Rigid PCBs that serve the purpose of making electronic devices operational are not compatible with high-tech advanced gadgets. As engineers are required to provide PCB solutions that match complex designs, shapes, and product specifications, flexible PCBs have better advantages for building a new era of products. The space and power demands of new-age products are challenging. We have to utilise bendability and freedom of component placement options to meet the product requirement.

In this article, we will focus on learning about flex boards. How are flex boards useful? What are the types of flex PCBs? What are the advantages? What are the application areas?

What are Flexible PCBs?

Flexible PCB as the name says are printed circuit boards that can be easily bent and twisted. They have other names, such as flex circuits, FPC (flexible printed circuit), flexible circuit boards and flex-circuits. They provide ease to keep the board shape and size in any form, due to the thin structure of PCB material. The PCB substrate for flex boards is made of plastic or polyimide, unlike rigid PCBs where fibreglass is the material. Flexible boards can tolerate more heat, and vibration than normal rigid PCBs. Flex circuits are now more in demand in the market because of the advantages they add to the PCBs. The polymer substrate for the fabrication of the boards is also easily available from the suppliers.

Benefits of Flex Circuits

Rigid boards are now being replaced by flexible boards due to various advantages. The electronic devices that are being developed in today’s industry have new demanding specifications such as fast production, low cost, simple processes, smaller shape and size, environment friendly and more. We should explore the advantages and usefulness of flexible boards to be able to utilise them in our future designs.

1.  Removes the Need for Heavy Connectors that Add Extra Weight to the PCB

Flexible cables are thin, and bendable and support multiple pin connections between the motherboard to daughter boards. In the application areas of automobile, medical and aerospace industries where the wire harness network is large, replacing too many wire cable connectors to FPC(flexible printed circuit) cables is a good approach.

2.  Easy Fitment in Compact and Complex Spaces

Folding at any angle (360 degrees)to place the PCB into a narrow shape is possible only with FPCs. Bending of flex circuits can be done more than once based on product applications, like wearable medical bands, robotic instruments, and medical equipment where durability and shapes require folding.

3.  The Small Form Factors of the Product

Reducing the size of rigid boards is not possible. These days, smaller shapes and sizes are insisted on for product development to match the competitor products. Engineers can use flex circuits to their advantage to transfer the components onto it, then reduce PCB size.

4.  Lighter and Durable Boards

As the flex circuits are built on thin complex polymer substrates, the weight of the PCB is lesser than that of a similar-sized rigid board. FPCs are reliable and highly durable. Considering the thickness is in microns, fabrication of the flex circuit is simple

5.  Freedom of Board Space Use

The flexible nature of FPCs allows for space-optimized boards. Designers can find more space to route the traces on the board. They can make multiple connections and route the interface through ribbon cables with multiple branches. The plastic and metal connectors used can be removed.

6.  Suitable for High-speed Boards

Potential signal integrity is possible with flexible PCBs. They provide low signal noises, distortions and reflections in high-speed digital communications. Signal transmission at high frequencies causes EMI/EMC issues that can be avoided with flex boards.

7.  Heat Management and Endurance to Vibration are Better

The polyimide substrate in flex circuits allows more heat handling capacity. Vibration is a normal phenomenon in the aerospace and medical industry. Flexible circuits are able to withstand more vibrations because they are divided uniformly in the PCB.

Types of Flex Circuit Boards

Based on the application area, testing and inspection levels of Flexible circuits, they are divided into categories of Classes. There are various options to select.

• Class 1

In this category, general consumer electronic products, use and throw and computer accessories are listed. These products require less inspection and testing.

• Class 2

In this category, devices are produced for industrial equipment, and machines for longer use. They require high-performance products. For testing and inspection requirements flexible boards undergo improved methods to provide continuous performance. These products are expensive as compared to Class 1

• Class 3

In this category, high-critical equipment is made. The condition is to provide high-quality boards to support life safety and medical care applications. In military and aerospace Class 3 flexible products are used. They are most expensive as they provide the highest grade of performance.

IPC Based FPC PCB Classification

The classification is based on the number of conductive layers, vias and holes present and the type of PCB material used.

• Type 1

The flex PCBs are single-sided. These are the most common types that are used. The conductive layer is only on one side. They do not have plated-through holes. It is easy to fabricate the board and it has an economical cost

• Type II

The flex boards are double-sided. The conductive layer is present on both sides. The layers are connected through plated holes. They have a higher density of components. Like rigid PCB manufacturing processes, Type II boards are etched, drilled and plated with dielectric inner layers.

• Type III

These flex PCBs are multi-layered. The conductive layers are present in more than 3 layers. They are fabricated by combining single and double-layered boards. Bonded by using adhesives. These flexible circuits are suitable for complex circuits that have a high density of components to be connected.

• Type IV

Type IV is a category of rigid-flex boards. A multi-layered flexible board with some portion of rigid board.  The combination of layers stacked up in rigid-flex boards includes inner layers of rigid and flexible material.

Flex Board Layer Stackup

All types of flexible boards are made from a structure of conductive copper layer, adhesive and dielectric material.

1. Dielectric Layer

Polyimide, PET (polyester) is the core substrate of flexible boards. The dielectric constant is between 2.5 to 3 at 100. The thickness is between 10um to 110 um. This layer will work as a core layer and insulating layer also in the stack up. The core material prevents more melting at high temperatures and maintains flexibility.

2. Copper Foil Layer

Flexible boards have a copper layer of 0.5oz to 1oz thickness. Traces, plated holes, and vias all use copper to define circuit conductivity in the board. The copper laminate foil is rolled and bonded with the core material， then etching is performed to transfer the circuit.

3. Adhesive Layer

Different types of bonding materials such as epoxy and acrylic are used to bind the layers together. Each material has different properties like strength and temperature.

Summary

Flexible PCBs are gaining more adoption for upcoming electronic products. Manufacturing is also becoming easier as the PCB board manufacturer has added more capability to match the industry demands.

Related articles:

• Rigid-Flex PCBs
• What are Printed circuit boards (PCBs)?
• The Manufacturing Process Behind 6 Layers PCB Board