Double Sided Flexible PCB Guide

In today’s electronics industry, flexible PCB has become the core of innovation and compact design. In this context, the concept of a double-sided flexible PCB has emerged to more effectively utilize surface area and provide greater design flexibility. However, adopting this technology is not without its challenges, with one of them being the increase in mechanical stress. This article will delve into the technical details of a double-sided flexible PCB to help readers better understand the essence of this innovative technology.

Definition of Double-Sided Flexible PCB

Double layer flexible PCB

The ultimate purpose of a PCB is the same – to provide a support platform for components, and a double-sided flexible PCB is no exception. For those new to the electronics industry, breaking down this term makes the concept and definition clear:

Double-Sided

Classifying PCB based on the position of components yields two categories: Single-Sided and Double-Sided. Single-sided means components are mounted on the top layer of the PCB, and assembly is completed with one soldering operation. In contrast, double-sided implies that components are located on both the top and bottom layers of the PCB, requiring soldering on both sides. Single-layer structures lack a double-sided structure since they have only one copper layer on the top. On the other hand, double-layer and multi-layer structures can choose between single-sided or double-sided based on requirements.

Flexible PCB

A type of circuit board that can be bent and folded, flexible PCB comes into play when rigid PCB may struggle to be accommodated in complex electronic internal environments. Additionally, these PCB are thinner and lighter, which is advantageous for wearable devices or small equipment. However, it is essential to note that constructing double-sided and multi-layer is challenging and comes with a higher cost.

Therefore, a double-sided flexible PCB is a PCB that can be bent and folded, containing components on both sides. The goal is to improve PCB spatial utilization, increase circuit density, and enhance connectivity performance.

Double-Sided Flexible PCB Stack-up Design

The most common stack-up structure for a double-sided PCB includes the following layers:

  • Coverlay
  • Adhesive
  • Copper Foil
  • Adhesive (optional)
  • Substrate
  • Adhesive (optional)
  • Copper Foil
  • Adhesive
  • Coverlay

Double-sided flexible PCB stack structure

Among them, polyimide is the most common material used to build the substrate and coverlay.

The copper foil forms the conductive copper layer by etching or other techniques to create the circuit pattern. In dynamic applications, a double-sided flexible PCB faces a significant challenge in mechanical stress. Therefore, it is recommended to use Rolled Annealed Copper Foil, which is prepared through rolling and annealing. Although the surface may appear slightly rough, it is softer and more ductile. For situations with high component density, Electro-Deposited Copper Foil is suggested, as it is prepared through electro-deposition and has a smooth surface.

ED VS RA

The adhesive can remove it, using a non-adhesive process to laminate a double-sided flexible PCB directly. This approach reduces PCB thickness and improves flexibility.

After the stack-up design, attention must be paid to the thickness and material information of various layers. It is recommended to obtain firsthand information directly from the collaborating manufacturer to ensure the PCB is producible.

Moreover, the minimum bending radius is crucial for a double-sided flexible PCB. According to IPC standards, a bend radius of 0 is suitable for static applications and requires higher thickness. For dynamic applications, the minimum bending radius should follow a specific formula:

R=(d/2+c)×[(100−EB)/EB−D]

Where:

  • R: Minimum bending radius
  • c: Copper foil thickness
  • D: Dielectric material thickness
  • EB: Copper foil elongation ratio
  • d: Thickness of laminated dielectric material

Applications of a Double-Sided Flexible PCB

Application of double-sided flexible PCB

As shown in the diagram, the applications of a double-sided flexible PCB vary significantly in different fields. The main differences are evident in consumer electronics, communications, and industrial applications, with relatively fewer uses in the aerospace sector. This difference can be explained by considering the characteristics of a double-sided flexible PCB.

For consumer electronic sectors:

A trend is observed – everyday items such as smartphones and computers are evolving towards being lightweight and compact. With technological advancements and changing demands, many products can achieve foldable functionality, and flexible PCB play a crucial role in realizing these requirements. Furthermore, consumer electronic products typically have a simple internal structure and do not require a higher number of PCB layers. The versatility of a double-sided flexible PCB shines in this field.

In industrial and communication sectors:

Three pain points exist – 1. Vibration caused by mechanical motion; 2. High-temperature working environments; 3. High-frequency radiation. A double-sided flexible PCB effectively addresses these issues by using high-performance materials like polyimide. Additionally, its double-sided structure helps optimize internal space and enhance overall performance.

In the aerospace sector:

PCB need greater robustness and more complex multi-layer structures to combat harsh external environments and achieve more advanced functions. Compared to rigid PCB, flexible PCB are less reliable, and constructing multi-layer structures, especially double-sided ones, is challenging. Therefore, a double-sided flexible PCB has relatively fewer applications in the aerospace industry compared to other sectors.

Ensuring the Quality of a Double-Sided Flexible PCB

Essentially, whether single-sided or double-sided, both follow flexible PCB manufacturing processes. The main difference lies in the assembly, requiring double-sided soldering. This demands higher temperature control to prevent issues such as insufficient melting of solder or substrate expansion due to excessive temperature. To ensure the quality of a double-sided flexible PCB, accurate manufacturing execution and necessary testing are recommended, such as:

Temperature Control:

Ensure the use of appropriate temperature, time, and pressure parameters during the soldering process to ensure complete melting of the solder and secure solder joints.

Visual Inspection:

Use automated optical inspection equipment to visually inspect a double-sided flexible PCB, including solder joints, component positions, and potential defects.

In-Circuit Testing (ICT):

Utilize ICT to perform circuit tests on finished products to verify circuit connectivity and basic functionality. This is a crucial step in ensuring the normal operation of the circuit.

Functional Testing:

Conduct specific functional tests based on the requirements of a double-sided flexible PCB. This ensures that the produced boards conform to design specifications, guaranteeing the expected performance of the final product.

Bend Testing:

Perform bend tests on flexible PCB, especially for applications that undergo bending and twisting during use. This helps assess the reliability of flexible PCB in real-world usage and ensures that component solder joints remain intact.

Ending

In summary, double-sided flexible PCB are suitable for products that require efficient utilization of internal space. However, they come with certain construction challenges and increased costs. When choosing to use them, ensure a genuine need and consistently adhere to international standards.

This article was compiled by TechSparks through online research and reference, and any infringement issues should be addressed for content removal. While I previously worked in a PCBA company and gained basic knowledge through factory visits and two years of learning, I wasn’t directly involved in production. If any factual errors are identified in this article, corrections are welcome!

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