Semi Flex PCB

Driven by continuous technological advances, rigid-flex PCB have emerged to address the challenges of traditional rigid PCB that are not flexible enough and flexible PCB that are not reliable enough.  However, the high cost of this advanced solution has made it less budget-friendly for some price-sensitive projects. In order to better meet cost requirements, semi flex PCB has gained prominence. This PCB is compatible with traditional rigid PCB manufacturing processes while utilizing more economical substrate materials, providing a more flexible choice for project costs.

semi flexible pcb

Table of Contents

What is a Semi Flex PCB

In our view, a semi flex PCB does not fall strictly into the categories of a flexible PCB or a rigid-flex PCB. Unlike traditional flexible and rigid-flex PCB that typically use polyimide as the flexible material, a semi flex PCB often uses FR4 material. Some readers familiar with PCB may have a question: Isn’t FR4 a rigid PCB material?

Indeed, FR4 is a rigid PCB material and the primary material used to construct a semi flex PCB. This is because the flexibility of a semi flex PCB is achieved through a milling technique that thins the areas that need to bend. Therefore, a semi flex PCB is also referred to as a semi rigid PCB, which is a derivative of rigid PCB. Depending on design requirements, the thickness of the bending area in a semi flex PCB can be reduced to 0.25mm +/- 0.05mm.

Semi-flexible PCB cross-section diagram

In terms of performance, a semi flex PCB inherits almost all the characteristics of FR4 PCB and adds a certain degree of flexibility. However, it’s important to note that the flexibility of a semi flex PCB cannot match that of a rigid-flex PCB. In most cases, it is designed for static bending to fit the corners inside electronic devices and optimize space. A semi flex PCB is not commonly used in precision electronics, and its layer count is usually less than 6. As mentioned earlier, a semi flex PCB exists for low-profit projects with lower precision requirements.

Flexible vs. Semi Flex vs. Rigid-Flex

PropertyFlexible PCBSemi Flex PCBRigid-Flex PCB
DefinitionConstructed with flexible substrate materials for bendabilityRigid PCB after processing using milling technologyCombination of rigid and flexible PCB connected to form a single unit
FlexibilityOptimal flexibility, capable of 360° folding to meet the demands of intricate internal spaces in microelectronicsBending usually less than 90° and is statically bent to accommodate device corner spacesOnly partial bending can be achieved, and the flexible part has the same bending performance as the flexible PCB
MaterialPolyimide film, LCP, PETFR4Varied combinations based on project requirements
ReinforcementCan choose PI, FR4, or even stainless steel based on project needsSufficient rigidity without the need for reinforcementNeed for reinforcement depends on specific application and design requirements
ReliabilityMost common flexible PCB use polyimide as a substrate, offering excellent thermal stability and chemical resistance, generally considered reliableWhile using rigid material, milled areas are fragile, requiring extra care during processing and useEssentially a combination of rigid and flexible PCB, with excellent reliability in the main portion but weaker in connection areas
Manufacturing DifficultyConductors made through electrodeposition or rolling annealingSuitable for traditional PCB manufacturing processesChallenges in layer-to-layer alignment and registration
CostMediumLowHigh

Key Considerations for Semi Flex PCB

In the design and manufacturing process of a semi flex PCB, adherence to principles of reliability, cost-effectiveness, and ease of operation is crucial. Although this PCB allows bending in both the X and Y directions, TechSparks recommends extending primarily in one direction to reduce manufacturing complexity and costs.

When considering vias design, ensure that these vias maintain at least one millimeter of space with the bending area. This not only helps reduce the risk of component-PCB collisions during bending but also prevents mechanical stress from damaging component pins. Additionally, this design decision provides more convenient conditions for manufacturing and assembly.

Regarding the thickness milling of the bending area, a mapping method can be adopted to ensure that the semi flex PCB has both reliability and sufficient bending performance. In the post-manufacturing 90-degree bending test, attention should be given to the copper thickness between the solder mask layer and L2, ensuring it falls within the range of 0.188mm to 0.213mm.

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