Every flex PCB design must take into account flex PCB stackup. Choosing the right stackup can affect the performance, reliability, and cost of your final product.
In this article, we’ll explore the 9 most common types of flex PCB stackup used in various industrial applications.
Whether you’re a PCB designer, engineer, or simply interested in the world of electronics, this guide will equip you with the knowledge you need to make informed decisions when designing your next flex PCB.
What is Flex PCB Stackup?
The positioning and layout of the many layers of materials that make up a flexible printed circuit board are referred to as “flex PCB stackup” (PCB). The stackup typically consists of several layers of copper and insulation materials, such as polyimide, that are laminated together to form a flexible, thin, and lightweight PCB.
Depending on the particular needs of the application, the design of a flex PCB stackup may be basic single-layer designs or complicated multi-layered arrangements.
A flex PCB’s stackup is a vital part of the entire design, and selecting the right configuration is essential to getting the best performance, dependability, and cost-effectiveness.
PCB Stackup
Why is Flex PCB Stackup Important?
A flexible printed circuit board’s performance and dependability depend on its stackup. The proper arrangement of copper and insulation layers in the stackup is crucial for maintaining signal integrity and preventing EMI issues.
Additionally, the thickness and placement of copper layers in the stackup can be adjusted to achieve specific impedance values, which is crucial for high-speed digital and RF applications.
The right stackup design can also help dissipate heat generated by the components, withstand mechanical stress, and minimize material costs.
By carefully considering these factors, designers can ensure that their flex PCBs will meet the requirements of even the most demanding applications.
Flex PCB File Format
There are three common file format types that are used in designing flex PCB stackup. These are the following:
Portable Document Format (PDF) is a universal file format that is commonly used for sharing documents.
It can also be used for flex PCB designs, as it allows designers to easily share their designs with manufacturers and other team members.
PDF files are not typically used for manufacturing flex PCBs, as they do not provide the necessary information on layer thicknesses, copper traces, and other critical design details.
DXF
Drawing Interchange Format (DXF) is a file format commonly used for exchanging 2D CAD drawings between different software applications.
It is sometimes used for flex PCB designs, as it provides a convenient way to exchange design files between different software packages.
Like PDF files, DXF files do not provide the necessary information for manufacturing flex PCBs.
Gerber
Gerber is a standard file format used for manufacturing printed circuit boards, including flex PCBs.
Gerber files contain detailed information on copper traces, solder masks, and other design features needed for manufacturing.
They are widely accepted by PCB manufacturers and are the preferred format for flex PCB designs.
PCB Stackup
Flex PCB Stackup Types and Description
The following are typical flex PCB stackups that are often used in the sector.
Single Layer
This is the simplest type of flex PCB stackup, consisting of a single layer of copper traces and insulation. It is commonly used in low-density applications where cost is a major consideration.
Single Layer PCB Stackup
Double Layer
Two layers of insulation and copper traces make up this kind of flex PCB stackup, and vias join the layers together. When greater routing space is required for more complicated applications, it is frequently employed.
Double Layer PCB Stackup
Multilayer
A multilayer flex PCB stackup contains three or more layers of copper traces and insulation, with vias connecting the different layers. It is used in high-density applications where space is at a premium.
4 Layer
A type of multilayer flexible printed circuit board that is commonly used in applications that require moderate routing space. It consists of four layers of copper traces and insulation, with vias connecting the different layers.
4 Layer PCB Stackup
6 Layer
A particular kind of flexible multilayer printed circuit board with six layers of copper traces and insulation connected via vias. The additional layers of copper and insulation in a 6 Layer Flex PCB allow for more complex designs and higher component densities.
6 Layer PCB Stackup
Rigid-Flex
Both flexible and rigid sections are included in this kind of flexible PCB stackup, enabling the integration of both rigid and flexible circuitry. It is commonly used in applications that require both flexibility and structural stability.
Rigid-Flex
Bookbinder Rigid-Flex
In this particular rigid-flex stackup, the flexible and rigid regions are organized in the form of a book. It is commonly used in applications where space is at a premium, such as in handheld devices.
Bookbinder Rigid-Flex
With ZIF Contact Fingers
This type of flex PCB stackup includes zero insertion force (ZIF) contact fingers, which allow for easy and secure connections between the flex PCB and other components.
With ZIF
With FR4 Stiffeners
The FR4 stiffeners used in this kind of flex PCB stackup offer further structural support to the PCB’s flexible regions. It is commonly used in applications that require a high level of durability and reliability.
With FR4 Stiffeners
Flex PCB Materials
The flex PCB’s overall performance, reliability, cost, and manufacturability can all be significantly impacted by the material selection. The following are typical forms of flexible PCB materials:
Base Flex Material
Serves as the foundation for the PCB and is typically made from flexible substrates such as polyimide or polyester. It must be able to withstand repeated bending and twisting without cracking or losing its mechanical and electrical properties.
Polyimide
A popular choice for base flex material due to its excellent thermal stability, resistance to chemicals and solvents, and high dielectric strength. It can also withstand repeated bending and flexing without cracking or breaking, making it ideal for use in a wide range of industrial and electronic applications.
Polyester Film
Another common base flex material, offers good flexibility, high tensile strength, and excellent dimensional stability. It is also resistant to moisture and chemicals, making it a popular choice for applications where environmental conditions may be a concern. While it may not offer the same level of thermal stability as polyimide, it is often more cost-effective and easier to process.
Polyimide
Copper Conductor
Used to create the circuits on the flex PCB, providing electrical conductivity and signal transfer between components. They are typically etched onto the base material and must be able to withstand bending and flexing without cracking or breaking.
ED
Electro-Deposited copper is a popular choice due to its high purity, excellent conductivity, and good adhesion to the substrate. It is typically deposited onto the base material through an electroplating process and is well-suited for high-frequency and high-speed applications.
RA
Rolled Annealed copper is another common type of copper conductor used in flex PCBs. It is a softer, more ductile material than ED copper, making it easier to process and bend. RA copper also offers good conductivity and adhesion to the substrate, making it a reliable choice for a wide range of applications.
Copper Conductor
Coverlay
A thin protective layer is applied over the copper conductors to protect them from physical and environmental damage such as moisture, dust, and chemicals.
Adhesive
Used to bond different layers of the flex PCB together, such as the base material, copper conductors, and stiffeners. They must be flexible enough to maintain the PCB’s flexibility while providing strong adhesion.
Epoxy
A popular choice for adhesive due to its high strength, chemical resistance, and excellent bonding properties. It can be applied in a variety of ways, including as a liquid or as a pre-preg material, and is well-suited for use in high-temperature and high-stress applications.
Acrylic
Another common choice, offering good bonding strength, flexibility, and resistance to environmental factors such as moisture and heat. It is typically applied as a pressure-sensitive adhesive or as a pre-applied film, making it a convenient option for many applications.
Stiffeners
Often used to provide additional support and rigidity to the flex PCB in areas where it is needed. They are typically made from materials such as FR4 or polyimide and are attached to the PCB using adhesives.
FR4
Made from a glass-reinforced epoxy material and are a popular choice due to their high strength, rigidity, and cost-effectiveness. They are often used in conjunction with polyimide base materials.
Polyimide
Made from the same material as the base layer and are well-suited for use in high-temperature applications due to their excellent thermal stability.
Aluminum
Lightweight and offer good thermal conductivity, making them a popular choice for applications requiring efficient heat dissipation.
Stainless
Strong, durable, and resistant to corrosion, making them ideal for use in harsh environments or applications requiring high mechanical strength.
Stiffeners
Flex PCB Material Specifications and Specialty Materials Sample
Flex PCBs often require specific material specifications to meet the requirements of their intended applications. Some common examples of specialized flex PCB materials include:
PSA(s): 3M9077, 3M467MP, etc.
PSA(s) (Pressure Sensitive Adhesives) like 3M9077, 3M467MP, and others, provide strong and reliable bonding for flexible PCB components. They can be used in a variety of applications, including those with high-temperature or high-stress environments.
EMI Shielding Films: Tatsuta SF-PC6000
Provides excellent electromagnetic interference (EMI) shielding for sensitive electronic components. This can be particularly important for applications in the aerospace, military, or medical industries.
Flex materials: DuPont TK, DuPont HT
Engineered to withstand high temperatures and harsh environments. These materials are well-suited for use in applications such as automotive, aerospace, and industrial equipment.
Epoxy Strain Relief: Eccobond 45/15 Flexible Formula
Used to provide additional mechanical support and strain relief for components on a flexible PCB. This helps to prevent damage or failure due to mechanical stress or vibration.
Conclusion
In conclusion, the number and configuration of layers in a PCB are determined by the Flex PCB Stackup, a crucial component of flexible PCB design. The PCB’s functionality, longevity, and performance can all be significantly impacted by the materials employed in the stackup.
The choice of Flex PCB Stackup and materials, as well as the design format used, are crucial factors in the development of a reliable, high-performance, and cost-effective flexible PCB solution for any industry application.
