Rigid-Flex PCB

What is rigid-flex PCB?

A rigid-flex PCB (Printed Circuit Board) is a type of circuit board that incorporates both rigid and flexible sections within the same board. This design allows the PCB to bend or flex without breaking, allowing for space-saving, durability, and design flexibility.

What is the stackup of rigid-flex PCB?
Rigid-flex PCB stackup
What materials are used in rigid-flex PCB?

Rigid Materials:

FR4 (Flame Retardant 4): FR4 is a common substrate material used in rigid sections of rigid-flex PCBs. It is composed of a fiberglass-reinforced epoxy laminate and offers good mechanical strength, thermal stability, and electrical insulation.
Polyimide (PI): Polyimide may also be used as a substrate material in rigid sections of rigid-flex PCBs. It provides excellent thermal resistance and dimensional stability.

Flexible Materials:

Polyimide (PI): Polyimide is the most widely used material for flexible sections of rigid-flex PCBs due to its excellent flexibility, high-temperature resistance, and good dielectric properties.
Adhesiveless Copper Clad Laminates (ACCL): ACCL is a type of flexible substrate material that consists of a thin copper layer bonded directly to a polyimide film without the use of adhesive. This construction enhances flexibility and reduces the overall thickness of the flexible sections.
Liquid Crystal Polymer (LCP): LCP is another flexible material used in some rigid-flex PCBs. It offers excellent thermal and mechanical properties, including high-temperature resistance and dimensional stability.

Conductive Materials:

Copper: Copper is the primary choice for conductive traces on both rigid and flexible sections of rigid-flex PCBs due to its high electrical conductivity and good solderability.
Plated Through-Holes (PTH): Plated through-holes are used to provide electrical connections between the rigid and flexible sections of the PCB. These holes are typically plated with copper to ensure reliable electrical continuity.

Coverlay:

Polyimide Coverlay: Polyimide film with adhesive backing is commonly used as a coverlay material in rigid-flex PCBs. It provides insulation and mechanical protection for the conductive traces while allowing flexibility.

Adhesive:

Flexible Adhesive: Adhesive materials are used to bond the rigid and flexible sections of the PCB together. These adhesives must be flexible and capable of withstanding mechanical stress and thermal cycling.

What is the manufacturing flow of multi-layer rigid-flex PCB?
rigid-flex PCB Flow
What issues are often encountered in making rigid-flex PCBs?

Issues Causes Solutions
Excessive Flow of Adhesive at Rigid-Flex Junction High flowability of the PPs between layers Change to the PPs  with lower or no flowability
Apply full pressure during lamination either too early or with excessive pressure Adjust the pressure parameters
Wrinkling of Flexible Board at Rigid-Flex Junction The flexible PCB is not flattened during lamination Adjust the height of the spacer to ensure proper flattening
Excessive pressure during lamination Adjust the pressure
High flowability of PPs between layers Change to the PPs  with lower or no flowability
Delamination, Whitening, or Layer Separation in Rigid Part of Rigid-Flex Board Moisture absorption in the flexible part before lamination Dry the flexible part before lamination
Use of acrylic adhesive or high flowability PP Change to the PPs  with lower flowability without use of acrylic adhesive
Excessive coverage of the flexible part’s film into the rigid part, and the surface is too smooth. Control the depth of coverage of the flexible  film into the rigid board and roughen the surface
The TG of PP and FR4 is low Change to use higher TG material of more than TG150℃