Flexible PCB Design Guide

PCB emerged in the early 20th century, reflecting technological advancements in semiconductor packaging that allowed industry professionals to invest in a more compact and efficient electronic product. Today, we see innovative developments such as medical devices tailored to the human body that can monitor health conditions through direct skin contact, or smartphones with a foldable display.

To achieve these advancements, the use of PCB flexível—beyond the constraints of traditional shapes—is essential. However, whether viewed from a design or manufacturing perspective, the construction of a flexible PCB presents greater challenges, requiring designers to invest more thought into the process. In this article, FS Technology provides a guide to flexible PCB design, aiming to help you refine the technical aspects of your flexible circuit design and reduce project costs.

Flexible PCB Design Basics

Let’s start by understanding the concept of flexible PCB and its uses to elicit design ideas!

PCB flexível, or flexible circuit, achieves bending and folding functionality by using thinner materials. Some of the materials required for construction include:

  • Conductive Copper Foil: Used as the main material for electrical conductivity in the circuit, it can be provided in fully annealed or low-temperature annealed forms, enhancing its flexibility.
  • Insulator: Common materials include polyimide, PET, PEN, PTFE, and aramid, requiring sufficient flexibility.
  • Cover Layer: Similar to the solder mask layer, its purpose is to protect the circuit, isolating the copper conductors as the outermost layer of the flexible circuit. Due to the need for compatibility with bending characteristics, the cover layer also needs to be flexible.
  • Adhesive: Cures after lamination to tightly bond the different layers of the PCB into a solid whole.
  • Adhesive-less: Depending on project requirements, adhesive-less materials can be used to avoid high-temperature cracks, moisture absorption, and increased copper layer thickness while providing greater flexibility.
 

Regardless of the tipo de PCB, the ultimate goal is the same: to secure electronic components on the outer layers and ensure reliable electrical connections between component terminals and conductive pads. To achieve this goal, we also need to perforate the board and perform electroplating to deposit copper layers onto the hole walls.

For some higher-precision devices, PCB with more layers is required. However, for flexible PCB, 2 to 6 layers are sufficient for most applications. Of course, if you do need a multilayer flexible PCB, FS Technology can meet the demand for up to 12 layers.

Flexible PCB Design Process

O Processo de conceção de PCB is generally standardized, with key steps including project understanding, schematic creation, circuit and component layout, design validation, file generation, manufacturing, and assembly.

  1. You should understand the project requirements first, such as current specifications, voltage requirements, capacitance, impedance, the necessity of shielding, wiring details, signal variations, etc.
  2. Create a schematic, detailing the circuit, including the connections and relationships of various componentes electrónicos.
  3. Consider the physical layout of the circuit, distances between components, and circuit traces based on static and dynamic applications, especially in the folding and bending sections.
  4. Ensure continuity in the project by adding labels and identifiers.
  5. Use simulation software or build prototypes to validate the flexible PCB layout.
  6. Generate manufacturing files, including Ficheiros Gerber, to ensure manufacturers can produce according to design specifications.
  7. Collaborate with additive manufacturing or mechanical engineers to ensure the compatibility of the flexible PCB throughout the device, considering the bending and shaping of the flexible PCB.

Flexible PCB Design Tips

Circuit Type

Flexible PCB can be used individually for a circuit, with some intended for minimal bending (“static”) and others for regular bending (“dynamic”). This ultimately depends on the material and thickness. Generally, a thicker flexible PCB has poorer flexibility, so for a dynamic flexible circuit (such as those in a laptop), the bending area must be kept thin.

Additionally, when multiple layers of the circuit board combine flexible and rigid materials, it is labeled as a PCB rígido-flexível. This type of board serves as a versatile 3D interconnect that can be molded and bent into various shapes. Utilizing rigid-flex technology, a designer can integrate multiple PCBs, interconnecting wires, or cables into a single, cohesive unit, providing enhanced performance and compactness.

Another concept is the semi-flex PCB, but in reality, it is not a flexible circuit. It uses deep milling technology to thin the area that needs to bend, giving it some flexibility. Essentially, it is still an FR-4 PCB.

Routing Layout

High-speed design for flexible PCB presents a challenge as the layer stackup poses impedance control challenges. Different layer counts offer various constructions, each with its pros and cons in terms of signal integrity and flexibility.

Various factors typically affect impedance, including insulation material, trace width, thickness, and the distance between signal trace and reference layer. A common practice is to use the ground layer as a reference plane to help control the impedance of signals and improve signal integrity.

To prevent electromagnetic interference (EMI) or unintended signal coupling, solid copper shielding, cross-hatching, or silver ink can be employed. Cross-hatching also helps enhance the flexibility of the flexible PCB and reduce weight.

In addition to these conventional considerations, the flexibility of flexible PCB introduces challenges related to bending. The bend ratio, comparing the bend radius to the circuit thickness, impacts bending durability, with smaller bend radii increasing the risk of failure. Analyzing potential bending impacts on conductor paths is crucial. Therefore, the key aspect of flexible PCB routing lies in bend areas.

When routing, a trace should cross these areas perpendicular to the bend line, while via and pin should be excluded to prevent damage. Stacking layers with adjacent traces can stiffen bend areas, so it’s preferable to stagger traces. Designers must also be mindful of layer structures and avoid routing on layers that may not be consistently available.

Layout do componente

Due to the components on the assembled PCB’s surface and the bending characteristics, the pads on the flexible circuit are prone to lifting. Manufacturers typically employ anchoring techniques, such as extending pads encapsulated from the cover layer.

In addition to manufacturers’ efforts, to alleviate the stress caused by bending, designers can increase the pad size, which is particularly effective for single-sided flexible PCB designs. Furthermore, introducing mechanical reinforcement ribs in specific areas can provide additional strength.

Depois de colocação de componentes, confirming the X, Y, and Z spacing of components on the flexible circuit is crucial not only in a flat state but also during the bending process. This can be checked using visualization tools.

Additional

As you approach the manufacturing stage, due to the long lead times of flexible circuit materials and their unique manufacturing challenges, it is critical to work with a manufacturer like FS Technology that specializes in producing flexible PCB. Intelligent data formats for file transfer, like IPC-2581, streamline the transfer process, which is more complex for flex designs than conventional PCB.

The choice of Ferramentas de desenho PCB greatly impacts the efficiency of flex circuit design. Tools like OrCAD PCB Designer by Cadence, equipped with 3D capabilities and tailored features for flex design guidelines, provided a more productive environment for designing flex and rigid-flex circuits.

Fim

Flexible electronics empower data retrieval within the body, showcasing the diverse potential of this technology. This serves as a foundational overview of flexible PCB design, emphasizing materials and manufacturing processes.

As we conclude this flexible PCB design guide, stay tuned for more intriguing content in the future. Your feedback and experiences are valuable — share them with us!

If you’re navigating the selection of a manufacturer for your flexible PCB project, explore FS Technology’s turnkey PCBA service. We offer end-to-end solutions, from design to assembly, ensuring the highest quality at the most competitive prices. Excited? Take action now! Click here for a flexible PCB quote and let’s bring your ideas to life.

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