Basic knowledge of softboard is the basement for a person who works in the PCB industry. With the constant increase of the production of soft PCB and the application and promotion of the new flexible PCB, it is now common to say that PCB with the soft, rigid or rigid flexibility is a few layers of PCB. Usually, we call the PCB with soft insulation substrate soft PCB or flexible PCB. And the newly flexible PCB is flexible PCB. It has adapted to the needs of today’s electronic products for high density and high reliability, miniaturization and lightweight direction. And also meets the need of strict economic requirements and market and technical competition.
In foreign countries, they widely use soft PCB in the early 1960s. Our country only began to produce applications in the 1960s. In recent years, with the global economic integration and open city, and the introduction of technology to promote its use is constantly growing. Some small and medium-sized rigid PCB factory aimed at soft hard do technology on this opportunity, tooling and process to make use of existing equipment improvement, transformation and adaptive soft PCB production consumption growing needs. To further understand PCB, we will discuss the soft PCB technology in this paper.
1. Soft PCB classification and its advantages and disadvantages
1). Soft PCB classification
Soft PCB is usually classified according to the number of layers and the structure of the conductor:
1.1 single-sided soft PCB
Single surface soft PCB, only one layer of conductor, surface can have cover layer or no covering layer.The insulating base material used varies with the application of the product. The commonly used insulation materials are polyester, polyimide, polytetrafluoroethylene, soft epoxy – glass cloth.
We can further divide the single-sided soft PCB into four categories:
1) No covering surface connection. We cover this type of flexible PCB by a wire pattern on the insulating substrate. Same as usual single-sided rigid PCB. This type of product is the cheapest, and we often use it in non-critical and environmentally protected applications. Then we achieve the interconnection by soldering, welding or welding. Early telephones commonly use this technology.
2) it has a veneer connection. This type is based on the customer’s requirement to have a layer of overlay on the surface of the wire, compared to the previous class. When covering, the solder shall be exposed, and the simple can not be covered in the end area. We can use the requirement of precision in the form of clearance hole and widely use it in soft PCB. We widely use it in automobile instrument and electronic instrument.
3) there is no covering layer. We can connect the interface of this type of connection on the front and back of the wire. In order to achieve this, we should open a access hole on the insulating substrate at the solder plate. Which we can use punching, etching or other mechanical methods in the desired position of the insulating substrate. We usually use it on both sides of the installation element. The device and the occasion that requires soldering, the passable area of the passageway is not insulated base material, such welding dish area is usually removed by chemical method.
4) there are double-side connections. This type is different from the previous type and has a layer of overlay on the surface. But there are access holes in the covering layer. Which also allow both sides to be able to connect, and still maintain the covering layer. We make the soft PCB with two layers of insulation and a metal conductor. We use it to insulate the surrounding devices from the surrounding devices, and to insulate themselves from each other.
1.2 double-sided soft PCB
Double-sided soft PCB has two layers of conductor. The application and advantages of this type of two-sided soft PCB are the same as those of single-sided soft PCB. Whose main advantage is the increase of the density of the wiring of the unit area. We can divide it into: A) non-metallized hole and no covering layer. B) without metal holes and covered layers. C). Has metallized hole and no covering layer. D) has metallized holes and covered layers. There is less application of double-sided soft PCB without covering layer.
1.3 multi-layer soft PCB
We can make Soft multi-layer PCB, such as rigid multi-layer PCB, into multi-layer soft PCB. The simplest multi-layer soft PCB is a three-layer soft PCB that has two layers of copper shielding layer on both sides of the single-sided PCB. This three-layer soft PCB is equivalent to a coaxial conductor or shielded conductor on electrical characteristics. The most commonly used multi-layer soft PCB structure is the four-layer structure. And we can realize the interlayer interconnect with metallized hole. And the middle layer is usually the power layer and the ground layer.
The advantages of multi-layer soft PCB are light weight and excellent electrical characteristics, such as low dielectric constant. Made of polyimide film as the backing material multi-layer soft PCB, than rigid epoxy glass cloth light weight about one-third of the multilayer PCB. But it lost the single side, double side PCB excellent soft flexible, we require most of these products is flexible.
We can be further divide multi-layer soft PCB into the following types:
1) multilayer PCB flexible insulating substrate composition, regulation of the finished product is can flexure. This structure is usually a lot of one-sided or two-sided microstrip flexible PCB bonding together two sides of the end. But the central part and at the end of the bonded together. Which is highly flexible.In order to have the desired electrical characteristics. Such as the characteristic impedance performance and the rigid PCB that it interconnects, each line layer of the multi-layer soft PCB component must design the signal line on the ground surface.
In order to have a high degree of flexibility, we can coat the conductor layer with a thin, suitable coating, such as polyimide, instead of a thick layer of laminated covering. The metallized hole enables the interconnection of the z surface between the flexible line layers. We best suit this multi-layer soft PCB for designs requiring flexibility, high reliability and high density.
2) it is composed of multi-layer PCB on soft insulation substrate, and its final product rules can be scratched. We make this type of multi-layer soft PCB from soft insulating materials, such as polyimide films. Which we can be press into multiple laminates. It loses its inherent flexibility after laminating. We use this type of soft PCB when designing requirements to maximize the insulation properties of the film. Such as low dielectric constant, uniform thickness of thickness, light weight and continuous processing. For example, the multi-layer PCB made of polyimide film insulation is about a third lighter than the rigid PCB of epoxy glass.
3) a layer of PCB is formed on the base material of soft insulation. Its finished product must be able to be formed rather than continuous flexure. This kind of multi-layer soft PCB is made of soft insulating material. Though it is made of soft material, but because of limited by electrical design. Such as for the conductor resistance, request with thick conductors, or to the required impedance or capacitance, requires the signal between the ground plane and a thick layer of insulation isolation. Therefore, it has when it is applied in product shape. The term “formable” is defined as multi-layer soft PCB parts have the ability to make the desired shape, and can not flexion in the application.
Application in the internal wiring of avionics unit. At this point, the stripline or three-dimensional space design, low resistance, capacitance coupling or circuit noise minimum. And at the end of interconnection can smoothly bending at 90 °. Multi-layer soft PCB made of polyimide thin film material has been realized. Because polyimide film is high temperature resistant, flexible, and general electrical and mechanical properties are good. In order to achieve all the interconnections of this part of the part. The walking part is further divided into multiple multi-layer flexible circuit parts. And together with adhesive tape, a printed circuit beam is formed.
1.4 rigid – flexible multi-layer PCB. This type is usually on one or two rigid PCB containing the soft PCB that is essential to the whole. We laminate the soft PCB layer in the rigid layer of PCB. Which is for special electrical requirements or to extend to the rigid circuit. In order to install the ability of the Z plane circuit. We widely use these products in electronic devices that make compression weight and volume key and ensure high reliability, high density assembly and excellent electrical characteristics.
Rigid-soft can also be a lot of one-sided or two-sided multilayer PCB flexible PCB bonding pressed together into rigid parts. At the end of the middle glue not become soft, rigid part of Z with metallized hole interconnection. We press the flexible line layer into the rigid multilayer plate and increasingly use this type of PCB is in situations. Where high density, excellent electrical characteristics, high reliability, and restricted volume are required.
We have designed a series of multi-layer soft PCB components for use in military avionics. Where weight and volume are critical. In order to meet the specified weight and volume limit, the internal packing density must be extremely high. In addition to high circuit density, we must shield all signal transmission lines to minimize crosstalk and noise. If you want to use a shielded separation wire, it is virtually impossible to encapsulate it economically. In this way, we have to use the mixed layers.
Soft PCB to achieve interconnection. This component will shield the signal line from the soft PCB of the flat strip. Which is a necessary part of the rigid PCB. At higher levels of operating situation, after completion of manufacture, PCB form a 90 ° S shape curve. Which provides the z plane of interconnection approach, and in the x, y, and z plane vibration under the action of stress, can eliminate the stress and strain on the tin solder joints.
Second, the advantages
2 .1 can be flexible
One of the obvious advantages of applying soft PCB is that we can use more easily in three-dimensional space. Curl within the allowable radius of curvature and can withstand thousands to tens of thousands of times without damage.
2.2 decrease volume
Even in the component, with the use of wire rope, soft PCB conductor cross-section thin and flat, reduce the wire size, and along the casing forming, make equipment more compact and reasonable structure, reduced the volume. Compared with rigid PCB, we can save space can by 60-90%.
2.3 weight loss
In the same volume, the soft PCB is less than 70% of the weight of the cable, and the weight loss is about 90% when comparing with that of the rigid PCB.
2.4 consistency of assembly
With soft PCB, we can eliminate the error of wire cable connection. As long as we check and approve the processing drawings, all subsequent circuits are the same.There is no wrong connection when loading the connecting line.
2.5 increase reliability
When we use the soft PCB, it can reduce the interconnection, increase the reliability of the whole system, and provide convenience for the fault inspection.
2.6 electrical parameters design controllability
According to the requirements, the designer can control the capacitance, inductance, characteristic impedance, delay and attenuation during the design of soft PCB. We design it to have transmission line characteristics. Because these parameters are related to wire width, thickness, spacing, thickness of insulating layer, dielectric constant, and tangent of loss Angle, this is difficult to do when using wire cable.
The end can be soldered by the whole like rigid PCB, soft PCB has the terminal welding disk. Which can eliminate the stripping and tin of the conductor, thus saving the cost. We connect the terminal solder plate with the element, device and plug. We can use it to replace the manual soldering of each wire by the soldering or wave soldering.
2.8 materials are optional
We can make Soft PCB according to different usage requirements and different base materials. For example, we can use a polyester film in an application that requires low cost. In high demand applications, it is necessary to have excellent performance.
2.9 low cost. With soft PCB, we can reduce the total cost. This is because:
1) due to the consistency of various parameters of flexible PCB. It is convenient to replace the errors and rework that occur frequently in cable wiring and the replacement of soft PCB.
2) the application of soft PCB simplifies the design of the structure. Which we can attach it directly to the components, reducing the line clamps and its fixing parts.
3) for wires that need to be shielded, the price of soft PCB is lower.
2.10 continuous processing
As we can continuously supply the soft cladding plateied, the continuous production of soft PCB can be realized. It also helps to cut costs.
Three and disadvantages:
3.1 high initial cost.
Since we design and manufacture soft PCB for special applications, the starting circuit design, wiring, and photographic underprinting costs are high. Unless there is a special need to apply soft PCB, usually a small number of applications, it is best not to use.
3.2 it is difficult to change and repair the soft PCB.
Once made, we must change the soft PCB from the base drawing or the drawing process prepared. It is difficult to cover the surface of a protective film, remove it before repaired, repair it and then restore it.
3.3 dimensions are limited.
We usually manufacture the soft PCB by intermittent process in the case of not yet. So it is limit by the size of the production equipment and can not be done very long and wide.
3.4 improper operation is liable to damage.
Improper installation of personnel is liable to cause the damage of the soft circuit, the management should use trained person to do the soldering and rework process.
