Flexible PCB

Using your smartphone or tablet is easy, but have you ever thought of the components?

My guess, I believe you have.

Well, among the various components making up your small portable devices is the flexible PCB.

In this guide you will learn about every interesting thing that you should know about the flexible PCB.

Our focus is to take you through the main characteristics of the flexible PCB.

These will include working principles, design, specifications and many more.

Let’s begin with the basics.

Flexible PCB Basics

Let’s start with the most fundamental aspects of flexible PCB

·What is Flexible PCB

A flexible printed circuit is a pattern of conductive traces on a flexible substrate.

Flexible PCB

You can as well describe it as the perfect solution for all of your electronic requirements.

Other names that you can describe them with include flex circuits, flexible circuits, and flexible PCBs.

It is basically a type of a printed circuit board with the ability to flex.  In this case, there are different forms of significant differences especially in fabrication, design and functionality.

Designing a flexible circuit does not follow the same rules as those of designing the rigid printed circuit boards.

In a way, the word “printed” is a misnomer as the mode of manufacturing is not the same. Most manufacturing processes depend on laser imaging and photo imaging as the method of defining the pattern.

You will learn more about the designing rules later on in this guide.

What Flexible PCB Consist of

Basically, the flexible PCB consists of a metallic layer and a dielectric layer bond together. There are many conductive metals that you can use but copper is the most common of the conductors.

You will be learning more about these features as we proceed.

Flexible Printed Circuit Board

Let’s look at some of the benefits of using the flexible PCB.

Advantages of Flexible PCB

Flexible PCB technology often has a wide range of possibilities for different designs and products.

The flexible feature is among the best attributes of the cables, wires, connectors and even PCBs.

Some of the advantages of using the flexible circuit include:

• Reduces the overall weight and size.
• A flex circuit can reduce the weight of a device by up to 70%.
• Improves the packaging of electronics
• It gives you the ability to solve interconnect and packaging issues as it can bend, form and move.
• Interconnect solution as it reduces the amount of interconnects such as cables, wires, pcbs and connectors.
• Conformability as the slender nature of the material allows for making of 3D packages
• Electrical integration – You can easily come up with custom solutions basing your design on a plethora of choices of materials. Apart from that, you have the freedom to choose from the myriad of designs and plating procedures.
• Heat dissipation ability or power – The polyimide dielectric has the ability to withstand very high temperatures. You can, therefore, use it for high power applications.
• Electrical and mechanical repeatability
• Cost saving as you can save up to 30% of the total cost of hand wiring and other assembly processes.
• You can also save on space by almost 30%
• It is more reliable as it does not have any wiring errors

Despite the numerous advantages, the flexible PCB also has its disadvantages.

Let’s have a look.

Disadvantages of Flexible PCB

Every electronic solution must have certain demerits.

Some of the disadvantages include:

Flexible PCB

• A higher initial cost – Since the flex circuit is often custom for particular electronics. The initial cost of designing, photographic plate and layout is very high. This limits you to using it on large quantities to save on cost.
• Flexible circuit repairs and changes are very difficult – The initial process is not just expensive but difficult to come up with. Once you have the base map, it becomes difficult to make any more changes. Removing and replacing the covering film for repair is also extremely difficult.
• It is often small in size – The batch process of making the flexible circuits often limits the size of the flexible circuit. The production equipment limits you from making wider and longer sizes.
• Easy to damage – Improper personal operations can easily damage the flexible circuits. Rework and soldering will require the expertise of a technician.
• The process of assembly is also very difficult.

Do not let the disadvantages kill your spirits.

Let us proceed and explore plenty of other interesting things about the flex PCB.

Types of Flexible Printed Circuit Boards

There are different types of flexible circuits in the market.

You will choose the type of flex pcb depending on the mode of application.

In this section you will learn more about the different types of flexible circuits.

·Single Layer Flex PCB

It is a type of flex circuit comprising of a conductive layer on a single side of the PCB.

 Types of flexible PCBs

Actually, it consists of one layer of copper between dielectrics of polyimide.

You may form holes in the base film by soldering to allow component lead to pass through.

A single layer PCB can pass through as a complete circuit without the cover coats or cover layers. However, the main practice in the manufacture of single layer flex PCBs often includes the cover layer.

The cost of fabricating a single layer flex PCB is high in comparison to that of single layer rigid PCB.

Advantages of the single layer flex PCB over the other types of flex PCB include:

• Despite the high cost, fabricating a single layer flex PCB is cheaper than fabricating the other types of flex PCBs.
• Since it has one layer, fabricating it easier in comparison to other types of flexible PCBs.
• It is easy to install and you will require less time for tracing and repairing the faults.
• There is a very low probability of short circuiting as the components are not close to one another.
• They are important for dynamic applications.

·Double Sided Flex PCB

It has two conductive layers with polyimide insulation between them. You can either decide to leave the outer part free or cover it with a copper pad.

The layers depend on plate through holes for the interconnection. You are free to use other methods as well.

Double sided flexible PCB

You can also include additional elements to the double sided flex PCB as it has more layers. The additional elements include connecters, pins and stiffeners.

Some of the main applications of the double sided flex PCB include:

• Power and ground plane applications
• Assembly of the dense surface mounting
• Shielding applications
• Digital cameras, LCD modules among others.

Why opt for a double sided flex PCB?

You can always go for the double sided flex PCB due to the following reasons

• It allows for more design variations more than the single layer flex PCB
• You can access the circuit traces from both sides of the board.
• Easy integration due to the flexible nature of the design
• The cost of assembly is lower in case you decide to ignore the interconnections.
• The turnaround time is lower in comparison to that of multilayer PCBs.
• With the low turnaround time, double sided flexible PCBs are cheaper.

·Multi-layer Flex PCB

A multilayer flex circuit features a combination of several single and double sided flex PCBs.

The multiple layers depend on complex interconnections, surface mounted and/ or shielding technologies to keep them together.

Multi-layer flex PCB – Photo courtesy LYNCOLEC

You may or may not continuously laminate the multiple layers together during the process of production. Since your design requires continuous lamination, you may not need to laminate the PCBs.

When will you need to use the multilayer flexible PCBs?

The multilayer flex PCBs provide the most efficient and effective solution in case of design challenges. Therefore, you will need them to:

• Control impedance with shielding
• Shielding applications
• EMI/RFI shielding
• Power plane and ground applications
• Increase the density of a circuit.
• Route the layout and density of a circuit which is not possible on one layer.

Let’s have a look at the benefits of a multilayer flex PCB

• Reduces the time and the cost of assembly as you will require relatively less manual labor for designing.
• Unlike the rigid boards, the multilayer flex PCB reduces the package weight and size.
• Has a larger surface area to volume ratio that is capable of increasing the dissipation of heat
• In applications with moving parts, the multilayer flexible circuit is the best option. It has the ability to move more than 500 times thus making it more durable.
• Increases the reliability of the system as it eliminates the interconnection points.
• You can use it for high density applications due to the fine lines that leave enough space.

·Rigid-flex PCB

This is a type of connection with a combination of both the flexible and rigid circuit technologies.

Most rigid flex circuits feature multiple layers of flexible circuits connected to one rigid board. The connection can either be internal or external depending on the nature of application.

Rigid flex PCB

The flexible circuits are often in a constant state of flex and you may find them on bends. It is a more challenging connection as the design has to take place in a 3D space.

This allows for greater and more spatial efficiency of the design.

Benefits of the rigid-flex circuits

• You can minimize the space requirement by using 3D applications
• You are capable of reducing the size of the device by eliminating the cables and connectors between individual parts.
• Lower count in parts and maximization of space.
• Simple test conditions making testing before installation possible.
• It follows a simpler PCB assembly process.
• Assembly and logistical costs reduce significantly with the rigid flex boards.
• Handling the rigid-flex circuits during assembly process is easier in comparison to flexible circuits.

Flexible printed circuit board are available in different types. With that in mind, let us proceed and look at the materials of the flexible PCBs.

Material of Flexible Printed Circuit Board

All the elements in a flexible circuit must meet all the requirements and life expectancy of the material.

In addition to that, the material has to work together with other parts of the flexible PCB. This will ensure the ease of manufacturing and reliability of the circuit.

In short, you have to improve the material performance to improve the performance of the circuit.

In the assembly of a flexible PCB, there are different materials that you will need.

This chapter will take you through some of the materials that you will need.

Parts of Flexible PCB – Photo courtesy: PCB Unlimited

·Substrate and Cover Overlay

The function of the substrate and the film will depend on the conductor carrier and medium of insulation. Furthermore, the substrate has to have the ability of bending and curling.

Usually the materials for the flexible PCBs often include the PI (polyimide) film and PET (polyester) films. Apart from that, you may find other types of polymer films such as PEN (polyethylene nphthalate), aramid and PTFE among others.

You should pick up the substrate material due to the cost and performance of the material.

The most common material that manufacturers use is the PI polyimide. It is a type of thermostat resin that can withstand very high temperatures thus eliminating the possibility of melting.

Apart from that, it can still remain flexible and elastic after thermal polymerization. It also features excellent electrical characteristics.

·Conductor Materials

You need to use conductor materials that will supply the current in the most efficient manner. In most of the flex circuits, the main conductor in use is copper.

Copper is a very good conductors and is also readily available. The cost of acquiring copper is also very low in comparison to other conductor materials.

Apart from electrical conductivity, heat dissipation also requires a good heat conductor. You can use materials that will eliminate the heat from the flexible circuit.

The thickness of the copper will also determine how much current it will be able to conduct.

The other types of conductors that you may consider include:

• Stainless Steel for the corrosion resistant properties
• BeCu for the spring properties
• Cupronickel for the high resistance properties

·Adhesives

In any flexible PCB, you will find adhesives between the polyimide film and copper. You may use the two main types of adhesives available which are epoxy and acrylic.

The adhesive material has to be strong enough to withstand high temperatures from the copper.

·Insulators

The insulators protect the user from shock when the electric current flows through the copper or conductor.

Polyimide film acts as the best material that you can use as an insulator.

In case of a rigid flex PCB, you will use a combination of FR4 and prepeg in the rigid part.

Let’s have a look at the prototyping process of the flexible PCB.

Flexible PCB Prototyping Process

Before ordering a fully running flexible circuit, you have to ensure that the design is working perfectly.

To achieve this cost effectively and efficiently, you have to create a prototype that your board will use.

A prototype is a simulation of your design so that you can test how good it performs.

Flexible PCB Design

So what does flexible PCB prototyping really entail?

Step One: Preparing for the Process of Prototyping

Make sure that you have all the information that you need about your flexible circuit.

While prototyping is very important, it might not be suitable for your designs.

The more information you provide on the prototype the better your design will be. Some of the information that you need to provide include:

• Number of flexible layers
• Dimensions of the substrate including the thickness
• Weight and thickness of the copper
• Minimum spacing and tracing
• Surface finishing

You will need to have various titles of software when you are submitting your design. After that, the manufacturer has to convert the file to Gerber which is the optimal type of file.

Once you have all the necessary information, the prototyping process can commence.

Step 2: Design

In this case, you have to come up with a suitable design.

You will use one of the software files to assist you in making a proper design.

Remember to inform the manufacturer of the version of the software you are using to make your designs.

Step 3: Schematic Design

The schematic design usually gives the manufacturer more crucial information to follow during the design process.

It includes more information about components, materials and hardware for the flex PCB. It also determines the function of the board, component placement and its characteristics.

Once you are done with the schematic, you need to run a preliminary check to ensure it functions properly.

Schematic

You will correct the defects in case of any and the run a simulation using special design software. This will include the conversion of the design into a netlist describing the interconnectivity of all electronic components.

Remember to run tests on the design more often during the prototyping process. It helps in fixing the problems as one moves from one step to the next.

Step 4: Creating a Bill of Materials

It is a list of all the materials that you need plus the information about the materials. Some of the information in the BOM includes quantity, reference designators, footprint, value and manufacturer part number.

 BOM

Once the schematic and BOM are complete, your manufacturer will inspect the document and gather all the parts.

Step 5: Routing the Design

You will design the routes by way of tracing that you will need for element connection.

You need to include various factors such as signal noise generation, power levels, and noise sensitivity.

Step 6: Creating the prototype

With all the information at hand, your manufacturer will create the flex PCB prototype. After that, he has to check if the prototype works perfectly.

With the prototype in hand, let us see how you will make the flexible PCB design and layout.

Step-by-step Flexible PCB Design and Layout

The flexible PCB design and layout process is not as different as that of the rigid PCB.

In this case, you have to account for and take note of mechanical complexity. For example, you have to ensure that the flexible PCB does not flex beyond its capability during installation process.

Therefore, it is proper to have a mechanical model and test it for functionality. It involves testing installation ergonomics, servicing and misalignments.

Step 1: Preparing the Board Thickness and Outline

You can test the idea of the flex circuit by cutting it out on a piece of paper.

Begin with the layout but do not draw the routes yet. After that, you will draw board outline and location of different thicknesses.

In case it is a rigid flex, you have to determine the areas that need stiffeners.

Avoid any unnecessary thicknesses as it will affect the bendability of the flexible circuit. The factors that determine the thickness of the flexible PCB include:

• Design and layout of the materials and the thickness of the material in use
• Copper layer count and weight of the base copper
• Adhesive and dielectric thickness.

Step 2: Understanding the bendability of your flexible PCB

You have to understand two important factors in relation to bend. This includes the number of times the circuit will bend and what it will be bending.

The number of times it will bend will be the determinant on whether it will be a dynamic or static bond.

A static bond is seen as a bend-to-install and will bend less than 100 times in its life time. The dynamic bond, however, is more robust and flexing happens more regularly.

Apart from that, you have to consider the bend radius of the circuit. You need to determine the minimum quantity of bendiness early in the design.

Flexible PCB Layout

It will ensure that the design allows the minimum quantity of bendiness without causing any damages. You can calculate the bend radios of the design by looking at the number of layers you need on the design.

Avoid 90 degrees bending of the flex circuit and plate through holes should not be on the bending areas.

Step 3: Increasing the Flexibility of the Ground Planes

You can increase the flexibility of the ground planes through two main ways.

You can either reduce the thickness of the dielectric as the thickness is directly proportional to flexibility.

Apart from that, you can also decide to reduce the thickness of the copper and the plane layers. You can do that by cross-hatching the plane

Step 4: Controlling the Impedance in the Flex Design

Cross hatching usually has significant impacts on the value of control impedance.

Flexible printed circuit boards

Since you will have to remove a certain percentage of the copper, you have to affect the shielding effect. As such, you will increase the controlled impedance of the flex circuit.

You can control the impedance through:

• Characteristic impedance with a micro-strip configuration with the conductor above ground plane.
• Differential impedance uses strip line configuration with the conductor running between two ground planes.
• Coplanar impedance

Step 5: Routing the Flex Traces

Longevity and performance of flex circuits is dependent on the circuitry of the design. You should not route the signals at right angles. I-beaming also reduces circuit flexibility and increases stress thus thinning copper circuits.

Use curves on the traces instead of sharp corners and the traces have to be perpendicular to the board.

Step 6: Annular Ring in the flex PCB

Allow for misregistration between the drilled holes and the internal pads. You should also consider minimum spacing between the drill holes and the tracks.

Step 7: Pad Design for the Outer Layers

You will make the circuits to be as large as possible to avoid overheating and melting the materials. You can also use anchors or spurs on the single circuits.

Apart from that, you can also use tear drops to eliminate potential stress concentration points.

Step 8: Flex Vias

To reduce the pealing on flex designs, you can do the following:

• Make the annular rings larger
• Do not teardrop the vias
• Add anchors or tabs to the vias.

The location of the vias are also important and you should place them away from the bending areas. You should also avoid the vias on dynamic applications.

Step 9: Drill to Copper

You should keep the distance between the via and copper feature in mind. You should, therefore, make tighter holes to copper and longer lead time.

Different types of vias

All these factors will play a great role in the assembly of the final design. You should also do thorough checks and rectify any mistake that would compromise the functionality of the design.

Let us proceed to the specifications of the flexible PCB.

Flex PCB Specification

In the process of designing a flexible circuit, you have to consider certain specifications.

These specifications will guide your manufacturer on the specific things that he has to include.

In this section, you are going to learn about some of the specifications to include.

Flex PCB

·Number of Layers

Since there are different types of flexible printed circuit boards, you should specify the type you need. The different types of boards have different number of layers.

It can either have one, two or multiple layers depending on your specifications.

·High TG PCB Board Dimensions

This features refers to the heat resistance of the materials you intend to use in manufacturing your flex PCB.

You need to specify the kind of resistance that you require for your design. This will depend greatly on the type of material that you will choose for your design.

·Surface Finish

The surface finish comes in different forms for you to choose from. There are no specific surface finishes that will work for particular applications.

This gives you the freedom of choosing from the available surface finishes in the industry.

Some of the surface finishes available include:

• Hot Air Solder Level (HASL)
• Electrolytic Tin Plating (RoHS compliant)
• Immersion Silver (RoHS Compliant)
• Immersion Tin
• Gold and Soldering
• Hard Gold over Nickel (RoHS Compliant)
• Electroplated Soft Gold over Nickel (RoHS compliant)
• ENIG (Electroless Nickel Immersion Gold) (RoHS Compliant)
• Organic Coating OSP (RoHS Compliant)

·Solder Mask

Solder mask or solder resist or solder stop mask coating are available in different forms. You have to specify the type of solder mask that you need and where you need it.

It can either be on one side of the flex PCB or both sides. The type of flex PCB will also guide the placement of the solder mask.

Flexible printed circuit board assembly

·Copper Weight

The weight of the copper also makes a significant requirement for your flexible PCB. You will determine the weight of the copper depending on your application.

Remember that the weight of the copper can also compromise the flexibility. You should, therefore, be careful not to compromise on the flexibility of the design.

·Finished Board Thickness

Apart from the weight, the thickness is the greatest determinant of the flexibility. The thicker the design, the less flexible it will be.

You should make the design as thin as possible but do not compromise on the quality and functionality of the design.

·Spacing

Spacing has a great effect on the functionality of the design. Small spacing can compromise functionality and may lead to short circuiting of the entire circuit.

In other words, your design specifications should be as wide as possible. The wider the spacing, the higher the functionality of device.

·Hole Sizes

You should make the holes tighter to avoid any drilling issues. This will hold the components you were mounting on the substrate tighter even when the circuit is flexing.

The size of the holes should also be smaller and ogre well with the flexibility of the device. It is a requirement that you have to specify on the double sided and multilayer flex circuits.

·Quality Grade

Your manufacturer needs to know the quality grade of your design.

You will determine this through the type of materials that you need for the flexible PCB. Apart from the material, there are various manufacturing grades for the flexible PCB.

The amount of money that you are willing to pay will determine the type of quality grade for your design.

·Quality Tests

Quality test is a crucial process that your design has to pass through.

The quality test will determine how good your design will perform. The manufacturer will take the design through the IPC guidelines for quality testing.

·Quality Compliance

It ensures that the design meets the right quality standards by different bodies. There are certain materials that may pause a great danger to the environment or user.

It is the responsibility of the manufacturer to ensure it is compliant to all quality standards. The main quality standards it has to comply with include RoHS and ISO9001:2000.

·Trace Widths

The trace widths of the flexible printed circuit vary depending on the type of application and design.

When specifying the trace widths, make sure that they are proper and will not affect the functionality of the design.

You should make the trace widths as wide as possible to avoid functionality issues.

Having knowledge on specifications takes you a step further to becoming a flex PCB expert.

Let us take you through the design rules of the flexible printed circuit.

Flexible PCB Design Rules

Designing a flexible PCB requires you to follow a few specific rules. These rules will guide you on the things that you need to do during the designing process.

Here are some of the key rules that you need to consider.

Flex PCB Assembly

·Bonding Sheet Layout

The bonding sheet layout should replicate the final design that you need to come up with. You should cut it out to fit the specific shape you need for the final design.

The size should also be wide enough to accommodate all the components of the flex circuit. Above all, the bonding sheet has to be flexible since you are making a flex circuit.

·Pattern Design of the Folder Area

The pattern design of the folder area should have curves. Any other form of patter will compromise the bendability of the flex circuit.

Apart from that, corners might also break when you are flexing the flex circuit. It is, therefore, important to make sure that they can ogre well with the flexible circuit.

·Silkscreen Description

The silkscreen description should be visible to the eye to allow you to see where to place the components. You should use colors that can reflect light to the eye and improve the visibility.

There are, however certain types of painting such as white and green that you can use for the silkscreen.

·Pattern Line Depth and Tolerance of Raw Material

The pattern line depth should not be very deep as it might compromise on the placement of the copper.

The raw materials have to be strong enough to withstand the high temperatures. The material should also be resistant to breaking no matter how much you flex it.

The material should also be a good conductor to dissipate the heat away from the device.

·Through Holes or Pads

You should make the through holes wide enough to allow for the fitting of the components. They should be deep enough to allow proper connection with the components on the other side.

Apart from that, you should make the tighter and away from the bending areas to avoid jeopardizing the flexibility.

·Stiffener and Tape Area Tolerance

The stiffener is useful when you are making rigid flex PCBs. The rigid areas of the rigid flex materials require you to add stiffeners to make them more rigid.

The tape area should also be strong enough to tolerate high temperatures as well as flexibility of the material.

·Teardrop Design

You can use the teardrop design when you are working on flex circuit designs with extreme mechanical and thermal stresses. It will assist in enhancing the structural integrity of the whole design whenever it is flexing or vibrating.

You might compromise the structural integrity of the design by drilling so you better use the teardrop process.

·Silkscreen Tolerance

The tolerance of the silkscreen should be able to withstand the working conditions of the flexible circuit. The flex circuit may work in extreme conditions of high temperatures.

You should, therefore consider these factors not to hinder the credibility of the silkscreen.

·Cover Overlay and Solder Resist Dimensions

The cover overlay and solder resist will assist you in making a good protector for the flex PCB design. It has to be strong enough to hold the components in place. You may also use a cover lay that conducts heat away from the flex circuit.

·Cover Lay Open Area Stiffener Specifications

You need to have a cover lay that can resist the harsh conditions of the flexible PCB. It has to be clear and very flexible to allow you to see through and flex the PCB appropriately.

You will need a cover lay that will not retain heat within the flex circuit system.

·Gap from Stiffener Edge to Hole

You should leave a considerable amount of space between the stiffener edge and the hole. It will reduce the chances of damages on the device whenever it flexes. Placing the holes closer to the stiffener edge may lead to damages on the flex circuit.

·Adhesive Specification

The adhesive which is the material in close contact with the copper conductors have to be strong. They have to tolerate the excess heat without melting. They should also be good resistors to keep the current within the flex circuit.

·Gold Finger Design

The gold fingers connect the flex PCB to the rigid part of the device. The gold fingers should be good conductors of both heat and electric current.

You can plate the gold fingers through the following processes:

• Electroless Nickel Immersion Gold (ENIG)
• Electroplated hard gold

·Stiffener and Pattern Design

You will use the stiffeners in areas that you do not want to have flexibility. It is more common in rigid flex printed circuits.

The pattern design varies according to your specifications. A good pattern design should consider enough spacing and depth.

Applications of Flexible Circuit Boards

A flex PCB is available in almost all the modern devices.

It is the most common technology that many device manufacturers choose to use in the modern era. Due to the numerous benefits, you will find the flex circuit in the most unlikely of places.

Figure 18 Consumer electronics

Some of the applications include:

• Cell phones
• Calculators
• Satellites
• Cameras
• Battery Packages
• Airbag Systems
• Antilock brakes
• Hearing aid devices
• GPS trackers
• Printers
• Motion Systems
• Fuel pumps
• Heart pacemakers and monitoring devices
• Automotive Engine Control Systems

Conclusion

I believe that your knowledge on the flexible printed circuit board is top notch.

You are now a flexible printed circuit board expert.

You can now make your own prototypes and designs with the help of your manufacturer.

What are you waiting for?

Contact your mates and come up with a good flexible PCB that will suit your needs.