Rigid Flex PCB Assembly: A Practical Guide to Design, Manufacturing, and Quality Control

Today’s electronics are requiring more from circuit boards than ever before. Devices need to fit into smaller spaces, withstand more mechanical stress and work reliably in difficult environments - all at the same time. This is when rigid flex pcb assembly is a true technical answer and not a design decision.

Rigid flex PCB assembly is the process of joining together rigid and flexible board components into one integrated structure. The rigid zones provide a safe surface to attach components and the flexible zones allow the board to bend, fold or wrap around mechanical restrictions. The end result is a package that removes cables and connectors, decreases the total weight and enhances long term durability in applications where vibration and frequent movement are constant issues.

This method has been seriously gaining popularity in industry. Rigid flex PCB assembly is used by medical device manufacturers to construct diagnostic equipment and implantables that need to fit the human body. It helps aerospace and defense developers save weight while maintaining strength. Consumer electronics companies like it for packing more function into slimmer gadgets. The adaptability of rigid-flex technology makes it so enticing and so technically difficult.

But stiff flex PCB assembly is not a simple extension of regular PCB manufacture. The combination of materials, complicated layer stack-ups and transitions between rigid and flexible zones generate design and manufacturing issues that require specialist understanding at every step. This guide is meant to provide the understanding of those problems – and knowing what to look for in a manufacturing partner.

What Is Rigid Flex PCB Assembly?

Rigid Flex PCB assembly is the process of creating, manufacturing and populating a circuit board that is composed of rigid FR-4 pieces and flexible polyimide sections into one continuous structure. A true rigid flex board is built as a single piece, not as a regular PCB with a flex connector attached.

The board's solid sections are constructed like normal PCBs, providing a solid surface for SMT and through-hole components. The flexible portions are made of thin polyimide substrates and generally contain routing traces and no components. The two zones are laminated together during manufacture and produce a board that may be bent or folded into a three dimensional shape prior to or after installation.

Rigid PCB vs. Flexible PCB vs. Rigid-Flex PCB

The table below summarizes the key differences between the three board types:

A rigid PCB is a FR-4 substrate. Solid all the way through. It gives very good mechanical stability and is the most frequent and cheapest choice for most applications. But it is not flexible, therefore joining several hard boards requires cables or connectors.

Flexible PCBs are constructed from a thin sheet of polyimide or similar material that can be bent repeatedly without damage. It is light and space-efficient, but provides less mechanical support for large or tall components.

The rigid-flex PCB is a combination of the benefits of both. Rigid parts provide structural stability and ease of component assembly. Flexible sections offer 3 dimensional routing and eliminate the need for inter-board connectivity. The trade off is more design and production costs. 

Common Applications of Rigid-Flex PCBAs

Rigid-flex technology is found across a wide range of demanding applications:

Why Rigid Flex PCB Assembly Is More Complex Than Standard PCB Assembly

Material, Bending, and Layer Stack-Up Challenges

Rigid flex PCB assembly guidelines are made using very different materials than standard boards. The flexible parts are usually made on polyimide substrates which respond differently from FR-4 under the effect of temperature and mechanical stress. It is important to match the coefficient of thermal expansion (CTE) between stiff and flexible portions. Delamination can occur due to soldering mismatch.

Another key intricacy is the stack-up configuration of the layers. A typical four-layer rigid-flex printed circuit board assembly is comprised of two or more rigid layers and one or more flexible layers. These layers are glued together from the inside out with special adhesive chemicals called prepreg. Things get considerably more complex with a 6-layer rigid-flex PCB assembly, usually needing blind and buried vias to manage routing between zones. With each added layer, the process control has to be tighter and the materials have to be chosen more carefully to maintain the impedance constant and to avoid warpage.

The table below shows how stack-up complexity and typical applications scale with layer count:

The major area of stress is the transition zone between the stiff segment and the flexible section. Sharp transitions tend to concentrate mechanical stresses and can lead to trace fracture or delamination in time. Good design and manufacturing practice involves smooth transitions and sufficient strain relief to reduce these hazards. 

Component Placement and Soldering Risks

Do not place components on or near the flexible parts of the board. To bend it would produce mechanical pressures that would soon destroy solder joints in there. All surface mount and through-hole components shall be placed in the rigid areas. If components must be mounted on a flex section, the area should be rigidized with a stiffener and kept outside the bend zone.

Reflow soldering profiles are to be adapted for rigid-flex boards. The disparity in thermal mass that exists between the rigid and flex parts is the root cause of the uneven distribution of heat that occurs throughout the reflow process. It is possible that an incorrect calibration of the profile could result in the overheating of flexible areas, the delamination of those sections, or damage to the polyimide substrate. When rigid sections are heated, it is possible that they will not reflow correctly.  

Main Steps in the Rigid Flex PCB Assembly Process

Design File Review and DFM Check

The correct way to assemble a stiff flex printed circuit board (PCB) begins with the first steps of analyzing the design files and running a Design for Manufacturability (DFM) analysis. At this stage the Gerber files, drill files and fabrication comments are checked for completeness and consistency. The DFM check considers bend radius rules (typically 10x the flexible layer thickness or greater), component layout relative to flex areas, route locations in transitions, and stack-up viability.

Problems detected at this stage are noted before the fabrication begins avoiding expensive rework down the line. The experienced manufacturer will also analyze the impedance requirements and verify that the suggested stack-up will fulfill the electrical performance standards.

SMT Assembly, Inspection, Testing, and Final Delivery

Once fabricated, the stiff parts of the board are assembled using surface mount technology (SMT). Solder paste is applied to the stiff zones using the stencil printing. Pick-and-place machines are used for placing the components and the board is passed through a controlled reflow oven. Add through-hole components as necessary, then wave or selectively solder the assembly.

Once assembled, the boards are tested with Automated Optical Inspection (AOI) to detect solder faults, missing components and misalignment. Complex assemblies – especially those that use BGA packages popular in quick turn rigid-flex PCB assembly service projects – are also subject to X-ray inspection to check hidden solder junctions.

Electrical testing verifies that the assembled board performs to specification. Connectivity and component values are checked using flying probe or in-circuit testing (ICT).

Key Quality Risks in Rigid Flex PCB Assembly

Solder Joint Stress and Flexible Area Damage

In projects to put together bendable printed circuit boards, the most common quality risk is a solder junction breaking because of mechanical stress. When a rigid-flex board is bent to its final shape, it can stress parts that are too close to the flex zones, which can cause solder connections to break. This risk is even higher for things that need to be bent over and over, like robotic arms or electronics that need to be folded.

Another important thing to think about is the damage to the flexible area during building. The polyimide base is pretty strong, but it can be broken by chemicals used in the building process, sharp bends, and holes. Handling rules must be followed correctly during production to avoid damage that might not be clear at first but could shorten the product's useful life. 

Bending Reliability, Delamination, and Testing Issues

The table below summarizes the most common quality risks and the controls used to address them:

This results in thermal stress during soldering that exceeds the bond strength between the rigid and flexible portions resulting in delamination or separation of layers in the stack up.  And that’s why the control of reflow profiles and selection of materials are so critical. The delamination may not be visible on the surface of the board and requires cross-section analysis or expert inspection to discover.

Flex reliability is proven by flex cycle testing where boards are repeatedly flexed over their intended range of motion to verify that traces and solder joints stay intact for the estimated life of the product. Consumer electronics often don’t require as much testing as boards used in medical or aeronautical applications.

What to Check Before Choosing a Rigid Flex PCB Assembly Manufacturer

Engineering Support and Rigid-Flex Manufacturing Experience

Not all companies that put together PCBs have the right tools, knowledge of the materials, or process experience to make rigid-flex boards reliably. You should make sure that the partner you choose has experience with the design you need, whether it's a 4-layer rigid-flex PCB assembly for a small consumer device or a 6-layer rigid-flex PCB assembly for an industrial application that needs to be very reliable. 

Check whether the manufacturer offers in-house DFM evaluation and engineering help. A good partner will be able to look at your stack-up, see where you may have problems with bend radius or transition zones and provide suggestions for improvement before you start production. Collaborating now can save expensive failures later.

Inspection, Testing, Traceability, and Communication

Ensure the manufacturer routinely performs AOI, X-ray inspection and suitable electrical testing as part of their usual process, not an optional extra. When it comes to medical rigid flex PCB assembly or aerospace applications, it is important to inquire about their quality management system certifications. These certifications should include compliance with IPC requirements as well as ISO 13485 or AS9100 certification, if relevant.

Traceability is essential for applications that require high levels of reliability. It is expected that your manufacturer would be able to provide complete traceability, which will link each board to its respective fabrication batch, materials, and test data. When working with tight deadlines and a fast turn rigid-flex PCB assembly service, it is extremely important to have communication that is both clear and responsive throughout the entirety of the project, beginning with the design review and ending with the delivery. 

How PCBasic Supports Rigid Flex PCB Assembly Projects

PCBasic provides comprehensive support for rigid flex PCB assembly, from first DFM evaluation to final delivery. Their technical staff works directly with customers to optimize designs for manufacturability including stack-up selection, bend zone requirements and advise on component placement.

Production capabilities include 4 layer and 6 layer rigid-flex configurations, including in-house AOI, X-ray and functional testing. PCBasic provides fast turn rigid-flex PCB assembly for projects with tight deadlines, supported by robust process documentation and traceability techniques to fulfill the needs of medical and industrial customers.

FAQ

What is the typical layer count for rigid flex PCB assembly?
The majority of rigid-flex configurations are 2 to 8 layers. Four and six layer layouts are the most often used, providing a fair trade-off between routing flexibility and manufacturing cost.

Can components be placed on the flexible sections?

None. Components are mounted exclusively on the stiff parts of the board. Components placed on flex zones exert an unacceptable mechanical stress on the solder junctions when the PCB is bent.

What is the minimum bend radius for rigid-flex boards?

The minimum bend radius is the general rule of thumb but depends on the number of flex layers and the polyimide material. The minimum bend radius is ten times the overall flex layer thickness. A DFM assessment from your manufacturer will verify the correct specification for your design.

Is rigid flex PCB assembly significantly more expensive than standard PCB assembly? Yes, rigid-flex assembly is usually more costly because of the need for particular materials, tighter process controls, and higher inspection requirements. But the deletion of cables and connectors that are more reliable often leads to a reduced total system cost over the product's life.

What industries most commonly use rigid flex PCB assembly?

Its main businesses are medical devices, aerospace and defense, consumer electronics, industrial automation and military communications. Rigid-flex is a viable contender for any application needing compact form factors, weight reduction or great durability in dynamic situations.