What are the Differences Between Rigid-Flex and Flexible PCB Assemblies?

PCB Basics

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Editorial Team - PCB Directory

May 4, 2023

A Printed Circuit Board (PCB) is a board made up of non-conductive material with conductive lines printed or etched. Electronic components are mounted on the board and the traces connect the components together to form a working circuit or assembly. PCBs are instrumental in any type of electrical device today, Without Printed Circuit Boards, our devices would not be able to function. They are essential in connecting different components and allowing for communication between them. Additionally, PCBs help protects these components from damage and interference.

With an ever-evolving electronics industry, PCB production and design have changed rapidly. Today, there are multiple variations of every single component of a Printed Circuit Board. For example, the non-conductive substrate that forms the basis of a PCB is available in various materials from fiberglass to Teflon. The conductive traces on a PCB can be copper, silver, gold, tin, or anything else.

Similarly, PCBs can be categorized on the basis of their flexibility. Based on their flexibility, PCBs can be categorized as:

Rigid PCBs
Rigid-Flex PCBs
Flexible PCBs

In this article, we will have a comprehensive discussion on two of these PCBs: Rigid-Flex and Flexible. This article will explore the different characteristics and applications, and how are both the PCB types are different.

Left: Rigid-Flex PCB; Right: Flexible PCB

Flexible PCBs

Flexible PCBs, also known as flex circuits, are a type of printed circuit board (PCB) that are made from flexible materials instead of rigid ones. They consist of a thin, flexible insulating polymer film, such as polyimide or polyester, which is coated with a thin layer of conductive material, typically copper.

Flexible PCB

Advantages of Flexible PCBs

Flexible PCBs offer a number of advantages over traditional rigid PCBs, including:

Flexibility: As the name suggests, flexible PCBs are highly flexible and can bend and conform to various shapes and sizes, making them ideal for use in applications with limited space or unusual form factors.
Weight and Size Reduction: Flexible PCBs are much lighter and thinner than traditional rigid PCBs.
Increased Reliability: Flexible PCBs are less prone to mechanical stress and vibration damage, making them more reliable.
Improved Signal Integrity: The thin and flexible design of flexible PCBs can help reduce electromagnetic interference (EMI) and signal loss.
Cost-Effective: In some cases when fewer flexible PCBs can replace multiple rigid PCBs to form an electrical component, they can significantly reduce the cost of production.

Applications of Flexible PCBs

Flexible PCBs are used in a variety of electronic applications, including:

Consumer electronics such as smartphones, tablets, and wearables
Automotive electronics such as car stereos, navigation systems, and dashboard displays
Medical devices such as pacemakers, heart monitors, and diagnostic equipment
Aerospace and defense such as satellites, missiles, and unmanned aerial vehicles (UAVs)
Industrial equipment such as sensors, robotics, and automation systems

Types of Materials Used in Flexible PCBs

Flexible PCBs can be made from a variety of materials, but the most common ones are:

Polyimide (PI): It has high-temperature resistance, good chemical resistance, and excellent mechanical properties.
Polyester (PET): This is a low-cost material that is used in consumer electronics applications.
Liquid Crystal Polymer (LCP): This material has a low dielectric constant and can be used for high-speed and high-frequency applications.

Rigid-Flex PCBs

Rigid-flex PCBs are a hybrid of both flexible and rigid PCBs. They consist of a flexible circuit substrate that is laminated to a rigid board, creating a single circuit board with both flexible and rigid areas.

Rigid-Flex PCB

Advantages of Rigid-Flex PCBs

Rigid-flex PCBs offer several advantages over traditional PCBs, including:

Increased Reliability: The combination of flexible and rigid areas in a single board makes rigid-flex PCBs more reliable, as they can withstand vibration, shock, and temperature changes better.
Space-Saving: Rigid-flex PCBs take up less space than a combination of separate rigid and flexible boards, making them ideal for compact designs.
Improved Signal Integrity: The rigid areas of the board provide a stable platform for components and connectors, reducing signal noise and improving signal integrity.
Simplified Assembly: Rigid-flex PCBs simplify the assembly process, reducing the need for connectors and cables, and decreasing the risk of errors.

Applications of Rigid-Flex PCBs

Rigid-flex PCBs are commonly used in applications that require high reliability and space-saving designs, such as:

Medical devices such as pacemakers, hearing aids, and diagnostic equipment
Aerospace and defense such as communication systems, missiles, and avionics
Industrial equipment such as robotics, sensors, and control systems
Consumer electronics such as smartphones, laptops, and wearables

Types of Materials Used in Rigid-Flex PCBs

Rigid-flex PCBs can be made from a variety of materials, including:

FR-4: This is the most commonly used material for rigid areas of the board due to its low cost and high-temperature resistance.
Polyimide: This is the most common material used for flexible areas of the board due to its high-temperature resistance, good chemical resistance, and excellent mechanical properties.
Copper: This is the most common material used for conductive traces and pads on both the flexible and rigid areas of the board.

Differences between Flexible and Rigid-Flex PCBs

Parameter	Flexible PCBs	Rigid-Flex PCBs
Mechanical	Flexible PCBs are fully flexible They can bend and twist in any direction.	These PCBs have both flexible and rigid areas PCBs have limited flexibility in the areas where the rigid board is attached to the flexible material.
Design	The design of flexible PCBs is generally simple. PCBs can be designed with a single layer of conductive material	They require laminating a flexible substrate to a rigid board. Hence, less simple. They typically have multiple layers of conductive material on both the flexible and rigid areas of the board.
Cost	Flexible PCBs are generally less expensive than rigid-flex PCBs because they require less material and have simpler designs.	Rigid-flex PCBs are typically more expensive than flexible PCBs due to the additional materials and manufacturing steps required.
Performance	Flexible PCBs have excellent flexibility, which makes them ideal for applications with limited space or unusual form factors. They also offer good resistance to vibration and mechanical stress. They are not suitable for applications that require high signal speeds or heavy components	Rigid-flex PCBs offer the flexibility of a flexible PCB with the stability and performance of a rigid PCB. They can support heavy components and high-speed signals while still offering some flexibility in specific areas of the board.

Conclusion

Concludingly, Rigid-Flex and Flexible PCBs are both crucial in their own aspects and should be used according to the requirements of a particular manufacturing project. On one hand, where flexible PCBs offer flexibility and ease of accessibility to a manufacturer, Rigid-Flex PCBs offer the right amount of strength required in tenuous electrical operations while also providing the flexibility that a rigid PCB might not provide. Cost and performance considerations also play a significant role in the decision-making process.