Flexible Printed Circuit Board - The Ultimate Guide to FPCB

Flexible Printed Circuit Board

Flexible PCB or Flex Circuits is the type of PCB or circuit board that has bending ability according to our required and wanted shapes. They are used especially for high-temperature and high-density applications. Typically, a thin polymer protects the conductor circuits by coating on top of the thin insulating polymer film. Since the 1950s, technology has been used for interconnecting electronic devices. Since then, it has been one of the most used and essential technologies used in most of today's electronics products.

Benefits of Flex PCB

Light weight and size reduction

Other solutions may not work in applications that require flexible circuit boards. Thin, lightweight, and able to fold and bend easily, the circuit boards can place in areas where other components are impossible. In addition, rigid-flex engineers often use 3D packaging geometry to reduce package size further.

Accurate Designs

Automated machinery designed and assembled the Flexible PCBs. The automated system helps reduce errors and ensures accuracy as the essential requirement for electronic devices.

Freedom of Design

Flex PCBs can have more layers providing design freedom to designers. It is easy to make flexible PCBs in several forms, including one-sided, two-sided, and multilayered. In addition, flexible configurations with multiple interconnections are possible with this product. Platted through-hole and surface-mounted components can flexible circuit boards accommodate.

Flexibility

During execution, the flexible circuit can connect to multiple planes. It reduces the space and weight faced by traditional circuit boards. At the same time, it can easily flex the flexible PCB according to our requirements to various levels at the installation time without damaging the board.

Improved Air Flow

Flexible circuits are designed to dissipate heat better and improve airflow. Circuits on rigid printed circuit boards stay cooler as a result of this. In addition to improving airflow, electronic circuit boards perform better over the long term.

High System Reliability

Early circuit boards had a lot of interconnections. As a result, circuit boards often fail due to interconnection failures. Nowadays, the number of interconnection points on PCBs can reduce according to requirements. Consequently, they have become more reliable under challenging conditions. Furthermore, using polyimide material improves this circuit board's thermal stability. 

Cost Savings

Having thin and flexible polyimide films reduces PCB assembly costs because they can fit into smaller areas. In addition to reducing testing time, routing errors, rejects, and rework time, flexible circuit boards also perform better under testing.

Structure of flexible PCB

The three Flexible PCBs include; single-layer, double and multi-layer circuits. The major element of single-layer FPCB has:

• Dielectric substrate film

It can be considered the base material of PCB. The prevalent material includes polyimide(PI), characterized by high-temperature resistance.

• Electric Conductors

It includes traces of the circuit. 

• Protective Finish

Cover lay, or cover coat, make it.

• Adhesive Material

The adhesive material helps in joining various parts of the circuit together.

Type of Materials for Rigid-Flex PCB

The commonly used rigid-flex PCB materials include: 

Substrate Materials 

Woven fiberglass is the primary material used in rigid-flex PCBs. Although epoxy imprinted fiberglass is a good option for some applications, it often fails to withstand vibrations, constant movement, and shocks. The rigid-flex circuit board manufacturing consists of the following materials:

Polyimide

Since polyimide is more versatile, tougher, and resistant to constant movements and vibrations than epoxy resins, it is preferred mainly over epoxy resins. In addition to its excellent heat resistance, we can also use the materials in highly fluctuating and elevated temperature environments.

Polyester(PET)

This substrate is typically available in a thickness of 25-125 microns and exhibits excellent electrical and chemical properties. In addition, substrate materials play a crucial role in the strength and durability of the entire assembly, which is one of the benefits of using them in aggressive industrial environments. 

Conductor Material

Rigid-flex, circuit boars are typically assembled with copper, materials that offer a variety of benefits such as excellent electrical characteristics and high workability. The copper foil is usually electro-deposited or rolled and is available in various thicknesses and weights for circuitry applications. For the foils to be as long-lasting as possible, Zinc treats them before being used in board assembly. Chemically treated foils also minimize bond degradation, improve adhesion, enhance bond strength, and prevent oxidation. 

Adhesive

Rigid-flex boards need adhesive to keep the substrate and conductor materials connected, extending their service life. Therefore, one of the crucial parts of a rigid-flex circuit board is to decide the adhesive type and thickness for bonding the conductor and substrate. 

The common adhesive used in manufacturing the rigid-flex board includes;

Polyimide Adhesive

The material has comprehensive temperature resistance quality. The heat resistance quality of this Polyimide Adhesive is used in power generation, defense and military. In addition, the production of multi-layer circuits requires this material's low coefficient of thermal expansion quality.

Polyester Adhesives

It is one of the cheapest adhesives used in flex PCBs. It features a low bond strength. This adhesive couldn't resist the fluctuating temperature. 

Type of Flexible PCB based on Layers

Single Sided Circuit

One of the most straightforward flexible printed circuit boards is a flexible circuit board with copper layers. Only one side of the circuit has access to the conductive copper layer. 

Single-Sided Flexible Circuit Boards with Dual Access

As their name suggests, single-sided flex circuits have copper sheets or conductor material accessible on both sides.

Double-Sided Flexible Circuit Boards

On each side of the polyimide base layer, there are two layers of conductors. Metalized plated through holes are used to connect two conductive layers.

Multilayered Flexible Circuits

Multilayered flex circuit boards are composed of multiple flexible circuits, some of which are double-sided and some of which are single-sided. The circuits are interconnected through cohesive surfaces or holes.

Types of Flexible PCB based on configuration

Rigid-Flex PCBs

As the name suggests, PCBs of this type combine the best features of both rigid and flexible boards. The rigid-flex PCB configuration usually features rigid circuits and flex circuits attached. Designers need to use hybrid circuits because they allow their circuits to be more reliable. 

In these circuits, the rigid areas mount the connectors, chassis, and other components. It has flexible areas, however, which can withstand vibrations and are flexible. As a result, these circuit boards offer PCB designers several advantages that enable them to develop circuit boards for challenging applications that meet their demands.

HDI Flexible PCBs

A high-density interconnect is called HDI PCB. Flexible PCBs designed for high-performance applications are ideal for this application. They offer better layouts, constructions, and designs with HDI flex circuit boards, which incorporate several features such as micro vias. In addition, the thinner substrate used by flexible PCB helps in reducing the package size and improves the electrical performance.

Applications of Flexible PCB

Flexible PCBs have a wide range of static and dynamic applications. Before any design works, the manufacturer confirms the application. The type of materials and components for building PCB also determined the application.

Consumer Electronics

Flex PCB has a wide range of applications in electronics. Some of the devices that use this flex PCB include laptops, cameras, flip cell phones, calculators, etc. 

Automotive

Flex PCB in the automotive industry not only increases reliability but also helps in weight saving compared to traditional PCB designs. Weight saving is one of the best methods of saving fuel; thus, flex PCB helps reduce the labor involved in the manufacturing of vehicles. While on the other hand, the flex PCB's resistance to vibrating makes them better for harsh environments. Some application areas of flex PCB in the automotive area include GPS, engine control, airbag systems, and antilock system. 

Medical

There has been a growing number of uses of PCBs with the advancement in technology in the healthcare industry. Flex PCBs are essential for medical applications as circuits must withstand extreme conditions in the human body. Some medical devices that use this flex PCB include pacemakers, hearing aids, exercise monitors, heart monitors, etc. In addition, the PillCam, a swallowable pill camera, uses a durable and non-toxic super-fine flex circuit. 

Researchers are not developing the process for printing the circuit in the flexible and organic material for body implants. This material is even more flexible and efficient.

Industrial

Flex PCBs can operate in stressful conditions and allow for machine design flexibility. In the industrial sector, the flex PCB is used in radio frequency communication, power circuits, industrial automation, etc. Using flex PCB opens up the door for various industrial spaces. 

Aerospace

Flex-rigid PCBs improve the reliability of industrial equipment and allow for more flexible machinery design. Its other applications include radio frequency communications, power distribution, and control circuits. In addition, compared to rigid boards, flex-rigid boards allow sensors to be much smaller and thinner. 

Military

The military is a sector that demands high performance with high precision PCB design. These PCBs must go through quality checks at each stage, starting from the fabrication of PCB, assembly, and final deployment of the product. Various factors influence the development process of military PCBs. 

Power Electronics

The flexibility and thinness of copper are also benefits of flexible printed circuit boards, which make them capable of handling heavy power currents. It is particularly important for devices in the power electronics industry, which require more power to operate fully.

High-Speed Digital, RF, and Microwave Applications

Flexible PCB is so reliable for handling high-frequency signals.

Pros and Cons of Flexible PCB

Flexible PCB contains various advantages and disadvantages that include:

Advantages of Flexible PCB

1. 1. It is flexible and can be bent, folded, and wound easily. It can thus be moved and expanded in 3D space, making it easier to integrate the component with an effective wire connection. 
2. 2. The size and weight of this Flex PCB required reduction, which is suitable for high-reliability and high-density products.
3. 3. It can provide the advantage of good heat dissipation, easy assembly, and good soldering advantage. 
4. 4. It can provide good signal integrity and have a good temperature range. 
5. 5. It can provide high mechanical resistance and resistance to the high atmospheric agent with better EMI(electromagnetic immunity).

Disadvantages of Flexible PCB

1. 1. The assembly process of this flex PCB is complex.
2. 2. It is difficult to repair any work and faults that arise in the PCB board.
3. 3. There is a high probability of damage while handling.
4. 4. Flex PCB is a bit more expensive compared to traditional PCB design.
5. 5. Flex PCB is sensitive to scratching.
6. 6. It requires proper storage conditions in sulfur-free plastic. 

Flexible PCB Manufacturing Process

The manufacturing process of PCB is structured. Its three significant steps are as follows:

Step 1: Flex PCB Build Up

It is one of the very first phases of this PCB manufacturing process, where the aim is to save the base materials. Polyimide is one of the primary materials in flex circuits. This polyimide material is costlier than FR-4 and requires extra care during use. In the nesting technique, the closed circuits help properly use polyimide. The prototype flex PCB manufacturing includes:

• Looping: 

It is acceptable to add a small amount of extra material beyond the requirements of the designers. For example, service loops enable circuit assembly and servicing lengths by adding material.

• Sizing Conductor

Dynamic applications recommend the thinnest copper, as it offers excellent flexibility.

• Etching

During the manufacturing process, this process compensates for any isotropic losses. Because of this process, almost twice as much copper foil thickness is lost in line width. Several factors affect line widths, such as copper types, etch masks, conductors, etc.

• Routing

The conductor's routing process is more accessible and quick. To bend and fold, get in a perpendicular position. This will then improve the bending and folding with an effective stress reduction.

• Ground Planes

If there is adequate electrical allocation, then create the ground areas cross-hatched. Reducing the weight of the circuit board improves the flexibility of the circuit.

Step 2: Flex PCB Fabrication Process

It is necessary to have an electrical conductor width of 375 micrometers for polymer thin films. At the same time, both silver-based polymer films and nominal polymer thick films conduct circuit currents. In addition, different designs and applications require different hole diameters in flexible PCBs.

• Hole Size

As well as creating tiny holes, the manufacturer creates flexible PCB layouts. As a result, the holes are as small as 25μm, and advanced technology can make it happen.

• Filleting

In this filleting process, you can multiply the pad area and divide the stress. All of the land termination and pads in the flex PCB require filleting. For a reliable solder joint, the plated through-hole is suitable.

• Button Plating

It requires the substitute plated through-hole. The manufacturer made the vias and through-hole using copper.

Step 3: Concentrate on Physical Constraints

This process of manufacturing deals with the cover coating problem and cover layers. Some of the few cover layers include:

• Adhesive Backed Flims

The raw materials present in this film are suitable for dynamic flex circuit apps. In the case of overcoating custom PCBs, it uses adhesive-backed films.

• Screen Printable Liquid Overcoats

Generally, screen-printable liquid overcoats use thick polymer films.

• Photo imaginable liquid and film polymers

We can consider it as one of the advanced over-coating methods with some surprising functionalities: 

• The solder can't connect circuit traces with this solder mask. 
• Both internal and external components protect the circuit.
• It helps to prevent the circuit from external electrification.

10 Tips for Your Flexible PCB Design

1. Solder, soft gold, nickel, hard gold, tin, electrolytes nickel, OSP and silver, electroless nickel with palladium gold, and organic palladium immersion gold allies with a flex circuit.

2. The minimum spacing and width are 0.038mm or 0.0015. The aspect ratio, which is the hole depth and diameter ratio, is to be 12:1 maximum.

3. Stacking, spacing, and width with your fab shop can control impedance in a flex circuit.

4. In bend areas, the routing of the conductor requires the parallel to perpendicular position as possible and maintains a constant thickness and width throughout. The conductor requires separate placement, not stacked with each other, to increase flexibility.

5. Whenever the circuit bends within 1" (25.4mm) of the termination pads, it must place fillets between conductors and termination pads.

6. The cross-hatched pattern compared to solid copper can be used if the circuit requires shield/ground planes.

7. In areas where the substrate or overlay bends, avoid discontinuities.

8. Tear stops or reliefs should protect slits in the circuit (90-degree relief slots). Tears can begin to propagate and start at the end of the slit.

9. Many materials make stiffeners, including epoxy-glass (FR-4), polyimide-glass, polyimide, copper, and aluminum. The required size of stiffener holes is 0.38mm or .015", more significant than the overlay access holes.

10. Around stiffeners, it allowed etched markings, while on the other hand, silkscreen markings are not.

Conclusion

Other conventional options cannot compare with the flexibility and accuracy of flex circuits. The flexible base material uses the circuit connections in flexible PCB. There are many flex circuits, from single to multi-layer PCB circuits. This includes many designs. In addition, several industries, including LED, medical, power systems, and more, are adopting flexible PCBs due to their durability and size and the declining size of electronics. 

NextPCB can provide you with all the types of flex PCB, from single-sided to multi-layer PCB. We provide you with the best quality of work for all of your complex layout designs. You can contact us for further details about the product and order the best-suited design per your requirement. We are happy to help you with your need.