In other countries, has been widely used soft PCB in the early sixties. Country, then in the sixties began production applications. In recent years, as the global economic integration and open the city to try to introduce technology to promote their usage constantly growing, some small and medium-rigid PCB Factory targeting this opportunity to make use of soft hardware technology, the use of existing tools and tooling equipment technology for improvement, restructuring production of soft PCB and flexible PCB dosage adaptation growing needs. To further understanding of PCB, PCB here for flexible process for an exploratory introduction.
With the increasing ratio of soft PCB production and rigid-flex PCB application and promotion, is now more common to say plus flexible, rigid or rigid-flex PCB to say when it is layers of the PCB. Typically, PCB made with soft insulation material called soft flex PCB or PCB, Rigid-flex PCB, said composite of rigid-flex PCB. It is adapted to the needs of today's electronic products to high-density and high reliability, miniaturization, lightweight direction of development, but also to meet the stringent requirements of the economy and market and technology competition.
The advantages and disadvantages of flexible PCB classification
1. Flexible PCB classification
Flexible PCB usually classified according to the following conductor layers and structures:
1.1 soft-sided PCB
Soft-sided PCB, only the conductor layer, the surface layer or covering can not cover. The insulating base material used, different applications with different products. Commonly used insulating materials are polyester, polyimide, polytetrafluoroethylene, soft epoxy - glass cloth.
Soft-sided PCB can be further divided into the following four categories:
1) No cover layer single-sided connection
Such flexible PCB conductor pattern on the insulating substrate, the surface of the wire without coating. As often as single-sided rigid PCB. These products are the cheapest one, usually in a non-vital and have environmental applications. Its interconnection with soldering, welding or bonding to achieve. It is commonly used in the early telephone.
2) connected to one side of the covering layer
Compared to the former category, and such, but according to customer requirements in multi-layer covering the surface of the wire layer. When the pad cover to be exposed, can be simple in the end regions is not covered. Requires the use of sophisticated clearance holes can form. It is a soft-sided PCB up to the application, the most widely used, widely used in automotive instruments, electronic instruments.
3) No cover layer double-sided connection
Such connections disk interface on the front and back of the wire can be connected. To do this, a via hole opening on the pads of the insulating substrate, the via hole may be in a desired position 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 path of the pad area without the insulating substrate, such land area is usually removed by chemical methods.
4) connected to the covering layer of double-sided
Such difference is that the former class of the surface layer of the coating layer. But covering layer vias, but also allows it to both sides can be terminated, and still maintain cover. Such PCB is made of two layers of flexible insulating material and a metallic conductor. Needs to be used in the cover layer and the insulating means surrounding each other and also their mutual insulation, and requires a positive terminal and negative connections are occasions.
1.2 soft-sided PCB
Soft-sided PCB, there are two conductors. Such soft-sided PCB applications and soft-sided PCB with the same advantages, its main advantage is increased wiring density per unit area. It may be there, and there are no metal holes without covering layer is divided into: a non-metallized holes, no cover layer; b non-metal hole, covering layer; c metallized holes, no cover layer ; d metallized holes with a covering layer. Soft-sided PCB without overlay rarely used.
1.3 Multilayer Flexible PCB
Multilayer PCB as soft as rigid multilayer PCB, multilayer lamination technology, can be made flexible multilayer PCB. The simplest multilayer soft-sided PCB on both sides of the PCB is covered with two layers of copper shield and the formation of three soft PCB. This three-layer flexible PCB or equivalent coaxial conductor shielded wire on the electrical characteristics. The most common structure is a four-layer PCB multilayer flexible structure, with inter-layer metal interconnection holes to achieve the intermediate floor is generally the power source layer and the ground layer.
Advantages of the multi-layer flexible PCB substrate film is lightweight, and has excellent electrical characteristics, such as low dielectric constant. Multilayer flexible PCB board as a base film made of polyimide, the weight ratio of the rigid multi-layer epoxy glass board PCB light about 1/3, but it lost sided, double-sided flexible PCB excellent The flexible, most of these products are not required can be flexible.
Multilayer Flexible PCB can be further divided into the following types:
A) a flexible insulating substrate constituting the multilayer PCB, which can be defined as deflection of the finished product: This structure is generally the many-sided or double-sided flexible microstrip PCB bonded to both surfaces of the end, but its center and bonding together the end portions, so as to have a high degree of flexibility. In order to have the desired electrical characteristics, such as the characteristic impedance of the interconnect performance and rigidity that it matches the PCB, each layer of the multilayer flexible wiring member of the PCB, the ground surface must be designed to 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 instead of a thicker coating layer. Z plane so that the hole metallization layer between the flexible printed circuit to achieve the desired interconnections. This is most suitable for multi-layer flexible PCB required flexibility, high reliability and high density designs.
2) constituting the multilayer dielectric substrate to the flexible PCB, the end of the finished product can be predetermined deflection: These soft multi-layer flexible PCB is an insulating material, such as polyimide film, a laminate of plywood. After laminating the loss inherent flexibility. When the design requirements to maximize the insulating properties of the film, such as low dielectric constant, the thickness of the homogeneous medium, lighter weight and other characteristics of continuous processing, the use of such a flexible PCB. For example, a polyimide film with a multilayer PCB insulating material around lighter than a rigid epoxy glass cloth of the third PCB.
3) is formed on a multilayer PCB flexible insulation material, the finished product can be formed instead of the continuous flexing: These soft multi-layer flexible PCB is made of insulating material. Although it uses a soft material, but due to restrictions of the electrical design, as for the required resistance of the conductor requires a thick conductor, or for the desired impedance or capacitance between the signal requires thick layer and the ground layer The insulating spacer, therefore, it has been applied in the finished molding. The term "forming" is defined as: Multi-layer flexible member having a PCB made of the ability of the required shape and can not flex in the application. Avionics unit in the internal wiring. In this case, requiring the design or 3D stripline conductor resistance is low, the capacitive coupling or interconnection circuit noise and small end portion can be smoothly curved in a 90 °. PCB multilayer flexible polyimide film of this material to achieve the routing task. Because high temperature polyimide film has flexibility, and a good overall electrical and mechanical properties. To achieve this section all the interconnecting member, wherein the alignment portion can be further divided into a plurality of multilayered flexible circuit member, together with adhesive tape to form a printed circuit beam.
1.4 Rigid - flexible multilayer PCB
This type is usually on a block or two rigid PCB, flexible PCB containing essential integral. Flexible PCB of the rigid layer is laminated on the multilayer PCB, which is to have special electrical requirements or in order to extend to the outside of the rigid circuit to circuit means connected dynasty capacity Z plane. Such that the compressed product in the weight and volume as the key, and to ensure high reliability, high density packaging of electronic equipment and excellent in electrical characteristics has been widely used.
Rigid - flexible multilayer PCB also be the end of the many-sided or double-sided adhesive flexible PCB pressed together to form a rigid part, the middle part is not bonded to become soft, Z surface of the rigid portion of the hole with metal interaction even. The flexible lines can be laminated to the rigid plywood. Such PCB increasingly used in applications requiring ultra-high packing density, excellent electrical properties, high reliability and strict restrictions on the volume of applications.
There have been a series of hybrid multilayer flexible PCB components designed for military avionics, and in these applications, the weight and volume are critical. In order to comply with the limits of weight and volume, have a high density inside the package. In addition to high circuit density outside, in order to make minimal crosstalk and noise, all the signal transmission lines must be shielded. To use separate shielded wire, then it is virtually impossible to economically packaged into the system. Thus, the use of a hybrid multilayer flexible PCB to achieve their interconnections. This part will be shielded signal lines included in the flat ribbon cable Flexible PCB, whereas the latter is an essential component of the rigid PCB's. In operations where relatively high levels, after fabrication, PCB to form an S-shaped bend of 90 °, thus providing a way to interconnect z plane, and in the x, y and z-plane vibrational stress, in a tin solder eliminating the stress - strain.
2. Advantage
2.1 flexible
Application of a soft PCB significant advantage is that it can more easily trace in three-dimensional space and equipment company, can also be curled or folded use. Just curl in the radius of curvature of the allowable range, and can withstand a few thousand to tens of thousands of times to use without damage.
2.2 reduce the volume
Even in the assembly in the same ratio using the guide wire, soft PCB conductor cross section of thin, flat, reducing the wire size, and shape along the chassis, making the device more compact structure, reasonable, even reduced loading volume. Compared with the rigid PCB space savings of 60 to 90%.
2.3 reduce weight
In the same volume, flexible PCB and the cable conductor than in the same ampacity, which reduce the weight of about 70% compared with the rigid PCB, weight reduction of approximately 90%.
2.4 installed even consistency
Even with a soft PCB assembly, eliminating wiring errors with wire cables. After a long working drawings proofread by all future production out of the circuit are the same around. Error does not occur when you install the cable connection.
2.5 increase the reliability
When using the device connected to the flexible PCB, since the X, Y, Z three plane wiring interconnection reducing adapter, increase the reliability of the entire system, and to check for faults, provided for convenience.
2.6 Design controllable electrical parameters
Depending on requirements, the designer during the design of soft PCB can be controlled capacitance, inductance, characteristic impedance, delay and attenuation. Can be designed to have a characteristic transmission line. Because these parameters and conductor width, thickness and spacing, the thickness of the insulating layer, the dielectric constant, dielectric loss tangent, etc., and when it is difficult to do using a wire cable.
2.7 overall soldering tip may
Rigid PCB Flexible PCB like the same, with the terminal pads can eliminate wire stripping head and the lining of tin, thus saving costs. Terminal pad element, device, plug connectors, solder dip or wave soldering can be used to replace each wire by hand soldering.
2.8 Material use alternative
Flexible PCB according to different requirements, use of different substrate materials to manufacture. For example, in a low-cost device connected to the application requirements, you can use polyester film. In demanding applications required to have excellent properties, polyimide film can be used.
2.9 Low Cost
Even with a soft PCB assembly, make the total cost decreased. This is because:
1) Due to the consistency of soft PCB wires of various parameters; implementation of the overall termination, eliminating the often happens even when the cable wires installed errors and rework, and more convenient to replace the flexible PCB.
2) Flexible PCB design simplifies application so that it can be adhered directly to the member, to reduce its clip and fasteners.
3) The need for shielded wire, with flexible PCB lower prices.
2.10 Continuity processing
As the soft foil covered board into a roll of continuous supply, thus enabling continuous production of soft PCB's. It also helps to reduce costs.
3. Shortcoming
3.1 one-time initial cost is high
Due to the special application of flexible PCB is designed, manufactured, so the start of the circuit design, layout and photographic negatives higher the cost. Unless there is a special need to apply soft PCB, but when the application is usually a small amount, it is best not used.
3.2 Flexible PCB difficult changes and fixes
Once the PCB is made of soft, you want to change must start from the base map or the preparation of light painting program, it is not easy to change. Its surface is covered with a protective film to be removed before the repair, but also restored after repair, it is more difficult to work.
3.3 size restricted
In the case of the flexible PCB is not ordinary, usually with a batch manufacturing process, and therefore the size of the production equipment is limited, you can not do very long, very wide.
3.4 are easily damaged by improper operation
The improper installation personnel to operate even cause damage to soft circuits, soldering and rework their needs trained personnel to operate.
