Introduction

In the field of PCB design, rigid-flex PCBs provide unique solutions for modern electronic devices thanks to their versatile shapes and compact sizes. Some rigid-flex boards—such as those used in hearing aid devices—are extremely small, making them particularly challenging to manufacture.

However, regardless of board size, the design of the transition area between the rigid and flexible sections has a direct impact on manufacturing feasibility and long-term reliability. This transition zone is subjected to both mechanical and thermal stresses, originating from fabrication, assembly processes, and real-world usage conditions.

Poor design in this area can introduce significant reliability risks. According to IPC-6013, the rigid-flex transition area is defined as a 3 mm-wide zone centered on the rigid board edge axis (see illustration).

This article explores the key design considerations and manufacturing challenges associated with this critical region, helping designers mitigate potential risks.

Unique Challenges in the Manufacturing Process

The manufacturing process of rigid-flex PCBs differs significantly from that of standard rigid boards.

After lamination of the flexible layers, all materials undergo a final lamination cycle to bond the rigid and flexible sections together. Before this second lamination, spacers are inserted into the stack-up to prevent resin from flowing into the flex area. These spacers are later removed through a milling process.

As shown in the illustration, certain defects within the transition area are considered acceptable. However, if any functional features are placed within this region, these defects may negatively affect the final performance of the rigid-flex PCB.

Common Defects and Their Impact

1. Crazing and Haloing

At the rigid-to-flex interface, defects such as material delamination, resin cracks, or copper fractures may occur. These are mainly caused by the thickness difference between the rigid and flexible sections, which creates a step structure after lamination. During bending or thermal cycling, stress concentrates along the transition line, increasing the likelihood of failure.

2. Lamination Voids

Lamination voids within the transition area are considered acceptable.

Rigid-flex PCBs typically combine FR4 and polyimide materials. In conventional rigid PCBs, prepreg flows to bond copper layers and eliminate voids. In rigid-flex PCBs, however, FR4 prepregs are often low-flow or no-flow, preventing resin from entering the flex region. As a result, lamination voids may appear in the transition area.

3. Resin Squeeze-Out

Although no-flow prepregs are used, resin may still squeeze out beyond the rigid board edge into the transition zone.

· For flex-to-install (one-time bending) applications, this may not be an issue.

· For dynamic-flex applications, hardened resin edges can damage the flexible circuit during repeated bending.

4. Protruding Rigid Dielectric

During milling of the flex area, lamination voids or resin squeeze-out may cause the rigid dielectric material to protrude slightly. This does not affect electrical performance and is considered acceptable under IPC-6013.

5. Copper Deformation

Material instability in the transition area can cause copper features to deform, crack, or delaminate. Copper may obstruct resin flow, leading to insufficient bonding. In addition, layer-to-layer registration accuracy can be compromised.

6. Coverlay Protrusion

Placing coverlay material into the transition area can cause reliability issues. Coverlay is not designed to bond with FR4 materials, and if it extends into the transition zone, poor adhesion may lead to delamination.

Feature Placement in the Transition Area: Risks and Considerations

A key question in rigid-flex design is:

How far can functional features extend into the transition area without compromising performance or causing excessive material stress?

The practical answer is: almost not at all.

As illustrated in Figure 2, although it is technically possible to design and manufacture features within the transition or flex areas, doing so is not considered a mature or low-risk practice.

Close collaboration with your PCB supplier is therefore essential. The supplier’s engineering team understands the limits of each manufacturing process and can advise how much usable space—if any—can be allocated within the transition area based on factory capability and specifications.

In Figure 2:

· The recommended values (left column) present the lowest risk.

· The “advanced” values (right column) require extreme caution and full alignment among all stakeholders regarding potential risks.

Miniaturization Trends and Advancing Manufacturing Capabilities

As electronic technology continues to evolve, increasing product demands drive rigid-flex PCBs toward greater miniaturization, pushing manufacturing processes to their limits.

READA encourages innovation and advanced PCB design—but only when:

· The application requirements are fully met

· All stakeholders clearly understand the associated risks

Some PCB manufacturers can support rigid-flex designs with smaller-than-standard transition areas, but we recommend a cautious approach and strict quality validation.

Design recommendations include:

· Avoid placing critical functional features within the transition area

· Communicate closely with suppliers to understand manufacturing limits

· Balance innovation with reliability based on application requirements

Engineering Best Practices for Rigid-Flex Transition Design

1. Early Risk Assessment and Communication

Engage suppliers early in the design phase.
Transition area specifications—such as width and material selection—vary by supplier capability. Some manufacturers can support transition zones smaller than the IPC-6013 3 mm standard, but this requires clear risk-sharing agreements.

Simulation and stress analysis:
Use FEA (Finite Element Analysis) to simulate mechanical bending and thermal cycling stresses. This is especially critical for dynamic-flex applications. Routing directions should avoid being perpendicular to the bending axis to reduce fracture risk.

2. Material Selection and Transition Optimization

Low-flow / no-flow prepreg trade-offs:
For critical projects, consider discussing hybrid adhesive solutions with suppliers to balance resin flow control and interlayer bonding strength.

Coverlay boundary control:
Coverlay extending into the transition area increases delamination risk. It is recommended to maintain at least 0.5 mm clearance between the coverlay edge and the rigid board boundary, and to confirm machining accuracy before production.

3. Manufacturing Quality Control

Acceptable defect standards:
Although IPC-6013 allows certain defects (e.g., lamination voids, resin squeeze-out), customers should request detailed cross-section analysis, especially for high-reliability applications such as medical or aerospace electronics.

Milling precision:
Milling accuracy directly affects dielectric protrusion and resin edge sharpness. In practice, milling-induced damage can often be resolved by adjusting parameters such as spindle speed and feed rate. Pilot runs are strongly recommended to validate process stability.

4. Matching Design to Application Scenarios

Flex-to-install vs. dynamic-flex:
Many failures occur because designers do not clearly differentiate between these two scenarios. Flex-to-install applications may tolerate limited resin squeeze-out, while dynamic-flex applications require strict control of sharp edges.

Miniaturization challenges:
For compact devices (e.g., wearables, hearing aids), transition area space is further constrained. Stack-up optimization—such as reducing rigid board thickness or adjusting flex positioning—can help free up space. Reliability testing, including bending cycle tests, is essential.

5. Common Failure Modes in Transition Areas

Typical failures include:

· Copper trace fractures

· Interlayer delamination

· Reduced dielectric strength

These issues are often linked to insufficient consideration of environmental factors such as temperature and humidity cycling. Accelerated aging tests are recommended during late-stage product development to simulate real-world conditions.

Closed-Loop Collaboration: From Design to Manufacturing

Designing rigid-flex transition areas is not only a technical challenge, but also a collaborative effort between design, manufacturing, and application requirements.

We strongly recommend:

· Proactive design: Consider mechanical and thermal stresses early, using simulation tools and supplier feedback

· Manufacturing control: Ensure processes such as lamination and milling meet design expectations, with quality checks at critical stages

· Application alignment: Adjust design strategies based on whether the application involves one-time or dynamic bending

For rigid-flex PCB technical support, please contact:
Tel: +86 181 2651 2285
Email: eng@readapcb.com

We are always happy to assist with your engineering challenges.