What Is Rigid-Flex PCB?

Nowadays, drones, laptop computers, wearable electronics, medical devices and testing equipment are a few of the applications that reply on rigid-flex PCB. What’s a rigid-flex PCB? Rigid-flex printed circuit board is composed of a combination of rigid and flexible circuit boards that are permanently laminated and interconnected to one another. Rigid-flex PCBs go across the boundaries of traditional rigid PCBs and the unique properties of flexible circuits that use high-ductility ED or RA copper photo-etched onto a flexible insulating film.

Flex circuits includes stack-ups made from a flexible Kapton polyimide and copper laminated together through heat, adhesive and pressure. As with conventional rigid PCBs, you can mount components on both sides of the rigid boards. Because of the integration that occurs between rigid and flex circuits, a rigid-flex PCB design does not use connectors or connecting cables between the sections. Instead, the flex PCBs electrically connect the system together.

The lack of connectors and connecting cables accomplishes several things:

  • Improves the ability of the circuit to transmit signals without loss

  • Accommodates controlled impedance

  • Reduces weight

  • Frees space for other components.

Every rigid-flex PCB is divided into zones that feature different materials and varying layer counts. Rigid zones may have more layers than flexible zones, and materials shift from FR-4 to polyimide in transition zones. Complex designs often transition from rigid to flex and back to rigid multiple times. As these intersections occur, the overlap of rigid-flex materials requires keeping holes away from the transition zone to maintain integrity. In addition, many rigid-flex designs include stainless include stainless steel or aluminum stiffener that provide additional support for connectors and components.


Design Rules of Rigid-Flex PCB

Different challenges offset the versatility and flexibility that allow you to build three-dimensional designs and products. Traditional rigid-flex PCB designs allowed you to mount components, connectors, and the chassis for your product to the physically stronger rigid part of the assembly. Again, in terms of traditional designs, the flexible circuit only served as an interconnect while lowering the mass and improving the resistance to vibration.

New product designs coupled with improved flex circuit technologies have introduced new design rules for rigid-flex PCBs. Your design team now has the freedom to place components on the flexible circuit area. Combining this freedom with a multilayer approach to rigid-flex design allows you and your team to build more circuitry into the design. However, gaining this freedom adds a few challenges in terms of routing and holes.

Flexible circuits always have bend lines that affect routing. Because of the potential for material stress, you cannot place components or vias close to the bend line.

And even when components are properly located, bending flex circuits places repeated mechanical stresses on surface-mount pads and through holes. Your team can reduce those stresses by using through-hole plating and by bolstering pad support with additional coverlay to anchor the pads.

As you design your trace routing, follow practices that reduce stress on your circuits. Use hatched polygons to maintain flexibility when carrying a power or ground plane on your flex circuit. You should use curved traces rather than 90° or 45° angles and use teardrop patterns to change trace widths.

These practices decrease stress points and weak spots. Another best practice distributes stress across traces by staggering the top and bottom traces for double-sided flex circuits. Offsetting the traces prevents the traces from laying over each other in the same direction and strengthens the PCB.

You should also route traces perpendicular to the bend line to reduce stress. When moving from rigid to flex and back to rigid, the number of layers from one medium to the other may differ. You can use trace routing to add stiffness to the flex circuit by offsetting the routing for adjacent layers.

 

Rigid-Flex PCB Materials

Rigid-flex PCBs are the vital components used in a lot of applications. The range of materials used to in the rigid-flex PCB fabrication is as diverse as its application area. There are numerous materials available for rigid-flex PCB assembly. However, the selection must be made after careful analysis of the board’s applications. Besides, the area of application, MADPCB suggests several other factors need to be considered when making the material selection such as:

  1. Reliability requirements

  2. Duration or shelf life of the application

  3. Manufacturing methods

  4. Cost and budget

Commonly Used Materials in the rigid-flex PCBs Manufacturing

The most commonly used rigid-flex PCBs materials are:

  • Substrate Materials -The basic material used for rigid-flex PCB manufacturing is woven fiberglass, which is thoroughly impregnated with epoxy resin. However, epoxy imprinted fiberglass is not always a reliable solution as it fails to resist frequent vibrations, shocks, and constant (Er or DK) movement. Hence, for applications in demanding environments, rigid-flex printed circuit boards are manufactured using:

    1. Polyimide (PI) -Polyimide is largely preferred over epoxy resins due to its versatility, toughness, and resistance to constant movements of elevated and fluctuating temperatures.

    2. Polyester (PET) -Polyester is usually available in 25-125 microns thickness, this substrate material is chosen for its excellent flexibility and electrical properties. This material also exhibits outstanding chemical and moisture resistance. So, it can be used reliably in aggressive industrial environments. Selecting the right substrate material is crucial as it determines the strength and durability of the entire flex circuit assembly. A substrate must be chosen after analyzing its dimensional stability, thermal resistance, electrical properties, flexibility, and chemical resistance, among others.

  •  Conductor Material -Copper is the most commonly used and readily available conductor material for rigid-flex circuit assembly. The material is preferred due to its benefits such as high workability and good electrical characteristics. For circuitry applications, two forms of copper foils are typically used – electro-deposited (ED) and rolled annealed (RA) cooper foil. Both these foil forms are available in various thickness and weights. They are subjected to surface treatment before they are used for the rigid-flex PCB assembly. Usually, a thin layer of zinc is applied to improve the longevity of the foils. Also, the foils are chemically treated to reduce bond degradation, increase adhesion, augment bond strength, and protect from oxidation.

  • Adhesive -Adhesive play an important role in extending the service life of rigid-flex boards. They are responsible for making secure connection between the substrate and conductor materials. Deciding on the type of adhesives and the required thickness to bond substrate and conductor is the most crucial part in rigid-flex circuit board manufacturing.

Among several adhesives available, the most common types used to manufacture rigid-flex PCBs are:

  • Polyimide Adhesives – Polyimide adhesives are largely chosen due to its excellent temperature resistance, extreme heat resistance. Polyimide adhesives are usually used in in defense, military, and power generation applications. Also, they are selected predominantly produce multi-layer rigid-flex PCB and flexible circuit due to their coefficient of thermal expansion.

  • Polyester Adhesives – This is relatively cheap adhesive used in the fabrication of simple rigid-flex PCB boards. This material features low bond strength. Also, it cannot resist elevated or fluctuating temperatures. Due to these reasons, today’s rigid-flex PCB manufactures are opting for modified polyester adhesives. The advanced forms of polyester material have excellent versatility and good heat resistance properties, making them reliable for use in the assembly of complex rigid-flex boards.

  • AcrylicAdhesives -This adhesive material has several superior features. Some of these include excellent thermal stability, outstanding chemical/corrosion resistance, and ease of application. Also, they are relatively cheap and readily available as compared to the other types of adhesives.

  • Epoxies -This is the most commonly used adhesive type for rigid-flex board manufacturing. They can withstand elevated/fluctuated temperatures and can resist solvents and other chemicals. Also, they are extremely flexible and possess excellent bonding stability. Today’s rigid-flex circuit manufacturers improve the flexibility of epoxies by adding a small amount of polyesters.

  • Protective Coatings -The surface of rigid-flex PCBs is comprehensively coated using protective films. This helps rigid-flex PCB resisting chemicals, oils, hydrocarbon solutions, dust, and other contaminations. The protective coating is selected after understanding the types of materials used in the rigid-flex PCB assembly, compatibility of PCB components with the coating material, and most importantly the application areas.

The most commonly used forms of coating are:

  •  Coverlay (Cover Layer) -When a flexible film like polyester or polyimide is combined with a suitable adhesive, the resulting product is a cover layer (coverlay). Coverlay has three major roles to play in a rigid-flex PCB assembly. First, to provide comprehensive protection to the entire assembly. Second, to access circuitry areas like circuit pads for further processing. Third, to augment the reliability and resilience of the circuitry.

  • Cover Coats -Unlike coverlay method, a thin coating of liquid acrylated epoxy and acrylated polyurethane solder mask ink is applied onto the circuitry surface. The liquid coating is applied using several methods, one of such is screen printing. The coating is then thermally cured.

Besides these materials, there are several other optional materials used in the fabrication of rigid-flex PCBs to augment their operational excellence and reliability. These include anti-tarnishing coatings, and back pressure sensitive adhesive (PSA). As a reliable Rigid-flex PCB manufacturer and provider, MADPCB chooses these materials as per the needs of customers, and specific application requirements.

The material used greatly determines the quality and overall functioning of the rigid-flex boards. As mentioned earlier, PCB materials must be carefully chosen after analyzing several criterial including cost, shelf life, and electrical requirements of the printed circuit board, among others. This helps produce rigid-flex PCBs that provide many years of reliable and trouble-free service.

 

Rigid-flex PCB Service Provider &Manufacturer

MADPCB provides quick-turn rigid-flex printed circuit board (PCB) manufacturing service, and the quality of our rigid-flex PCB prototypes and high-volume production are leading and reliable. No finding more, we are the trustable rigid-flex PCB provider and manufacturer in China. The list of IPC standards below applies to rigid PCBs and flex circuits. Take note that this list is not exhaustive, and additional IPC standards may need to be considered. You should consult the ipc.org website for a full list of available IPC standards.

IPC-2221A, Generic Standard on Printed Board Design

IPC-2223, Sectional Design Standard for Flexible Printed Boards

IPC-4101, Specification for Base Materials for Rigid and Multilayer Printed Boards

IPC-4202, Flexible Base Dielectrics for Use in Flexible Printed Circuitry

IPC-4203, Adhesive Coated Dielectric Films for Use as Cover Sheets for Flexible Printed Circuitry and Flexible Adhesive Bonding Films

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

IPC-6013, Qualification and Performance Specification for Flexible Printed Wiring