FPC, like many industrial technologies, has undergone a long process of transformation and innovation, and it has become quite perfect today. The functions of FPCB can be divided into four types: lead line, printed circuit, connector, and integration of function. So the FPC is available in computers, computer peripheral auxiliary systems, consumer livelihood appliances, and automobiles. So we can see FPCB is closely related to our lives. 

Catalog

I Brief Introduction

Flexible Printed Circuit(FPC) also known as "soft boards," is a printed circuit made of flexible insulating substrates, which has a lot advantages that rigid printed circuit boards do not have.

Ordinary conductor and FPCB are generally glued together, although there are currently glueless copper foil materials. Since the dielectric constant of the FPCB is lower than that of a conventional PCB, it can provide good insulation performance and impedance performance to the conductor. At the same time, the FPCB is very thin and flexible, and also has good tensile strength, versatility and heat dissipation.

The biggest difference between a FPCB and a conventional hard circuit board is that the FPCB can be bent, folded or repeatedly moved in many ways. In order to realize all the possibilities of the FPCB, designers mostly use a variety of structures to meet the various needs of different products, such as single-layer board, double-layer board, multi-layer board and rigid- flexible board.

There are two types of FPCB currently on the market: printed (etched) board and silk screened board. The silk-screened FPCB is also called polymer thick film FPCB, which is different from the common printing etching technology. The FPCB technology uses other processes to directly use silk screen conductive ink as a conductor on the dielectric film to form the circuit we need. Although the application of FPCB is expanding, the printed FPCB is still the most widely used of the two.

II The Basic Structure of FPCB

Based on the analysis of the basic structure of the FPCB, it is found that the materials that make up the FPCB are insulating substrate, adhesive, metal conductive layer (copper foil) and covering layer.

2.1 Copper Film

(1) Copper foil: basically divided into the electrolytic copper and the rolled copper. The common thickness is 1oz 1/2oz and 1/3oz.

(2) Substrate film: the common thickness is 1mil and 1/2mil

(3) Glue (adhesive): the thickness is determined according to the requirements of the customer.

2.2 Cover Film

(1) Cover Film: for surface insulation. The common thickness is 1 mil and 1/2 mil.

(2) Glue (adhesive): the thickness is determined according to the requirements of the customer.

(3) Release paper: avoid the adhesive adhering to foreign objects before pressing, and make it easy to operate.

2.3 PI Stiffener Film

(1) PI Stiffener Film: stiffener the mechanical strength of FPC to facilitate surface mounting operation. The common thickness ranges from 3 mil to 9 mil.

(2 )Glue (adhesive): the thickness is determined according to the requirements of the customer.

(3) EMI: electromagnetic shielding film, which protects the circuit board from external interference (strong electromagnetic area or susceptible area).

III Classification of the FPCB

3.1 Classification according to the Combination of Substrate and Copper foil

(1) Glued FPCB

It means that the copper foil and the substrate are glued together, which is a commonly used one.

(2) Glueless FPCB

Compared with the glued FPCB, its flexibility, bonding between copper foil and substrate, pad flatness are better because it is combined by pressing the copper foil and the substrate together with a hot press. But its price is relatively high, and it is generally only used in places with high requirements, such as COF(CHIP ON FLEX).

3.2 Classification according to the Structure of FPCB

(1) Single-layer FPCB

After putting the material of the single-side PI copper clad plate on the circuits, then cover it with a coat of protective film, and form a soft circuit board with only a single-layer conductor.

(2) Ordinary Double-layer FPCB

After putting the material of the double-layer PI copper clad plate on the double-side circuits, then cover a coat of protective film on both sides, and form a circuit board with double-layer conductor.

(3) Substrate Generation of Single-layer FPCB

In the process of making circuit, use the pure copper foil, then cover a coat of protective film on both sides, and form a circuit board with only a single-layer of conductor but exposed on both sides of the circuit board.

(4) Substrate Generation of Double-layer FPCB

Press two layers of single-layer PI copper clad plate in the middle with the adhesive glue, and become a double-layer conductor circuit board with a two-layer separation structure in the local area to achieve a circuit board with high flexure in the layered region.

IV The Manufacture Process of FPCB

4.1 Single-layer Board

The Structure of Single-layer board

Cutting → Drilling → Drying film → Alignment → Exposure → Development → Etching → Stripping → Surface treatment → Covering film → Pressing → Curing → Surface treatment → Precipitating nickel gold → Printing characters → Shearing → Electrical test → Punching → Final inspection → Packaging → Shipping

The Overlay Composition of Single-layer Board

4.2 Double-layer Board

The Structure of Double-layer Board

Cutting → Drilling → PTH → Plating → Pretreatment → Drying film → Alignment → Exposure → Development → Pattern plating → Stripping → Pretreatment → Drying film → Alignment exposure → Development → Etching → Stripping → Surface treatment → Covering film → Pressing → Curing → Precipitating nickel gold → Printing characters → Shearing → Electrical test → Punching → Final inspection → Packaging → Shipping

The Overlay Composition of Double-layer Board

V Features of the FPCB

(1) Short: short assembly time.

All lines are configured, eliminating the connection of redundant lines.

(2) Small: smaller than PCB.

Can effectively reduce product volume and increase portability.

(3) Light: lighter than PCB(rigid board).

Can reduce the weight of the final product.

(4) Thin: thinner than PCB.

Can improve the flexibility and strengthen the assembly of three-dimensional space in the limited space.

5.1 Advantages

(1) It can be freely bent, wound, folded, can be arranged arbitrarily according to the spatial layout requirements, and can be arbitrarily moved and expanded in three-dimensional space, thereby achieving integration of component assembly and wire connection.

(2) The use of FPC can greatly reduce the size and weight of electronic products, and is suitable for the development of electronic products in the direction of high density, miniaturization, and high reliability. Therefore, FPC has been widely used in aerospace, military, mobile communications, laptop computers, computer peripherals, PDA, digital cameras and other fields or products.

(3) FPC also has the advantages of good heat dissipation and solderability, easy assembly, and low cost. The design of soft and hard combination makes up for the slight shortage of flexible substrate in the bearing capacity of components.

5.2 Disadvantages

(1) High one-time initial cost: because flexible PCB is designed and manufactured for special applications, the cost of starting circuit design, wiring and photographic plate is high. Unless there is a special need to apply a soft PCB, usually a small number of applications, it is best not to use it when applied in small quantities.

(2) It is difficult to change and repair soft PCB: once the flexible PCB is made, it is difficult to change it because it must be changed from the base map or the compiled light painting program. It is difficult to cover the surface with a protective film, remove it before repairing and restore it after repairing.

(3) Limited size: the FPCB is usually manufactured by batch process under the condition that it is not popular, so it is limited by the size of production equipment and can not be made very long and wide.

(4) Easily damage because of the improper operation: It is easy to cause damage to the FPCB if the operator operates improperly. So the soldering and rework require the trained personnel to operate.

VI The Applications of FPCB

Due to the rapid development of the times, people have more and more demand for electronic information, and the FPCB play an essential role in this case.

(1) Mobile phone

Emphasize the light weight and thinness of FPCB, it can effectively save the product volume, and easily make the battery, microphone, and key connect together.

(2) Computer and LCD screen

Take advantage of the integrated circuit configuration of FPCB, as well as the thickness, convert the digital signals into pictures and present them through the LCD screen. 

(3) CD Walkman

Emphasize the three-dimensional assembly characteristics and thinness of FPCB, transform the huge CD into a companion. 

(4) Disk drive

Whether it's a hard disk or a floppy disk, it relies heavily on the high softness and the ultra-thin thickness of 0.1mm of the FPC to complete fast data reading.

VII The Prospects of FPCB

7.1 The Flexibility and Reliability of FPCB

At present, four kinds of flexible circuit boards are prevalent: single-layer board FPCB, double-layer FPCB, multi-layer FPCB and rigid-flexible FPCB. The single-layer FPCB has a layer of chemically etched conductive pattern, and the conductive pattern layer on the surface of the flexible insulating substrate is a rolled copper foil. The insulating substrates used of the flexible assembly may be Kapton, PET, Nomex, and PVC.

The double-layer FPCB has a conductive pattern formed by etching on both sides of the base film. The metallized holes connect the two sides of the insulating material to form a conductive path to meet the flexural design and functions. The covering film protects the single-layer and double-layer wires and indicates where the components are placed.

The multi-layer FPCB is formed by laminating three or more layers of single-layer FPCBs or double-layer Fpcbs, and forming metallized holes by drilling and electroplating to form conductive path between different layers. In this way, there is no need to adopt complex welding procedures. Although the number of conductive layers designed for this FPCB can be infinite, the interaction of the assembly size, the number of layers, and the flexibility should be taken into account in designing  the layout in order to ensure the convenience of assembly.

Conventional rigid-flexible FPCB is made up of rigid and flexible substrates that are selectively laminated together. The structure is tight, and the metallized holes form an electrically conductive connection. Given reliability and price factors, manufacturers should try to keep as few layers as possible.

The FPCB industry is in the process of small scale but rapid development. PTF is a highly efficient, low-cost process for producing circuit boards. This process selectively prints conductive polymer inks on cheap flexible substrates. Its representative flexible substrate is PTE. PTF conductors include silk-printed metal fillers or carbon powder fillers. The PTF itself is clean and uses lead-free SMT adhesives without etching.

Schematic Illustrations of the PTF

There is also a FPCB of hybrid structure which is also a multi-layer board, but the conductive layers of the multi-layer board are composed of different metals. This hybrid structure is mostly used in the relationship between electrical signal conversion and heat conversion, and in the case of relatively harsh electrical performance at low temperatures. In this case, the FPC is the only feasible solution.

Whether the construction of these FPCBs is cost-effective and is optimally utilized can be evaluated by the convenience and total cost of the internal design. The overall way of interconnection is different. The mobile phone is in the form of a block layout; the portable computer is in the X-Y orientation locatable layout; the printer is in the form of a rigid-flex PCB. These products are made from different materials at different prices to reduce the cost of each interconnection. Each design is typologically evaluated to achieve the best performance-price ratio.

7.2 The Economy of FPCB

If the design of the circuit is relatively simple, the total volume is small, and the space is suitable, the traditional interconnection method is mostly cost-effective. But if the circuit is complex, handles many signals, or has special electrical or mechanical performance requirements, the FPCB is a better design choice. FPCB also has another potential cost-saving over rigid materials, which is eliminating connectors.

7.3 The cost of FPCB is Reducing

Some of the newer materials are now available with finer lines due to the thinner copper layer. The thinner copper layer make components lighter and lighter, while the lighter and thinner assembly makes the flexible component more suitable for smaller spaces. In the past, we used a roll-press process to adhere copper foil to an adhesive-coated medium. Today, copper foil can be produced directly on the media without the use of an adhesive. These techniques can be obtained with a few micron copper layers, giving the industry a fine line of 3 mils or even narrower widths. Removing adhesives from the FPCB enables it to have flame retardant properties.

This not only speeds up the UL authentication process, but also further reduces costs. When FPCB continue to devolve rapidly from the initial military industrial applications to civilian and consumer applications, it is even more important to obtain the UL certification.

7.4 Continue to Innovate

FPC should continue to innovate in four aspects in the future, mainly in the followings:

(1) Thickness

The thickness of FPC must be more flexible and be thinner.

(2) Folding resistance

The ability of bending is the inherent property of FPC. In the future, FPC must have stronger folding resistance, which must exceed 10,000 times. Of course, this requires better substrates.

(3) Price

At this stage, the price of FPC is much higher than that of PCB. If the price of FPC comes down, the market will certainly be much wider.

(4) Technological level

In order to meet various requirements, the FPC process must be upgraded, and the minimum aperture, minimum line width and line spacing must meet higher requirements.

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