A Must-Know Guide for What is Flex PCB

As the name suggests, a flexible PCB is a particular type of circuit board that can bend to the desired application shape. Compared to the regular rigid circuit boards, this type places the conductive pathways and electrical components on a flexible base material.

The flexible design has several benefits that come in handy when building modern compact electrical devices, such as phones, cameras, hearing aids, etc. Therefore, if you are a technician working on such a project, here is an in-depth guide to flexible PCBs and where to buy them.

Rigid vs. Flexible PCBs

Rigid vs. Flexible PCB: What's the Difference?

While the purpose of both rigid and flexible PCBs is to connect electronic components in a compact circuit design, the two are different.

The main difference is in the substrate material. Flexible PCBs have a flexible substrate, usually consisting of polyimide films, while the rigid type features a fiberglass base.

Because of the bending, manufacturers use flexible rolled annealed copper instead of rigid electro-deposited copper to make the conductive layer.

A flexible circuit board with a white connector

 

When manufacturing the pieces, flexible PCBs go through an overlay process to protect the exposed circuitry, while the rigid type uses a solder mask.

The former is also more expensive, but the overall product cost might be cheaper because you can shrink the size of the device you plan to make.

A rigid circuit board for a computer

 

Rigid vs. Flexible PCB: which one is better?

It depends on the application. Flex PCBs are ideal for compact electronic devices and appliances, such as phones, hearing aids, and wearables.

However, a rigid board is better for devices with lots of space, like computers and TVs.

You can combine the two to form a rigid-flex board with half of the design in a rigid board and the rest in a flexible circuit.

 

A rigid-flex board

 

Combination of Rigid and Flexible PCB: Means to Make a Rigid-Flex PCB

 

Basic rigid PCBs only need Gerber file layers for the design stage. However, rigid-flex boards need 3D design software to create the board in a 3D space before manufacturing.

They have more dimensions and need precise definitions of the flex-to-rigid transition points, so a 3D design is essential.

The flexible side's base film is polyimide, on which a copper foil and overlay holding film layer are above it. However, rigid PCBs have FR4 as the base material.

The first step is to apply accessible glue on the copper layer during assembly. After that, you can use the copper plating method or the overlay process for the next stage.

Once complete, use the precise laser penetration method to bore tiny holes on the flex substrate.

Next, fill copper into these holes using the through-open plating procedure (synthetic copper plating).

 

A PCB copper layer electroplating machine

Source: Wikimedia Commons  

The following step involves applying photosensitive engraving opposes covering on the flex surface, then carving of the copper film. After these two procedures, the draw oppose gets expelled from the circuit board.

Next, apply the overlay (usually polyimide) to the top and base layers of the flexible substrate.

After this, the board goes through the blanking process, which cuts the substrate into the required sizes. The most commonly used procedures for blanking are hydraulic punch or kick the bucket set.

Lastly, the cut flex blanks are overlaid between the rigid layers. The final piece can then be tested to ensure it works correctly.

The Bendability of Flex PCB

Flexible PCBs can bend, but there is a limit to how many times you can bend them and the extent to which they can flex.

When you consider the number of times it can bend, static boards can only flex 100 times or less because manufacturers consider them as bend to install.

In most cases, you’ll only bend them during assembly, leaving a big allowance of flex cycles to play with later during repairs or maintenance.

But dynamic PCBs can withstand thousands or tens of thousands of bends because they flex regularly.

The primary factor that determines this level of flexibility is the material thickness. The thinner the board, the more pliable it will be and the more flexing cycles it will handle.

When considering the extent to which they can bend, we’ll look at the bend radius.

You can determine the minimum angle at the flexible region can bend early on during PCB design.

Advantages of Flexible PCBs

 

• Save space and weight.

A flex PCB. Note how it takes up less space when folded or rolled.

 

• Increased stability and reliability
• Eliminates the use of connectors and wire harnesses and minimizes wiring errors
• Possible to stack them in various configurations
• High tensile strength & high resistance to radiation, chemicals, and extreme temperatures
• Durable (ideal for harsh environments)
• Easy to install
• Delivers a strong signal quality
• Improves impedance control

 

Disadvantages of Flexible PCBs

 

• It has a higher cost compared to rigid PCBs in specific applications
• It uses a complex assembly process
• Difficult to repair or rework
• Needs proper storage conditions
• Easily damaged through improper handling

 

Types of Flexible PCBs

 

Single-Sided Flex

As the simplest type, the single-sided flex circuit has a single conductive layer on one side of the dielectric layer (polyimide or polymer film).

Double-Sided Flex

Double-sided flex PCBs have a conductive copper layer on each side of the dielectric material. Metalized holes connect the two copper sides.

Multi-Layer Flex

This type contains multiple copper layers separated and encapsulated by dielectric layers. Each of the metal layers is connected through holes.

Rigid-Flex Circuits

As stated earlier, rigid-flex PCBs are multi-layer circuits that combine rigid and flexible boards, creating a hybrid design with a high circuit density.

HDI Flexible PCB

High-Density Interconnect (HDI) circuits are efficient and reliable boards with multiple electronic components. They deliver better electrical performance at a reduced package size due to their thin, non-conductive substrate material.

Sculptured Flex Circuit

Sculptured flex circuits feature circuit traces that vary in thickness throughout the circuit design. As such, they can have varying copper thickness in some areas. 

Polymer Thick Film Flexible PCB

Polymer Thick Film is a simple and affordable way to make reliable FPCBs via screen printing using stencils. You can find such PCBs in low-voltage applications because they are not powerful enough to handle high voltage.

Double Access/Back Barred Flexible PCB

This is a special single-sided PCB with conductive tracks accessible from either side of the board.

Two things make this possible. First, the metal layer gets a pre-punched dielectric film applied over it. Second, the conductor layer etching runs under the access points (openings) on the flexible film (dielectric).

Flex PCB Prototype Correct Thickness

You can determine the most suitable flexible PCB thickness of the prototype circuit by considering several factors in the equation.

For instance, PCB weight is a significant aspect because heavy materials are stiffer, but they have some performance benefits.

Light materials are cheaper and more flexible, but they usually have lower heat dissipation properties and high dielectric constants.

A flexible PCB with a chip and some components soldered onboard

If your board needs hot active components, you must use stiffeners or the entire circuit must be on a thick substrate to hold heat sinks. Similarly, boards that require chips like microcontrollers and microprocessors must have stiffened areas.

However, thick PCBs require more materials to fabricate the layers, which increases manufacturing cost per square foot.

Reliability is also critical, and the determining factor is the board's thickness.

Thin boards are more flexible than thick ones, which makes them more reliable because they have a larger bend radius and can withstand more flex cycles.

Generally, circuit boards with a thickness of less than 1 oz./sqft are the most flexible and reliable if the application is in a flexible environment.'

 

Typical Flexible PCB Schematic Diagram and Stackup

 

A flex PCB consists of several layers, but the number varies depending on the type. These structure diagrams and stack-ups can help you during the design process when building one.

 

Single Layer Flex PCB

 

Single-layer FPCBs have a simple stack-up that comprises a flexible base layer (usually polyimide), then an adhesive layer followed by copper. A protective layer (cover lay) consisting of an adhesive and polyimide covers the metal. However, the conductive part cannot be left exposed.

A single-sided flex PCB

 

Double Access/Back Barred FPCB

 

Like the single-layer electrical circuit, this one has five layers but with one difference. The polyamide outer layer on the base has a laser opening to access the copper circuitry.

A double access flex PCB

 

Double Layer FPCB

 

This PCB features a flexible laminate material as the base, separating the conductive layers. Each metal layer has its cover lay to protect the circuitry and copper-plated through holes to connect the two sides.

A double layer flex PCB

 

Multi-Layer Flex PCB

 

Multi-layer flexible circuits combine single and double-sided flexible circuitry designs. Inside, the middle section has dual-sided flex circuits, while the surface has a single-side conductive design covered in a protective film.

A multilayer flex PCB

 

Materials Used in Flex Circuits

 

The four main layers in flexible boards can have the following materials.

 

Conductors:

 

The metal used in this layer is usually copper due to its superb electric properties and low cost. Heat dissipation is also a key factor to consider, and copper does this quickly.

 

A flex PCB with a copper conductive layer

 

Other suitable conductor materials include stainless steel, Beryllium copper, and Cupronickel.

 

Adhesives

 

A glue layer containing acrylic or epoxy is between the copper and insulating film (polyimide). Acrylic bonding adhesive materials offer better heat resistance but have poor electrical properties and low binding strength.

On the other hand, epoxy does not give the best heat resistance, but it has better electrical properties and stickiness.

 

Epoxy adhesive

Source: Wikimedia Commons  

 

Insulators

 

The insulator is also known as the base material or dielectric and is the flexible material that gives this circuit board its bending properties. Polyimide is the most commonly used material, but some boards have polyester, polyetherimide, various copolymers, or fluoropolymers.

 

Finishes

 

An exposed copper circuitry can shock you or short circuit if an electrical conductor touches the pathways. A cover film usually consisting of a polyimide film keeps the copper sealed and protected.

 

Flexible PCB with an orange/golden polyimide film finish

 

Applications

 

• High-speed digital/RF/microwave connectors
• Industrial sensors
• Instrumentation consumables
• Medical devices - wearables
• Automotive
• Satellites
• Avionics
• Consumer electronics

Flexible PCB Design: How To Design Flex Circuits

Check and Understand the Electrical Parameters

These parameters include the maximum currents and voltages, capacitance limitations, impedance characteristics, etc.

The Create the Circuit Schematic

Create a general representation of the circuit while considering its bendability and type.

Create the Layout Design

The layout diagram shows exactly how the flexible board will look like.

A PCB layout design

Design the Stack-Up

Stack-ups help determine the impedance of the copper traces.

Define the Rules and Requirements

Consider the IPC standards while defining the rules and requirements for PCB manufacturing.

Place your Components

Discuss the design and layout guidelines for component placement with your layout provider, who should have datasheets for all components.

Insert Drill Holes

Most flexible boards have two copper layers, so they need an electrical connection through the substrate.

Route the Traces and Add the Labels and Identifiers

Route the traces, then add the labels, markings, identifiers, and reference designators to the layout design.

Conductor tracing on a multilayer circuit board

Generate Design and Layout Files

Generate the flex PCB design and layout files, which contain all the information about the board.

Flexible PCB Manufacturing Process

Flexible Laminate and Copper Fabrication and Lamination

This process involves brushing the PI film with adhesive and laminating it to form the overlay.

The copper foil also gets pressed to the required thickness for the board. These two layers are mated to make the overlay laminate the copper foil.

Drilling and VCP Electroplating

Drill holes through the copper layer to connect them if the board has two or more.

These holes are electroplated using copper on the vertical continuous plating line to create an even PTH thickness.

Circuit Transfer

The copper laminate gets a UV-sensitive photoresist layer printed above it, then a film of the circuit’s negative image is placed above this laminate and the copper gets exposed to UV light.

Etching

The etching machine uses a sodium hydroxide solution to dissolve the areas with the uncured photoresist layer. After that, this machine washes away the cured photoresist layer.

Positioning Hole Drilling

A mechanical drill bores three positioning holes diagonally on the board.

Electrical Testing

This testing process evaluates the electrical performance of the etched circuit using a power-on self-test.

A circuit board undergoing flying probe testing

Cleaning and Scrubbing

Cleaning removes the residual sodium hydroxide, metal oxides, and moisture, while scrubbing makes the surface glossy.

Layer Alignment and Lamination

This step aligns the copper and overlay layers before exposing them to high temperatures.

Silkscreen Printing and Baking

The silk screen printing machine places the required logos, marks, and symbols on the board; then the silkscreen undergoes baking to dry the layer.

Surface Finish Application

Protect the copper pads from oxidation using a surface finish like ENIG.

Finished Board Electrical Testing

This second electrical test checks the integrity of the circuit after lamination and silkscreen printing.

Profiling

Profiling means cutting the edge rails to separate the individual boards on the production line.

Visual Inspection

Experienced technicians inspect the complete flexible boards visually to check for scratches and contamination. They also count the boards to check if the production quantity is correct.

Sample Testing for Quality Control

The last testing step involves picking a sample of up to 10% of the manufactured flex PCBs randomly from the produced batch for testing.

Packaging and Storage

Once manufacturing and testing are complete, the fabricator should package the flex board batches according to your requirements and keep them in a warehouse before delivery.

Custom Flex PCB: Why Use Stiffeners in Flex Circuit Applications

Stiffeners improve the flex board’s reliability and durability, and they fall into three usage categories.

Rigidizing Connector and Component Stiffeners

These stiffeners create a rigid localized area suitable for mounting components or connectors.

They also protect the solder joints for these components from bending, which can damage the electrical link.

A PCB with a rigidized area for mounting a chip

ZIF Stiffeners

ZIF stiffeners create a thick, localized section at the ZIF’s fingers to meet its connection requirements.

Localized Bend Constraints

These stiffeners restrict bending at specific locations to meet the end application requirements, such as to allow final component assembly.

Factors Affecting the Cost and Turn-Around Time of Your Flex PCB

The primary factors affecting a flexible PCB's cost include the following.

• Materials
• Size and shape
• Number of layers
• Dimensional tolerance
• Board class or grade
• Via-hole features
• Drill hole size
• Copper foil thickness
• Circuit construction type
• Stiffener type
• Plating requirements
• Drill-to-copper distance

The manufacturing turn-around time is also important, and the factors that affect it include the following.

• Incomplete design data requirements
• Lacking information on the flexible-to-rigid transition zones
• Inappropriate bend requirements
• Missing ZIF connector specifications
• Other design-related issues

 

IPC Standards for Flexible Circuit Board Manufacturing

No.	Serial Number	Name	Description
1	IPC-4203	Cover and Bonding Material for Flexible PCBs	The standard classifies dielectric films (cover/base material) and adhesives, plus the qualification and quality requirements.
2	IPC-4204	Flexible Metal-Clad Dielectrics for use in Flexible Circuit Board Fabrication	Metal-clad dielectrics are materials used in fabricating flexible PCBs. The standard classifies these materials and describes the quality requirements.
3	IPC-6013	Performance and Qualification Specification for Rigid-Flex and Flexible PCBs	This standard specifies the qualification and performance for flexible circuit boards designed according to IPC-2221 and 2223.
4	IPC-FC-231C	Flexible Base Dielectrics for use in Flexible Printed Wiring	Revision C from April 1992
5	IPC-FC-241	Flexible Metal-CladDielectrics for use in Flexible Printed Wiring Fabrication	IPC-FC-241 describes the dielectrics for FPCBs and their quality requirements.
6	IPC-2223	Flexible/Rigid-Flex Printed Board Sectional Design Standard	The standard highlights the requirements, interconnecting structures, and component mounting forms for flexible and rigid PCBs.
7	IPC-ML-990	Performance Specification for Flexible Multilayer Wiring	1972 Edition (September 1972)
8	IPC-FC-250A	Single and Double-Sided Flexible Printed Wiring Specification	This standard specifies the conductor patterns on the flexible base. They might have stiffeners or interfacial connections.
9	IPC-RF-245	Rigid-Flex Printed Board Performance Specification	The 1987 Edition of IPC-RF-245, April 1987

How To Choose a Flex PCB Manufacturer

Flexible PCB manufacturers have increased over the years, but only a few meet the highest quality standards in manufacturing and storage.

You can determine the best by checking if they have quality, cutting-edge equipment for flex PCB manufacturing.

Also, ensure your manufacturer has trained, high-caliber technicians to handle the flexible circuit board fabrication process.

Ask if you can get a prototype made before going into full-scale production.

The manufacturer should be able to manufacture any flexible PCB you want for your project, whether single-layer, double-layer, multilayer, static, or dynamic.

Also, analyze the quality control measures they implement and check if they have the necessary accreditation and certification.

At a minimum, they should adhere to the industry’s manufacturing standards.

Remember to ask for a quote for a price comparison. Flexible PCB production shouldn’t be too costly, but an extremely low price is a reason to worry because price and quality go hand in hand.

Lastly, the manufacturer should provide proper storage or warehousing after manufacturing to keep the PCBs free from oxidation-related damage.

OurPCB: A Leading Flexible PCB Manufacturer

 

OurPCB is the top PCB manufacturer, and we have the expertise required to build any PCB type, including the flexible one.

We specialize in multi-layer flex and rigid-flex manufacturing and provide SMT component assembly for these boards.

We assure you of quality, reliability, and durability, meaning you can use our products in high-temperature, high-density applications worry-free.

Furthermore, our prices are reasonable, and we have a fast lead time of 5 - 6 days and 3 - 14 days for flexible and rigid-flex PCBs, respectively. The build time varies depending on the design.

 

Summary

 

In conclusion, flexible PCBs are ideal for building compact packages and devices due to their bending properties.

If you have a project requiring this type of circuit, contact us for more details about flex circuit boards and get free quotations for your designs.