PCB News - Basic knowledge of soft board for pcb copy board

With the continuous increase in the output ratio of flexible PCBs and the application and promotion of rigid-flex PCBs, it is now more common to add soft, rigid or rigid-flex when talking about PCBs, and say that it is a PCB with several layers. Generally, a PCB made of a flexible insulating substrate is called a flexible PCB or a flexible PCB, and a rigid-flex composite PCB is called a rigid-flex PCB. It meets the needs of today's electronic products in the direction of high density and high reliability, miniaturization, and light weight. It also meets the strict economic requirements and the needs of market and technology competition.

In foreign countries, flexible PCB has been widely used in the early 1960s. In my country, production and application only began in the mid-1960s. In recent years, with the promotion of global economic integration and the opening of the market, the use of imported technologies has continued to increase. Some small and medium-sized rigid PCB factories aim at this opportunity to adopt soft and hard craftsmanship, and use existing equipment to improve tooling and tooling. The process has been improved to transform the production of flexible PCBs and adapt to the ever-increasing demand of flexible PCBs. In order to further understand the PCB, here is an exploratory introduction to the soft PCB process.

First, the classification of flexible PCB and its advantages and disadvantages

1. Flexible PCB classification
Flexible PCBs are usually classified as follows according to the number and structure of conductors:
1.1 Single-sided flexible PCB
Single-sided flexible PCB has only one layer of conductor, and the surface can have a covering layer or no covering layer. The insulating base material used varies with the application of the product. Commonly used insulating materials include polyester, polyimide, polytetrafluoroethylene, and soft epoxy-glass cloth.
Single-sided flexible PCBs can be further divided into the following four categories:
1) Single side connection without covering layer
The wire pattern of this kind of flexible PCB is on the insulating substrate, and the wire surface has no covering layer. Like the usual single-sided rigid PCB. This type of product is the cheapest one, usually used in non-critical and environmentally friendly applications. The interconnection is realized by soldering, welding or pressure welding. It is commonly used in early telephones.

2) One-sided connection with cover layer
Compared with the previous type, this type only has an extra layer of coating on the surface of the wire according to customer requirements. The pads need to be exposed when covering, and it can simply be left uncovered in the end area. If precision is required, the form of clearance hole can be adopted. It is the most widely used and widely used single-sided flexible PCB, and is widely used in automotive instruments and electronic instruments.

3) Double-sided connection without covering layer
This type of connection board interface can be connected on the front and back of the wire. In order to achieve this, a via hole is opened on the insulating substrate at the pad. This via hole can be punched, etched or made by other mechanical methods at the required position of the insulating substrate. It is used for two-sided mounting of components, devices and occasions where soldering is required. There is no insulating substrate in the pad area of the via. Such pad area is usually removed by chemical methods.

4) With cover layer connected on both sides
The difference between this type and the previous type is that there is a covering layer on the surface. However, the cover layer has via holes, which allows termination on both sides and still maintain the cover layer. This kind of flexible PCB is made of two layers of insulating materials and a layer of metal conductors. It is used in the occasions where the covering layer and the surrounding devices need to be insulated from each other, and the ends need to be connected to both the front and back sides.

1.2 Double-sided flexible PCB

Double-sided flexible PCB with two layers of conductors. The application and advantages of this type of double-sided flexible PCB are the same as those of a single-sided flexible PCB, and its main advantage is to increase the wiring density per unit area. It can be divided into with or without metallized holes and with or without covering layer: a without metallized holes, without covering layer; b without metallized holes, with covering layer; c with metallized holes, without covering layer ; D There are metallized holes and covering layers. The double-sided flexible PCB without covering layer is rarely used.

1.3 Multilayer flexible PCB

Flexible multilayer PCB, like rigid multilayer PCB, adopts multilayer lamination technology to make multilayer flexible PCB. The simplest multilayer flexible PCB is a three-layer flexible PCB formed by covering two copper shielding layers on both sides of a single-sided PCB. This three-layer flexible PCB is equivalent to a coaxial wire or a shielded wire in electrical characteristics. The most commonly used multilayer flexible PCB structure is a four-layer structure, which uses metallized holes to realize interlayer interconnection. The middle two layers are generally the power layer and the ground layer.

The advantage of multilayer flexible PCB is that the base film is light in weight and has excellent electrical properties, such as low dielectric constant. The multi-layer flexible PCB board made of polyimide film as the base material is about 1/3 lighter than the rigid epoxy glass cloth multi-layer PCB board, but it loses the excellent single-sided and double-sided flexible PCB. Most of these products do not require flexibility.
Multilayer flexible PCB can be further divided into the following types:

1) A multilayer PCB is formed on a flexible insulating substrate, and the finished product is specified to be flexible: this structure usually bonds the two sides of many single-sided or double-sided microstrip flexible PCBs together, but the center The parts are not glued together, so it has a high degree of flexibility. In order to have the desired electrical characteristics, such as the characteristic impedance performance and the rigid PCB to which it is interconnected, each circuit layer of the multilayer flexible PCB component must be designed with signal lines on the ground plane. In order to have a high degree of flexibility, a thin, suitable coating, such as polyimide, can be used on the wire layer instead of a thicker laminated cover layer. The metallized holes enable the z-planes between the flexible circuit layers to achieve the required interconnection. This multilayer flexible PCB is most suitable for designs that require flexibility, high reliability, and high density.

2) A multilayer PCB is formed on a flexible insulating substrate, and the finished product can be flexed: this kind of multilayer flexible PCB is laminated with a flexible insulating material, such as polyimide film, to make a multilayer board. The inherent flexibility is lost after lamination. This type of flexible PCB is used when the design requires maximum use of the insulating properties of the film, such as low dielectric constant, uniform thickness of the medium, lighter weight, and continuous processing. For example, a multilayer PCB made of polyimide film insulating material is about one-third lighter than a rigid PCB with epoxy glass cloth.

3) A multilayer PCB is formed on a flexible insulating substrate, and the finished product must be shapeable, not continuously flexible: this type of multilayer flexible PCB is made of soft insulating materials. Although it is made of soft materials, it is limited by electrical design. For example, for the required conductor resistance, a thicker conductor is required, or for the required impedance or capacitance, a thicker conductor is required between the signal layer and the ground layer. The insulation is isolated, so it is already formed in the finished application. The term "formable" is defined as: a multilayer flexible PCB component has the ability to be shaped into the required shape and cannot be flexed in the application. Used in the internal wiring of avionics units. At this time, it is required that the conductor resistance of the strip line or three-dimensional space design is low, the capacitive coupling or circuit noise is extremely small, and the interconnection end can be smoothly bent to 90°. Multi-layer flexible PCB made of polyimide film material achieves this wiring task. Because the polyimide film is resistant to high temperatures, flexible, and has good overall electrical and mechanical properties. In order to achieve all the interconnections of this component section, the wiring part can be further divided into a plurality of multilayer flexible circuit components, which are combined with adhesive tape to form a printed circuit bundle.

1.4 Rigid-flexible multilayer PCB

This type is usually on one or two rigid PCBs, and it contains the soft PCB that is necessary to form a whole. The flexible PCB layer is laminated in a rigid multi-layer PCB. This is to have special electrical requirements or to extend outside the rigid circuit to dynamize the Z-plane circuit mounting capability. This type of product has been widely used in electronic equipment that takes compression of weight and volume as the key, and must ensure high reliability, high density assembly and excellent electrical characteristics.

Rigid-flexible multi-layer PCBs can also bond and press the ends of many single-sided or double-sided flexible PCBs together to form a rigid part, while the middle is not bonded to form a soft part. The Z-side of the rigid part is interconnected with metallized holes. even. The flexible circuit can be laminated into the rigid multi-layer board. This type of PCB is increasingly used in applications that require ultra-high packaging density, excellent electrical characteristics, high reliability, and strict volume restrictions.

There have been a series of mixed multi-layer flexible PCB components designed for military avionics. In these applications, weight and volume are critical. In order to meet the specified weight and volume limits, the internal packaging density must be extremely high. In addition to the high circuit density, in order to minimize crosstalk and noise, all signal transmission lines must be shielded. If you want to use shielded separate wires, it is practically impossible to economically package them into the system. In this way, a mixed multi-layer

Flexible PCB to realize its interconnection. This component contains the shielded signal line in a flat stripline flexible PCB, which in turn is an essential part of a rigid PCB. In relatively high-level operating situations, after the manufacturing is completed, the PCB forms a 90° S-shaped bend, thereby providing a way for z-plane interconnection, and under the action of vibration stress in the x, y and z planes, it can be used in the solder joints. To eliminate stress-strain.

2. Advantages

2.1 Flexibility

A significant advantage of the application of flexible PCB is that it can be more easily routed and assembled in three-dimensional space, and it can also be crimped or folded for use. As long as it is curled within the allowable radius of curvature, it can withstand thousands to tens of thousands of times without being damaged.

2.2 Reduce the size

In the assembly and connection of components, the conductor cross-section of the flexible PCB is thin and flat compared with the use of conductive cables, which reduces the size of the conductor and can be formed along the casing, making the structure of the device more compact and reasonable, and reducing the size of the assembly. volume. Compared with rigid PCB, space can be saved by 60~90%.

2.3 Reduce weight

In the same volume, the flexible PCB can be reduced by about 70% compared with the wire and cable under the same current carrying capacity, and the weight can be reduced by about 90% compared with the rigid PCB.

2.4 Consistency of installation and connection

Use flexible PCB to install the connection, which eliminates the error when wiring with wires and cables. As long as the processing drawings are proofread and passed, all the winding circuits produced later will be the same. There will be no wrong connection when installing the cable.

2.5 Increased reliability

When flexible PCB is used for assembly and connection, because it can be wired on the three planes of X, Y, and Z, the switching interconnection is reduced, the reliability of the whole system is increased, and the inspection of faults is provided to provide convenience.

2.6 Design controllability of electrical parameters

According to the application requirements, the designer can control the capacitance, inductance, characteristic impedance, delay and attenuation when designing the flexible PCB. Can be designed to have the characteristics of a transmission line. Because these parameters are related to wire width, thickness, spacing, insulating layer thickness, dielectric constant, loss tangent, etc., it is difficult to do this when using wire cables.

2.7 The end can be soldered as a whole

Flexible PCB, like rigid PCB, has terminal pads, which can eliminate wire stripping and tinning, thereby saving costs. The terminal pads are connected to the components, devices, and plugs. Dip soldering or wave soldering can be used to replace the manual soldering of each wire.

2.8 Material use is optional

Flexible PCB can be manufactured by using different base materials according to different usage requirements. For example, polyester film can be used in assembly applications that require low cost. In demanding applications, excellent performance is required, and polyimide film can be used.

2.9 Low cost

With flexible PCB assembly, the total cost can be reduced. This is because:

1) Due to the consistency of the various parameters of the flexible PCB wires; the overall termination is implemented, which eliminates the errors and rework that often occur when the cable wires are installed and connected, and the replacement of the flexible PCB is more convenient.
2) The application of flexible PCB simplifies the structural design, it can be directly adhered to the component, reducing the clamp and its fixing parts.
3) For wires that need to be shielded, flexible PCBs are cheaper.

2.10 Continuity of processing

Since the flexible foil-clad laminate can be continuously supplied in rolls, the continuous production of flexible PCBs can be realized. This also helps reduce costs.

3. Disadvantages

3.1 High one-time initial cost

Because the flexible PCB is designed and manufactured for special applications, the initial circuit design, wiring and photographic masters require higher costs. Unless there is a special need to apply a flexible PCB, it is usually best not to use it for a small number of applications.

3.2 It is difficult to change and repair soft PCB

Once the flexible PCB is made, it must be changed from the base map or the light drawing program, so it is not easy to change. The surface is covered with a protective film, which must be removed before repair and restored after repair, which is a relatively difficult task.

3.3 Size is restricted

Flexible PCBs are usually manufactured by batch process when they are not yet common, so they cannot be made very long and very wide due to the limitation of the size of the production equipment.

3.4 Improper operation is easy to damage

Improper operation of the assembly and connection personnel can easily cause damage to the flexible circuit, and its soldering and rework need to be operated by trained personnel