Before the advent of PCBs, circuits were composed of point-to-point wiring. This method had low reliability because as circuits aged, ruptures in the wiring would lead to breaks or shorts at the line nodes. Wire-wrapping technology was a major advancement in circuit technology, and this method improved the durability as well as the replaceability of the wiring by wrapping small-diameter wires around the posts at the connection points.

As the electronics industry evolved from vacuum tubes and relays to silicon semiconductors and integrated circuits, the size and price of electronic components declined. Electronics were appearing more and more frequently in the consumer space, prompting manufacturers to look for smaller as well as more cost-effective solutions. Thus, the PCB was born.

PCBs are used in all electronic products, and the market direction of PCBs is almost a weathervane for the electronics industry. However, with the development of high-end, miniaturised electronics such as mobile phones, laptops and PDAs, there is an increasing demand for flexible PCBs (FPCs), and PCB manufacturers are accelerating the development of FPCs with thinner thicknesses, lighter weights and higher densities. 

A single-layer FPC

Has a layer of chemically etched out conductive graphics, conductive graphics layer on the surface of the flexible insulating substrate is calendered copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid fibre ester and polyvinyl chloride. Single-layer FPCs can be further divided into the following four subcategories: 

1. Single-sided connection without cover layer

Wire graphics on an insulating substrate, wire surface without a cover layer, the interconnection is achieved by soldering, fusion soldering or pressure welding, commonly used in the early telephone. 

2. Single-sided connection with covering layer

Compared with the former category, just in the wire surface more than a layer of cover. Covering the pads need to be exposed, simple can not cover the end area. Is a single-sided flexible PCB in the most widely used, one of the most widely used in automotive instruments, electronic instruments.

3. Double-sided connection without covering layer

Connection disc interface in the front and back of the wire can be connected, in the pad at the insulating substrate to open an access hole, the access hole in the insulating substrate can be punched, etched or made of other mechanical methods in the desired location. 

4. Covered double-sided connection

The former class is different, there is a layer of surface covering layer, covering layer with access holes, allowing both sides can be terminated, and still maintain the covering layer, made of two layers of insulating material and a layer of metal conductor.

Second, double-sided FPC

Double-sided FPC in the insulating base film on each side of a layer made of etched conductive graphics, increasing the density of wiring per unit area. Metallised holes in the insulating material on both sides of the graphic connection to form a conductive path to meet the flexural design and use of the function. The overlay film protects single and double-sided wires and indicates where components are to be placed. The metallisation holes and overlay are optional according to demand, and there are fewer FPC applications in this category. AIOT Big Data

Three, multi-layer FPC

Multi-layer FPC is 3 or more layers of single- or double-sided flexible circuits laminated together, through the drilling of solenoids, plating to form metallised holes, forming conductive paths between different layers. This eliminates the need for complex soldering processes. Multilayer circuits make a huge functional difference in terms of higher reliability, better thermal conductivity and easier assembly properties.

The advantages are the light weight of the substrate film and excellent electrical properties, such as a low dielectric constant. Made of polyimide film as the substrate for the multilayer flexible PCB board, than the weight of rigid epoxy glass cloth multilayer PCB board is about 1/3 lighter, but it lost the single-sided, double-sided flexible PCB excellent flexibility, most of these products are not required to be flexible. Multilayer FPC can be further divided into the following types: 

1. Flexible insulated substrate finished product

This category is manufactured on a flexible insulating substrate, the finished product is specified to be able to flex. This type of structure is usually highly flexible by bonding the ends of the two sides of a number of single- or double-sided microstrip flexible PCBs together, but the centre portion of them is not bonded at the end. In order to have a high degree of flexibility, the conductor layer can be a thin, suitable coating, such as polyimide, instead of a thicker layer of laminated cover layer.

2. Flexible insulating substrate finished products

This category is manufactured on a flexible insulating substrate, the end of the finished product can be flexed. This type of multilayer FPC is a soft insulating material, such as polyimide film, laminated into a multilayer board, in the laminate after the loss of the inherent flexibility.

FPC/PCB basic knowledge

1.Introduction to circuit boards

1.Flexible Printed Circuit Boards

Flexible Printed Circuit Board (FlexPrintCircuit, referred to as ‘FPC’), is the use of flexible substrate made of single, double or multi-layer line of printed circuit boards. It has a light, thin, short, small, high density, high stability, flexible structure, in addition to static bending, but also for dynamic bending, curling and folding.　

2. Rigid printed circuit board

Rigid printed circuit board (PrintedCircuieBoard, referred to as ‘PCB’), is not easy to deform the printed circuit board made of rigid substrate material.

Rigid substrate material made of printed circuit boards, in use in the spread state. It has high strength

It has the advantages of high strength, not easy to warp, chip components installed firmly.　　

3. Rigid-flex boards

Rigid-flexible combined board (RigidFlex), is composed of rigid-flexible and flexible substrate selective lamination together, the structure is close to metallised holes to form an electrical connection between the special flexible printed circuit board. It has the characteristics of high density, fine line, small aperture, small size, light weight, high reliability, and its performance is still stable under vibration, shock, and humid environment. It can be flexed and installed in three dimensions to effectively use the installation space, and is widely used in portable digital products such as mobile phones, digital cameras and digital video cameras. Rigid-flexible combination boards will be used more in the field of reducing packaging, especially in the consumer field.