, two resin systems (epoxy and polyimide) are used for rigid copper clad laminates. These materials are generally referred to as:
For epoxy resin systems (most common): IPC-4101/21, /24, /26, /28
For polyimide resin systems: IPC-4101/40, /41, /42 (/41 preferred)
