Semi-Flexible Printed Circuit Boards
12 December 2025
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Semi-flexible printed circuit boards (PCBs) are an innovative solution that bridges the gap between rigid printed circuit boards (PCBs) and flexible circuits. This hybrid structure combines rigid sections with flexible interconnects, offering unique advantages for modern electronic applications.
What is a Semi-Flexible PCB?
A semi-flexible PCB, also known as a rigid-flex hybrid PCB, integrates rigid and flexible substrates into a single component. Unlike purely rigid circuit boards or fully flexible circuits that can be continuously bent, semi-flexible PCBs have independent rigid areas connected by flexible sections. This hybrid architecture allows certain areas to remain structurally stable while other sections can be bent or folded to accommodate space constraints.
The rigid sections typically use FR-4 material, providing mechanical strength and a stable component mounting platform. The flexible sections use polyimide films, typically 25-125 micrometers thick, capable of withstanding repeated bending. These two distinct areas are laminated together using specialized manufacturing processes to form a unified electrical and mechanical structure.
Key Structural Features
Semi-flexible PCBs have several significant structural features that distinguish them from other types of circuit boards. The transition between rigid and flexible regions requires a smooth transition using variations in the thickness of reinforcing material, ensuring the board can bend without breaking at these critical connections.
Flexible regions have fewer copper layers than rigid regions because excessively thick copper layers reduce flexibility and increase the risk of fatigue failure. Most flexible regions use one or two copper layers, ranging in thickness from 12 to 35 micrometers. Copper traces in flexible regions follow specific design rules, including minimum bending radius requirements and restrictions on trace orientation relative to the bending axis.
The stack-up structure of semi-flexible circuit boards differs from that of rigid PCBs. The flexible portion uses a glue-free structure, where copper is directly bonded to polyimide without an adhesive layer. This glue-free method improves flexibility and heat dissipation while reducing overall thickness. Rigid portions can use a standard multilayer stack structure, containing multiple signal layers, power layers, and ground layers.
Manufacturing Process
The production of semi-flexible PCBs differs from standard PCB manufacturing. The process first manufactures the flexible and rigid portions separately. The flexible circuitry uses a polyimide substrate and copper overlays, with the circuit pattern formed through subtractive etching or additive manufacturing processes. The rigid sections follow traditional PCB manufacturing methods. A critical step occurs during lamination, where the rigid and flexible sections are precisely aligned and bonded together under controlled temperature and pressure.
Cover layer coating is another crucial manufacturing step. In the flexible areas, a cover film replaces the solder mask used on the rigid board, providing insulation and protection while maintaining flexibility.
Drilling on semi-flexible PCBs requires special drilling parameters to prevent delamination or material tearing, especially when dealing with through-holes that penetrate both rigid and flexible sections.
Design Considerations
Designing a semi-flexible PCB requires consideration of several factors. The location of the transition areas from flexible to rigid significantly impacts the board's durability. Engineers must place these transition areas away from high-stress areas and ensure that components mounted near these areas are adequately supported.
The minimum bending radius is six times the total thickness of the flexible section in dynamic applications, or three times the thickness during static bending. Violating these guidelines can lead to circuit breakage.
In semi-flexible designs, component placement requires strategic planning. Heavier components are mounted only on the rigid sections. Components near flexible areas must have sufficient clearance to prevent interference during bending. Near flexible areas, surface-mount components are preferred over through-hole components.
Trace routing in flexible areas follows specific rules. Traces should be as perpendicular to the bending axis as possible, as this direction reduces bending stress. Bending traces are better than sharp-angled traces, and staggered routing patterns help distribute stress across multiple conductors. Shaded ground planes are typically used instead of solid ground planes to improve flexibility while maintaining adequate shielding.
Advantages and Applications
Semi-flexible PCBs offer significant advantages in numerous applications. They eliminate the need for connectors and cables between rigid sections, reducing assembly complexity and overall weight. Integrated flexible interconnects enable three-dimensional packaging, allowing electronic components to fold into compact configurations impossible with rigid circuit boards alone.
The flexible portions absorb mechanical stress and control vibration, making semi-flexible designs ideal for aerospace, military, and automotive applications.
Medical devices frequently utilize semi-flexible PCBs, which conform to the body's contours while providing a stable platform for sensitive electronic components. Wearable technologies, hearing aids, and implantable devices all benefit from the flexibility and functional density they offer.
Consumer electronics products also largely utilize semi-flexible technology, such as smartphones, tablets, and cameras.
Cost and Complexity Factors
Due to complex manufacturing processes and lower production volumes, semi-flexible PCBs typically cost more than rigid PCBs of equivalent specifications. The price premium can be as high as 50% for rigid PCBs, depending on the complexity of the design, the number of layers, and the quantity of components. Designing semi-flexible PCBs is also more challenging than designing rigid PCBs, requiring experienced designers with expertise in circuit design principles.
