FPC Basics

- Aug 09, 2016-

With the increasing production of flexible PCB and flex-rigid PCB than the application and promotion, is now more common to say plus flexible, rigid or rigid-flex PCB to say when it is layers of PCB. Typically, PCB with soft insulation material made of PCB called soft or flexible PCB, rigid-flex PCB said compound of flex-rigid PCB. It adapted to the needs of today's electronic products to high density and high reliability, small size, lightweight direction of development, but also to meet the strict economic and technical requirements and market competition.

In other countries, flexible PCB has been widely used in the early 1960s. Our country, in the sixties began production applications. In recent years, as the global economic integration and opening up the city to try to introduce technology to promote their usage continues to grow, some small and medium-rigid PCB factory aimed at the opportunity to make use of soft hardware technology, the use of existing tools and tooling equipment processes improvement, transformation of the production of soft PCB and flexible to adapt to the needs of the growing amount of PCB. To further understanding of PCB, flexible PCB technology here to make a study of the introduction.

First, the advantages and disadvantages of flexible PCB classification

1. Soft PCB classification

Soft PCB usually classified according to the following structure of layers and conductor:

1.1 single-sided flexible PCB

Soft-sided PCB, only one conductor surface may have a covering or not covering. Insulating base material used, with different applications and different products. Commonly used insulating materials are polyester, polyimide, Teflon, soft epoxy - such as glass cloth.

Single-sided flexible PCB can be further divided into the following four categories:

1) connected to one side uncovered

Such flexible PCB conductor pattern on the insulating substrate, the surface of the wire without a covering layer. Like conventional single-sided rigid PCB same. These products are the cheapest kind, usually in a non-key and have environmental protection applications. Its interconnection with soldering, welding or bonding to achieve. It is commonly used in the early telephone.

2) connected to one side with a covering layer

Such classes compared to the previous, just according to customer requirements in the surface of the wire multi-layer coating. When required to cover exposed pad, simply can not cover the end zone. You can use requires sophisticated forms clearance hole. It is a single-sided flexible PCB in most applications, the most widely used, widely used in automotive instruments, electronic instruments.

3) uncovered duplex connection

Such connections disk interface on the front and back conductors can be connected. To do this, open a hole in the insulating substrate via the pad at this via holes in the desired location on the first insulating substrate punching, etching or other mechanical methods is made. It is used on both sides of the installation element, devices and require soldering occasions, passage of the land area without the insulating substrate, such pad region typically removed by chemical methods.

4) with a covering layer of double-sided connection

Such differences with the former category is the surface layer of the cover layer. But the cover layer vias, also allows both sides can be terminated, and still maintain the cover layer. Such flexible PCB is made of two layers of insulating material and a layer of metal conductors. Need to be used in the cover layer and the insulating means around each other, but also insulated from each other and themselves, we need to end the positive and negative are connected to the occasion.

1.2 double-sided flexible PCB

Soft-sided PCB, has two conductors. Such double-sided flexible PCB applications and advantages of the single-sided flexible PCB same, its main advantage is to increase the wiring density per unit area. It may have no metal of the hole and have no covering layer is divided into: a metal-free holes, no cover layer; b metal-free holes with a covering layer; c metallized holes without a covering layer ; d metallized holes with a covering layer. Double-sided flexible PCB without overlay is seldom used.

1.3 Multilayer PCB soft

Multilayer PCB soft as rigid as a multilayer PCB, multi-layer lamination technology, can produce a multilayer flexible PCB. The easiest multi-layer flexible PCB is single-sided PCB on both sides covered with two layers of copper shield formed by three flexible PCB. This is equivalent to three soft PCB coaxial conductor or shield wire on the electrical characteristics. The most common multi-layer flexible PCB structure is four-layer structure with metallized holes to achieve the interconnection between layers, the middle floor is generally the power and ground planes.

Advantages Multilayer PCB soft base film is lightweight and has excellent electrical characteristics, such as low dielectric constant. Multi-layer PCB board flexible polyimide film to the substrate made of the weight ratio of the rigid epoxy glass cloth multilayer PCB board about 1/3 of light, but it lost its single-sided, double-sided flexible PCB fine the flexible, most of these products are not required to be flexible.

Multilayer PCB soft can be further divided into the following types:

1) constituting the flexible insulating substrate is a multilayer PCB, defined as the finished product can flex: This structure is usually put a lot of single or double flexible PCB microstrip both sides end bond together, but its center and the end of the bonded portion, which has a high degree of flexibility. In order to have the desired electrical characteristics, such as characteristic impedance and performance rigid PCB interconnect it matches, each layer of the multilayer flexible wiring PCB components, the design must be in ground plane signal lines. In order to have a high degree of flexibility, the layer may be a thin wire, a suitable coating, such as polyimide, laminated in place of a thicker coating layer. Metallized holes so that z surface layer between the flexible printed circuit can achieve the desired interconnection. This multi-layer flexible PCB best suited for applications requiring flexibility, high reliability and high-density designs.

2) constituting the multilayer PCB in the flexible insulating substrate, the end of which the finished product can be predetermined deflection: This is a multi-layer PCB soft flexible insulating material, such as a polyimide film, a laminate of plywood. After lamination of missing inherent flexibility. When the design requirements to maximize the use of the insulating film properties, such as low dielectric constant, the thickness of the homogeneous medium, light weight and other characteristics of continuous processing, the use of such soft PCB. For example, a polyimide film with an insulating material of a multilayer PCB with epoxy glass cloth weight is lighter than the third rigid PCB.

3) constituting the multilayer PCB in the flexible insulating base material, the finished product can be shaped, rather than continuously deflectable: These soft multi-layer PCB is formed of a soft insulating material. Although it uses flexible materials, but due to restrictions of the electrical design, such as required for the resistance of the conductor requires a thick conductor, or for the desired impedance or capacitance is required between the ground layer and a signal layer thick insulation isolation, therefore, at the time of the application it has finished forming. The term "forming" is defined as: the ability to multi-layer PCB soft member having a desired shape is formed, and in the application can not flex. Application avionics unit internal wiring. At this time, the strip line requires three-dimensional design of the conductor or low resistance, or capacitive coupling circuit noise and a small portion at the end can be smoothly interconnecting bent 90 °. Multilayer PCB soft polyimide film material implements this routing task. Because of high temperature polyimide film, there is flexibility, and good overall electrical and mechanical characteristics. To achieve this interconnection of all parts of the cross-section, wherein the connector area is further divided into a plurality multilayer flexible wiring member, and together with the adhesive strip to form a printed wiring bundle.

1.4 Rigid - flexible Multilayer PCB

This type is usually on a piece or two rigid PCB, soft PCB containing integral essential. PCB soft layers being laminated in the multilayer rigid PCB, which is to have special electrical requirements, or in order to extend outside the rigid circuit to circuit means connected dynasty Z plane capacity. Such products that compress the weight and volume as the key, and to ensure high reliability, high-density packaging of electronic equipment and excellent electrical characteristics has been widely used.

Rigid - flexible Multilayer PCB may be the end of many single or double sided adhesive soft PCB pressed together into a rigid part, the middle part is not bonded to become soft, Z surface of the rigid portion of the hole with a metal cross even. The flexible circuit can be laminated into the multilayer rigid. Such PCB increasingly used in applications requiring ultra-high packaging density, excellent electrical characteristics, high reliability and strict restrictions on the volume of applications.

Already we have a series of hybrid multilayer flexible PCB components designed for military avionics, and in these applications, weight and size are critical. In order to meet the required weight and size limits, internal packaging density must be very high. In addition to high-density circuit outside, in order to minimize noise and crosstalk, all signal transmission lines must be shielded. To use separate shielded wire is virtually impossible to economically packaged into the system. Thus, the use of a hybrid multilayer

Flexible PCB to achieve their interconnection. This component will shield signal lines included in the flat ribbon cable in flexible PCB, which in turn is a necessary part of the rigid PCB. In case of relatively high levels of operation, after fabrication, PCB form an S-shaped bend 90 °, thus providing a way to interconnect z plane, and in the x, y and z plane vibrational stress, in tin solder eliminate stress - strain.

Second, the advantages

2.1 flexible

Application of flexible PCB of a significant advantage is that it can make it easier to trace in three-dimensional space and even loaded, can be curled or folded use. Just curl in the radius of curvature of the allowable range, may be subjected to tens of thousands of times to use without damage.

2.2 reduce the volume

Even in the assembly is mounted, with the use of guide cables than conductor cross section flexible PCB Thin, flat, reducing the size of the wire, and forming along the chassis, making the device more compact structure, reasonable, and even reduced loading volume. Compared with the rigid PCB space savings of 60% to 90%.

2.3 weight

In the same volume, flexible PCB and wire cable than at the same carrying capacity, its weight can reduce about 70%, compared with the rigid PCB, weight reduction of about 90%.

2.4 installed even consistency

Even with soft PCB mounted, eliminating wiring errors wire cables. Just after the drawing process by proofreading, after all produced around the circuit are the same. Error does not occur when you install the cable connection.

2.5 increase the reliability

When connected to a flexible PCB mounted, since the X, Y, Z three planes wiring, reducing the interconnect adapter that increases the reliability of the whole system, and check for faults, provided for convenience.

2.6 Design of electrical parameters controllable

Depending on requirements, the designer during soft PCB design, control capacitors, inductors, characteristic impedance, attenuation and delay. It can be designed to have a characteristic transmission line. Because these parameters with the wire width, thickness, spacing, an insulating layer thickness, dielectric constant, loss tangent and so on, which is in the use of wire cable is difficult to do this.

2.7 overall soldering tip may

Soft PCB as rigid PCB as having terminal pads can eliminate wire stripping head and the lining of tin, thus saving costs. Terminal pads yuan, device, plug, dip soldering or wave soldering can be used in place of each of the wires by hand soldering.

2.8 use alternative materials

Flexible PCB according to different requirements, the choice of different substrate materials to manufacture. For example, in the low-cost requirements of the application device connected, you can use a polyester film. In demanding applications, we need to have excellent performance, the use of polyimide film.

2.9 Low Cost

Even with soft PCB mounted, make the overall cost is reduced. This is because:

1) Due to the consistency of soft PCB conductors of various parameters; implementation of the overall termination, eliminating the often happens even when installed cable wire errors and rework, and more convenient replacement of the flexible PCB.

2) application enables flexible PCB design simplified, it can be adhered directly to the member, and to reduce its fixed clamp member.

3) the need for shielded wire, with flexible PCB lower prices.

2.10 Continuity Processing

Since the soft foil covered board into a roll of continuous supply, so you can achieve continuous production of flexible PCB. Which will also help reduce costs.

Third, shortcomings

3.1 Disposable high initial cost

Because soft PCB for a particular application is the design, manufacture, so the beginning of the circuit design, wiring and photographic negatives higher the cost. Unless there are special applications that require flexible PCB, but usually a small application, it is best not used.

3.2 soft PCB difficult changes and fixes

Once the flexible PCB made to change must start from the base map or making light painting program, it is not easy to change. Its surface covered with a protective film to be removed before the repair, but also restored after repair, this is a more difficult task.

3.3 Size Restricted

In the case of the flexible PCB is not P, usually batch manufacturing process, and therefore limited by the size of the production equipment, can not do a very long, very wide.

3.4 improper operation easily damaged

Even loading personnel improper operation can lead to damage to the flexible circuit, the soldering and rework need trained personnel to operate.

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