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Double-Sided Flexible PCB

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Welcome to globalwellpcba

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OVERVIEW

Item Flexible PCB
Max Layer 8L
Inner Layer Min Trace/Space 3/3mil
Out Layer Min Trace/Space 3.5/4mil
Inner Layer Max Copper 2oz
Out Layer Max Copper 2oz
Min Mechanical Drilling 0.1mm
Min Laser Drilling 0.1mm
Aspect Ratio(Mechanical Drilling) 10:1
Aspect Ratio(Laser Drilling) /
Press Fit Hole Ttolerance ±0.05mm
PTH Tolerance ±0.075mm
NPTH Tolerance ±0.05mm
Countersink Tolerance ±0.15mm
Board Thickness 0.1-0.5mm
Board Thickness Tolerance(<1.0mm) ±0.05mm
Board Thickness Tolerance(≥1.0mm) /
Impedance Tolerance Single-Ended:±5Ω(≤50Ω),±10%(>50Ω)
Differential:±5Ω(≤50Ω),±10%(>50Ω)
Min Board Size 5*10mm
Max Board Size 9*14inch
Contour Tolerance ±0.05mm
Min BGA 7mil
Min SMT 7*10mil
Surface Treatment ENIG,Gold Finger,Immersion Silver,Immersion Tin,HASL(LF),OSP,ENEPIG,Flash Gold;Hard gold plating
Solder Mask Green,Black,Blue,Red,Matt Green
Min Solder Mask Clearance 3mil
Min Solder Mask Dam 8mil
Legend White,Black,Red,Yellow
Min Legend Width/Height 4/23mil
Strain Fillet Width 1.5+0.5mil
Bow & Twist /
Table of Contents
Primary Item (H2)

Double-sided flexible PCBs are the key to making modern electronic devices thin, flexible, and efficient. These versatile circuit boards are used in smartphones, wearable technology, aerospace, and automotive applications, offering incredible design flexibility and improved signal integrity.

Hand-holding-a-rolled-flexible-PCB-with-connectors

Understanding Double-Sided Flexible PCBs

Double-sided flexible PCBs, also known as 2-layer boards or bilayers, feature two conductive layers on each side of a flexible base material. This design allows for more complex and compact circuits compared to traditional rigid PCBs.

Definition and Basic Structure

Double-sided flexible PCBs consist of two conductive copper layers separated by an insulating layer, usually made of polyimide. These layers are bonded together using an adhesive, although some designs use adhesiveless construction for better flexibility. The copper layers can have traces on both sides, connected through plated through holes (PTH), enabling more intricate circuit designs.

The flexibility of these PCBs allows them to bend and twist without damaging the conductive paths. This characteristic is particularly beneficial in applications where space is limited or where the PCB must move during operation.

Learn more about designing flexible PCBs: A Guide for Flex PCB Design Requirements

Comparison with Single-Sided and Multilayer Flexible PCBs

Yellow flexible PCB sheet with multiple cut-out designs

Single-Sided Flexible PCBs

  • Structure: Single-sided flexible PCBs have one conductive layer on one side of the insulating substrate. The other side remains bare or covered with a protective layer.
  • Applications: Used in simple, low-density designs such as basic consumer electronics and appliances.
  • Advantages: Easier and cheaper to manufacture, making them ideal for cost-sensitive applications.
  • Limitations: Limited circuit complexity and fewer connection options compared to double-sided or multilayer designs.

Double-Sided Flexible PCBs

  • Structure: These PCBs have conductive layers on both sides of the insulating substrate. They allow for more complex and compact circuits.
  • Applications: Suitable for mid-range complexity designs, including telecommunications, automotive, and medical devices.
  • Advantages: Greater design flexibility, higher wiring density, and the ability to mount components on both sides.

Limitations: More complex and costly to manufacture than single-sided PCBs, but still more economical than multilayer PCBs.

Orange-flexible-PCB-with-components-and-a-magnified-section

Multilayer Flexible PCBs

  • Structure: Multilayer flexible PCBs have three or more conductive layers separated by insulating layers. These layers are interconnected through vias, creating a more complex structure.
  • Applications: Used in high-density, high-performance applications such as advanced medical devices, aerospace systems, and high-end consumer electronics.
  • Advantages: Highest wiring density, optimal performance, and maximum design flexibility.
  • Limitations: Most complex and expensive to manufacture, requiring advanced fabrication techniques and materials.

Read about multilayer PCBs: Comprehensive Guide on Multilayer PCB

Configurations and Types of Double-Sided Flexible PCBs

Double-sided flexible PCBs come in various configurations and types, each tailored to meet specific needs. Understanding these configurations and types can help you choose the right PCB for your application.

Configuration Details

A double-sided flexible PCB features two conductive layers, one on each side of a flexible base material. Typically, this base material is polyimide, known for its excellent flexibility, heat resistance, and durability. The polyimide layer acts as the foundation, providing a stable yet bendable surface for the copper traces.

Dual-Layer Flexible Circuit Structure

In a dual-layer flexible circuit, the conductive layers are separated by the polyimide base. Custom trace patterns are etched onto both sides of this substrate. These patterns can be interconnected using copper-plated through-holes (PTH), which facilitate electrical connections between the two layers. This structure allows for more intricate circuit designs and enhanced functionality without increasing the overall thickness of the board.

Types of Double-Sided Flexible Circuits

Double-sided flexible circuits can be customized based on the presence of protective layers and PTHs. Here are the main types:

With a Cover Layer, Without PTH

This type features a protective cover layer on one or both sides but does not include PTHs. The cover layer adds durability and protects the circuitry from environmental factors like dust and moisture. However, since there are no PTHs, connections between the two layers must be made using alternative methods, such as surface mount technology (SMT).

Without PTH or a Cover Layer

These circuits lack both PTHs and a cover layer. They are generally used in applications where the circuit does not require much protection from external elements and where layer-to-layer connections are unnecessary. This configuration is simpler and can be more cost-effective for specific uses.

With PTH and Cover Layer

This type includes both PTHs and a protective cover layer. The PTHs allow for easy electrical connections between the two conductive layers, while the cover layer offers added protection and durability. This configuration is ideal for applications requiring robust and reliable connections, such as in automotive or aerospace industries.

With PTH, Without a Cover Layer

These circuits have PTHs but lack a cover layer. The PTHs provide the necessary interconnections between layers, making them suitable for designs that require multiple connections. However, without the cover layer, these circuits might be more susceptible to damage from environmental factors. They are typically used in controlled environments where protection is less of a concern.

Materials Used in Double-Sided Flexible PCBs

Choosing the right materials is crucial for the performance and durability of double-sided flexible PCBs. Different materials offer various strengths, making them suitable for specific applications. Let's explore the common materials used and their properties.

Stack of orange flexible PCBs with multiple connectors

Overview of Materials

Double-sided flexible PCBs are made from a variety of materials, each selected for its unique properties. The main materials include polyimide (PI), FR-4, PET, LCP, and PEN, along with other options like steel and aluminum for specific needs.

Detailed Material Properties

Polyimide (PI)

Polyimide is a popular choice for double-sided flexible PCBs due to its excellent flexibility and resistance to heat and chemicals.

  • Flexibility: Polyimide is highly flexible, making it perfect for applications that require the PCB to bend or twist.
  • Heat Resistance: It can withstand high temperatures, which is essential for applications exposed to heat.
  • Chemical Resistance: Polyimide resists various chemicals, ensuring longevity and reliability in harsh environments.

Applications: Wearables, medical devices, aerospace electronics.

FR-4 (Flame Retardant 4)

FR-4 is a rigid laminate material commonly used in standard PCBs and as a stiffener in flexible PCBs.

  • Electrical Insulation: FR-4 provides excellent electrical insulation, protecting the circuitry.
  • Cost-Effectiveness: It is affordable, making it a cost-effective choice for many applications.
  • Heat Resistance: It can endure high temperatures without degrading.

Applications: Power electronics, automotive electronics.

PET (Polyethylene Terephthalate)

PET is a common plastic used in flexible PCBs for its chemical resistance and affordability.

  • Chemical Resistance: PET withstands various chemicals, ensuring durability.
  • Lightweight: It is lightweight, contributing to the overall reduction in the device's weight.
  • Affordability: PET is cost-effective, making it a popular choice for budget-sensitive applications.

Applications: Membrane keyboards, sensors, flexible displays.

LCP (Liquid Crystal Polymer)

LCP is a high-performance material known for its excellent chemical and thermal resistance.

  • High Performance: LCP offers superior performance compared to other materials.
  • Chemical and Thermal Resistance: It resists chemicals and high temperatures, ensuring reliability.
  • Dimensional Stability: LCP maintains its shape and size under different conditions.

Applications: Automotive and industrial electronics.

PEN (Polyethylene Naphthalate)

PEN strikes a balance between cost and performance, making it suitable for applications requiring moderate flexibility and temperature tolerance.

  • Flexibility: PEN is flexible enough for various applications.
  • Heat Resistance: It can withstand moderate heat, making it versatile.
  • Optical Clarity: PEN offers good optical clarity, which is useful in specific applications.

Applications: Flexible displays, solar cells, medical devices.

Other Materials

Steel and aluminum are also used in double-sided flexible PCBs, primarily for their strength and heat dissipation properties.

  • Steel and Aluminum: These materials provide high strength and are excellent for heat dissipation. However, they add weight and reduce flexibility compared to other materials.

Applications: High-strength and heat dissipation requirements.

Explore more about PCB materials: Most Common Types of PCB Materials

Structure and Design Considerations

Designing double-sided flexible PCBs requires a keen understanding of their structure and thoughtful consideration of various design elements. This ensures optimal performance and reliability.

Layer Structure

A double-sided flexible PCB comprises several layers. Starting from top to bottom, these include:

  • Coverlay
  • Adhesive
  • Copper Foil
  • Adhesive (optional)
  • Substrate
  • Adhesive (optional)
  • Copper Foil
  • Adhesive
  • Coverlay
Diagram of a flexible PCB showing layers and components

Design Tips

When designing double-sided flexible PCBs, certain considerations can significantly impact the effectiveness and reliability of the final product.

Choosing the Right Copper Foil

The choice of copper foil is crucial for the performance of the PCB:

  • Static Applications:

Rolled Annealed Copper Foil: This type of copper foil is soft and ductile, making it ideal for static applications where the PCB is not subjected to frequent bending. It helps withstand mechanical stress and maintains integrity over time.

  • High Component Density Applications:

Electro-Deposited Copper Foil: For applications requiring fine circuit patterns and higher component density, electro-deposited copper foil is preferred. It has a smoother surface, allowing for more precise and intricate designs.

Selecting the Appropriate Laminate Process

The laminate process affects the flexibility and overall thickness of the PCB:

  • Non-Adhesive Process:

Advantages: This process reduces the overall thickness of the PCB and improves its flexibility. It’s ideal for applications where space and flexibility are critical.

Considerations: Requires careful selection of compatible materials to avoid issues during manufacturing.

  • Adhesive Process:

Advantages: More commonly used due to easier manufacturing processes. It provides strong adhesion between layers.

Considerations: Adds to the overall thickness of the PCB, which might affect flexibility. It’s crucial to balance the adhesive thickness to maintain desired flexibility.

Learn about PCB design basics: Comprehensive Guide to PCB Design and Basics

Understand PCB layout: PCB Layout: A Comprehensive Guide

Advantages of Double-Sided Flexible PCBs

Double-sided flexible PCBs offer numerous advantages that make them indispensable in various modern electronic applications. Their unique properties provide significant benefits, enhancing the performance and reliability of electronic devices.

Curved flexible PCB with labeled sections

Design Flexibility

One of the standout features of double-sided flexible PCBs is their design flexibility. These PCBs can accommodate intricate layouts, allowing designers to create complex circuits that would be impossible with rigid boards. 

The ability to bend and flex repeatedly without damaging the circuits makes them ideal for applications that require movement or tight spaces. Whether it's a foldable phone or a compact wearable device, double-sided flexible PCBs provide the necessary adaptability to fit into innovative designs.

Versatile Component Mounting

Double-sided flexible PCBs support a wide range of component mounting configurations, enhancing their versatility:

  • Through-Hole Components: These PCBs can easily accommodate through-hole components, providing robust connections that are essential for certain applications.
  • Surface Mount Technology (SMT): They support SMT components, which are crucial for high-density applications where space is a premium.
  • Wire Bonding: The ability to use wire bonding adds another layer of flexibility, allowing for various interconnection techniques.

This versatility ensures that double-sided flexible PCBs can meet the needs of diverse electronic projects, from simple gadgets to complex industrial equipment.

Compact and Lightweight

The slim profile of double-sided flexible PCBs is a significant advantage, especially in the age of miniaturization. Modern electronic devices are getting smaller and lighter, and these PCBs play a crucial role in this trend. 

By using thinner substrates and flexible materials, designers can reduce the overall size and weight of the device. This is particularly beneficial in portable electronics, where every gram and millimeter counts. As a result, devices become more compact and easier to carry, without sacrificing functionality or performance.

Reduction in Wiring Errors

Manual wiring is prone to errors, which can lead to faulty circuits and increased production costs. Double-sided flexible PCBs significantly reduce these errors through automated production processes. Automated manufacturing ensures precision and consistency, minimizing the risk of human error. 

This leads to higher reliability and quality in the final product. Moreover, automated processes can handle complex designs that would be challenging or impossible to achieve manually, further enhancing the capabilities of double-sided flexible PCBs.

Improved Signal Integrity

Signal integrity is crucial in modern electronics, especially for high-speed and high-frequency applications. Double-sided flexible PCBs offer improved signal integrity due to their design:

  • Shorter Traces: With two layers available for routing, designers can create shorter traces, reducing signal delay and loss.
  • Isolation from Noise: The two-layer structure allows for better isolation of sensitive signals from noise sources, enhancing overall performance.
  • Dedicated Ground Planes: Having an extra layer enables the use of dedicated ground planes, which further reduces interference and improves signal clarity.

These factors contribute to cleaner signals with less interference, making double-sided flexible PCBs ideal for applications like radio tuners, high-speed data processing, and other sensitive electronics.

Flexible PCB being processed in an FPC puncher machine

Applications of Double-Sided Flexible PCBs

Double-sided flexible PCBs are incredibly versatile, making them suitable for a wide range of applications across various industries. Their unique properties enable them to meet the demands of both everyday consumer electronics and specialized industrial equipment.

Communication

In communication networks, there are three problems: 

  • Machines shake and vibrate, which can damage electronics.
  • Some places get hot, which can fry circuits.
  • Strong radio waves can mess with signals.

With the help of double-sided flexible printed circuit boards (PCBs), these issues can be resolved easily. As these are made of specialized materials such as polyimide, they can bend and handle heat. Also, they have two sides for wires, which helps save space and makes them work better.

Aerospace

Rigid PCBs are preferred more in space and airplanes as things can get rough due to extreme temperatures and vibrations. Since they are tougher, they can handle it better. 

Another reason they are used is their ability to build many layers for complex functions. Though flexible PCBs can be bent and fit in tight spaces, they are not as strong as their multi-layered counterparts. 

Therefore, in space and airplanes, rigid PCBs, with their strength and layered complexity, are used more than flexible ones.

Used where weight and space are crucial. They are also a common choice for satellite applications.

Consumer Electronics

As smartphones and computers shrink and become lighter, foldability is emerging as a key trend. Flexible PCBs have thus become vital for this industry, as they allow for compact, bendable designs without compromising functionality. In simpler electronics with fewer layers, double-sided flexible PCBs are an ideal solution.

Used in Foldable phones, wearables, VR headsets

Automotive industry

Unlike traditional single-sided flex PCBs, double-sided ones offer greater design flexibility and can accommodate more complex circuitry. This makes them particularly well-suited for various applications in the demanding automotive industry.

Used in Seat control panels, steering wheel electronics

Other Industries

Double-sided flexible PCBs are also used in a variety of other industries, demonstrating their versatility:

  • Medical: Used in medical devices for their reliability and flexibility.
  • Industrial: Ideal for industrial equipment that requires durable and adaptable PCBs.
  • Military: Suitable for military applications due to their robustness and ability to function in extreme conditions.
  • Displays: Used in flexible displays, allowing for innovative screen designs.
  • Robotics: Essential for robotic systems that require precise and flexible circuitry.

Explore the advantages and applications of flexible PCBs: What is a Flexible PCB: Advantages & Applications

Challenges in manufacturing

It is difficult to:

  • Ensure precise details on both sides of the flexible circuit, even during repeated bending.
  • Maintain perfect alignment between layers throughout the bending process.
  • Prevent layers from separating or peeling apart under stress.
  • Create reliable connections through multiple layers with sufficient surrounding space for strength.
  • Manage electrical resistance and reactance within the circuit as it flexes.
  • Prevent stiff sections from breaking off during bending.
  • Control the forces and strains generated by heat and bending.
  • Ensure electrical connections remain intact throughout flexing cycles.
  • Handle, process, and assemble delicate circuits safely and efficiently.
  • Use components within the circuit that can bend without compromising performance.
  • Predict and account for changes in electrical properties due to various bending configurations.

Conclusion

Double-sided flexible PCBs offer numerous advantages, making them essential in modern electronics. Their design flexibility, compactness, and improved signal integrity make them a superior choice for innovative and reliable circuitry.

Contact GlobalWellPCBA today to learn more about our double-sided flexible PCB solutions and how they can enhance your electronic projects.

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