Rigid-Flex boards are a combination of rigid boards and flexible circuits that are permanently laminated together and interconnected through through-holes. Rigid-Flex boards are also written as Flex-Rigid. Rigid-Flex boards straddle the boundaries of traditional rigid boards and the unique properties of flexible circuits, using highly ductile ED or RA copper photolithography onto a flexible insulating film.

Flex circuits consist of a stack of flexible Kapton polyimide and copper laminated by heat, adhesives, and pressure. Rigid-Flex circuits offer higher component density and better quality control. As with traditional boards, you can mount components on both sides of a rigid-flex board.

Accommodates controlled impedance
No additional components such as ZIF connectors and connecting cables are required
Improves the circuit’s ability to transmit signals without loss.
Reduces weight
More valuable component assembly and routing space
Reduces logistics complexity
Can solve difficult contact issues and simplify assembly
Significantly improve the reliability of the entire PCB system
Can be combined with HDI PCB microvia technology (microvias, buried and blind vias, finest conductor lines)
Can be combined with heat sink technology
Functional testability is significantly improved

Rigid-Flex PCB Challenges

Different challenges offset the versatility and flexibility that enables you to build rigid-flex PCB designs and products. Traditional rigid-flex board designs allow you to mount components, connectors, and product chassis to the physically stronger rigid part of the component. Likewise, in terms of traditional designs, the flex circuit is used only as an interconnect, while reducing mass and improving vibration resistance.

New product designs combined with improved flex circuit technology have introduced new design rules for rigid-flex boards. Your design team now has the freedom to place components on the flex circuit area. Combine this freedom with the multilayer approach of rigid-flex PCB design, and you can build more circuitry into your design. However, gaining this freedom adds some challenges with routing and vias.

What are the rigid-flex PCB design rules?

Flex circuits always have bend lines that affect routing. You can’t place components or vias near bend lines because of the potential for material stress.
Even with the correct component placement, bending a flex circuit can cause repeated mechanical stresses on surface mount pads and vias. Your team can reduce these stresses by using through-hole plating and using additional coverlays to secure pads for enhanced pad support.
When designing traces, follow practices that reduce stress on your circuits. Use mesh polygons to maintain flexibility when carrying power or ground planes on a flex circuit. You should use curved traces, not 90° or 45° angles, and use teardrop patterns to vary trace widths.
These practices reduce stress points and weak spots. Another best practice is to spread stress on traces by staggering the top and bottom traces of a double-sided flex circuit. Offsetting the traces prevents the traces from overlapping each other in the same direction and increases the strength of the PCB.
You should also route the traces perpendicular to the flex lines to reduce stress. The number of layers from one medium to another can vary when going from rigid to flexible and back to rigid. You can use routing to increase the rigidity of a flex circuit by offsetting the traces of adjacent layers.

What are Rigid-Flex PCB Materials?

Rigid-Flex boards are important electromechanical components used in many applications. The range of materials used in rigid-flex manufacturing is as varied as its application areas. There are many materials that can be used for rigid-flex assembly. However, the selection must be made after a careful analysis of the application and thickness requirements of the board. In addition to the application area,KKPCB recommends that there are several other factors to consider when selecting materials, such as:

Reliability requirements
Duration or shelf life of the application
Manufacturing method
Cost and budget

Materials Commonly Used in Rigid-Flex PCB Manufacturing

The most commonly used rigid-flex (PCB) materials are:

Substrate Material – The basic material used in rigid-flex PCB manufacturing is woven glass fiber, which is fully impregnated with epoxy resin. However, epoxy-imprinted fiberglass is not always a reliable solution as it cannot withstand frequent vibrations, shocks and continuous (Er or DK) motion. Therefore, for applications in demanding environments, rigid-flex PCBs are manufactured using the following materials:
Polyimide (PI): Polyimide is preferred over epoxy due to its versatility, toughness and resistance to continuous high and fluctuating temperatures.
Polyester (PET): Polyester is typically available in thicknesses of 25-125μm (1-5mil) and this substrate is chosen for its excellent flexibility and electrical properties. However, PET is rarely used in FPC manufacturing today.
Conductor Materials: Copper is the most common and readily available conductor material in rigid-flex circuit assembly. This material is preferred due to advantages such as its processability and good electrical properties. For circuit applications, two forms of copper foil are commonly used – electrodeposited (ED) and rolled annealed (RA) copper foil. Both foils are available in various thicknesses and weights. They undergo surface treatment before being used in rigid-flex PCB assembly. Sometimes, Constantan foil is used in Constantan FPC manufacturing. The foil is chemically treated to reduce bond degradation, increase adhesion, enhance bond strength, and prevent oxidation.
Bonding Sheets: Bonding sheets (also known as adhesives) play an important role in extending the life of rigid-flex boards. They are responsible for creating a strong connection between the substrate and conductor materials. Determining the type of adhesive and the thickness required to bond the substrate and conductors is the most critical part of rigid-flex PCB manufacturing.
PP: Prepreg is used to bond FR-4 CCL, copper foil in rigid components. In complex circuit board designs, KKPCB rigid-flex PCB manufacturers tend to use low-flow prepregs during the manufacturing process to ensure high quality.

Rigid-Flex PCB Surface: Cover Layer vs. Solder Mask

The surface of the rigid-flex board is fully covered with a protective film, called a protective coating. This helps the rigid-flex board resist chemicals, oils, hydrocarbon solutions, dust, and other contaminants. Choose the protective coating after understanding the types of materials used in rigid-flex assembly, the compatibility of PCB components with the coating material, and the most important application areas. The most commonly used forms of coating are:

Coverlay: When a flexible film such as polyester or polyimide is combined with a suitable adhesive, the resulting product is a coverlay (cover layer). Coverlays play three major roles in rigid-flex PCB assembly: (1) Provides comprehensive protection for the entire assembly. (2) Access to circuit areas (such as circuit pads) for further processing. (3) Enhances the reliability and resilience of the circuit.

Soldermask: Unlike the coverlay method, a thin layer of liquid acrylic epoxy and acrylic polyurethane solder mask ink is applied to the surface of the circuit. The liquid coating is applied using a variety of methods, one of which is screen printing. The coating is then heat cured. In some complex rigid-flex boards, where the coverlay openings on the flexible components are very dense, we recommend the use of flexible soldermask.

In addition to these materials, several other optional materials are used in the manufacturing process of rigid-flex boards to improve their operating performance and reliability. These include anti-rust coatings and back pressure sensitive adhesives (PSAs). As a reliable rigid-flex board manufacturer and supplier, KKPCB selects these materials based on customer needs and specific application requirements.

The materials used largely determine the quality and overall functionality of a rigid-flex board. As mentioned earlier, PCB materials must be carefully selected after analyzing several criteria such as cost, shelf life, and the electrical requirements of the printed circuit board. This helps produce a rigid-flex board that will provide many years of reliable and trouble-free service.

IPC Standards for Rigid and Flexible PCBs

The following list of IPC standards applies to both rigid PCBs and flexible circuits. Please note that this list is not exhaustive and other IPC standards may need to be considered.

IPC-2221A, General Standard for Design of Printed Circuit Boards

IPC-2223, Division Design Standard for Flexible Printed Circuit Boards

IPC-4101 Specification for Rigid and Multilayer Printed Board Base Materials

IPC-4202, Flexible Base Dielectrics for Flexible Printed Circuits

IPC-4203, Adhesive Coated Dielectric Films for Use as Cover Films and Flexible Adhesive Films for Flexible Printed Circuits

IPC-4204, Flexible Metal Clad Dielectrics for Use in the Fabrication of Flexible Printed Circuits

IPC-6013, Qualification and Performance Specification for Flexible Printed Circuit Boards

KKPCB conducts research on special processing technologies such as ordinary double-sided boards, thick copper circuit boards, high-frequency circuit boards, HDI circuit boards, rigid-flexible circuit boards, FPC flexible boards, buried blind hole circuit boards, and IC carrier boards. Provides PCB design, PCB layout, PCB prototyping and PCB assembly services