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Flexible Printed Circuit Board (FPC) and Surface Plating

Flexible Printed Circuit Board (FPC) and Surface Plating
Issue Time:2018-09-18

Generally, a PCB made of a soft insulating substrate is called a flexible PCB, and a rigid-flex type PCB is called a rigid flexible PCB. It adapts to the needs of today's electronic products to the direction of high density and high reliability, miniaturization and light weight, and also meets the strict economic requirements and the need of market and technology competition.

Basic Introduce About FPC

flexible pcbFlex PCBs are usually classified as follows according to the number of layers and structure of the conductor:

1 Single-sided flexible PCB

A single-sided flexible PCB with only one layer of conductor and a surface with or without a cover. The insulating substrate material used will vary from application to product. Commonly used insulating materials are polyester, polyimide, polytetrafluoroethylene, soft epoxy-glass cloth.

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

* Uncovered one-sided connection
The wire pattern of such a flexible PCB is on an insulating substrate, and the surface of the wire has no cover layer, like a normal single-sided rigid PCB.

These products are the cheapest and are often used in non-critical and environmentally friendly applications. The interconnection is achieved by soldering, welding or pressure welding. It is commonly used in early telephones.

* Covered one-sided connection
Compared with the previous class, this type only has a layer of cover on the surface of the wire according to customer requirements. The pad needs to be exposed when covering, and it can be easily covered in the end area. If precision is required, it can be in the form of a clearance hole.
It is the most widely used and widely used single-sided flexible PCB, and is widely used in automotive instruments and electronic instruments.

* Double-sided connection without cover
This type of land interface can be connected to both the front and the back of the wire. To this end, a via hole is formed in the insulating substrate at the pad, which can be formed by etching, etching or other mechanical means at a desired position of the insulating substrate.
It is used for two-sided mounting elements, devices, and where soldering is required. There are no insulating substrates in the pad area at the vias. Such pad areas are typically removed chemically.

* Covered with double-sided connection
This type differs from the former class in that it has a layer of cover on the surface. However, the cover layer has via holes, which also allows both sides to be terminated and still maintain the cover layer.
This type of flexible PCB is made of two layers of insulating material and a layer of metal conductor. It is used when the cover layer and the surrounding devices are required to be insulated from each other, and they are insulated from each other, and the ends need to be connected to both the front and the back.

2 double-sided flexible PCB

Double-sided flexible PCB has 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. The main advantage is that the wiring density per unit area is increased.

It can be divided into: no metallized holes and no or no cover layer: a) no metallized holes, no cover layer; b) no metallized holes, with a cover layer; c) with metallized holes, no cover layer ;d) has metallized holes, with a cover layer.

3 Multilayer flexible PCB

Flexible multilayer PCBs, such as rigid multilayer PCBs, use multilayer lamination technology to create multilayer flexible PCBs.
The simplest multi-layer flexible PCB is a three-layer flexible PCB formed by coating two layers of copper shielding on both sides of a single-sided PCB. This three-layer flexible PCB is equivalent in electrical characteristics to a coaxial wire or a shielded wire.
 The most commonly used multilayer flexible PCB structure is a four-layer structure in which interlayer interconnections are realized by metallized holes, and the middle two layers are generally a power supply layer and a ground layer.
The advantage of a multilayer flexible PCB is that the substrate film is lightweight and has excellent electrical properties such as a low dielectric constant. A multilayer flexible PCB board made of polyimide film is about 1/3 lighter than a rigid epoxy glass multi-layer PCB board, but it loses its single-sided, double-sided flexible PCB. Flexibility, most of these products are not required for flexibility.

FPC plating & FPC electroless plating & FPC hot air leveling

FPC Plating

1 Pretreatment of FPC plating
The surface of the copper conductor exposed by the flexible printed board FPC after the coating process may be contaminated by adhesive or ink, and there may be oxidation and discoloration caused by the high temperature process. In order to obtain a tight adhesion with good adhesion, it is necessary to Contamination of the conductor surface and removal of the oxide layer clean the surface of the conductor. However, some of these pollutions are very strong in combination with copper conductors, and they cannot be completely removed with weak cleaning agents. Therefore, most of them are often treated with a certain amount of alkaline abrasive and brushing. The overlay adhesives are mostly rings. Oxygen resin and poor alkali resistance, which will lead to a decrease in bonding strength. Although it is not obvious, in the FPC plating process, the plating solution may infiltrate from the edge of the coating layer, and in severe cases, the coating layer may be peeled off. In the final welding, the phenomenon of soldering the drill to the underside of the cover layer occurs. It can be said that the pre-treatment cleaning process will have a major impact on the basic characteristics of the flexible printed board, and the processing conditions must be given full attention.

2 Thickness of FPC plating
When plating, the deposition rate of electroplated metal is directly related to the electric field strength. The electric field strength varies with the shape of the line pattern and the positional relationship of the electrodes. The thinner the line width of the wire, the terminal of the terminal part. The sharper the distance from the electrode, the greater the electric field strength and the thicker the coating at that location. In the applications related to flexible printed boards, many wire widths in the same line are extremely different. This makes it easier to produce uneven coating thickness. To prevent this, a shunt cathode pattern can be attached around the line. Absorbs the uneven current distributed on the plating pattern to maximize the uniform thickness of the coating on all parts. Therefore, it is necessary to work on the structure of the electrode. Here, a compromise scheme is proposed, which is strict with respect to the requirements of the uniformity of the thickness uniformity of the coating, and relatively relaxed standards for other parts, such as the lead-plated tin for fusion welding, the gold plating layer for metal wire bonding, etc. High, and for the general anti-corrosion of lead-plated tin, the coating thickness is relatively relaxed.

3 Smudges and dirt on FPC plating
There is no problem with the plating state just after plating, especially the appearance, but some surfaces will appear smudges, dirt, discoloration, etc., especially when the factory inspection does not find any abnormality, but when the user receives the inspection, Found that there are appearance problems. This is caused by insufficient drift, and a residual plating solution on the surface of the plating layer, which is caused by a chemical reaction slowly over a period of time. In particular, the flexible printed board is not very flat due to its softness, and the recess is prone to "accumulation" of various solutions, and then it will react and discolor at this portion. In order to prevent this from happening, not only sufficient drifting but also full drifting is required. It is also necessary to carry out sufficient drying treatment, and it is possible to confirm whether or not the drifting is sufficient by a heat aging test at a high temperature.

FPC electroless plating

When the line conductor to be plated is isolated and cannot be used as an electrode, only electroless plating can be performed. Generally, the plating solution used for electroless plating has a strong chemical action, and the electroless gold plating process is a typical example. The electroless gold plating solution is an alkaline aqueous solution having a very high pH. When such an electroplating process is used, it is easy to occur under the coating of the plating liquid, especially if the quality of the lamination process is not strict and the bonding strength is low, which is more likely to occur.

Electroless plating of the displacement reaction is more likely to occur under the coating layer due to the characteristics of the plating solution, and it is difficult to obtain ideal plating conditions by electroplating using this process.

FPC hot air leveling

The hot air plain is a technology developed for the coating of lead-tin on rigid printed circuit boards. Due to the simplicity of this technology, it is also applied to flexible printed board FPC. The hot air leveling is to immerse the board directly in the molten lead-slot, and the excess solder is blown off by hot air. This condition is very demanding for the flexible printed board FPC. If the flexible printed board FPC cannot be immersed in the solder without any measures, the flexible printed board FPC must be clamped to the middle of the screen made of titanium steel. Then, it is immersed in the molten solder. Of course, the surface of the flexible printed board FPC is also cleaned and coated with flux.

Due to the harsh hot air leveling process conditions, it is easy for the solder to be drilled from the end of the cover layer to the underside of the cover layer, especially when the bond strength between the cover layer and the copper foil surface is low, and this phenomenon is more likely to occur frequently. Since the polyimide film is easy to absorb moisture, when the hot air leveling process is used, the moisture absorbing moisture will cause foaming or even peeling of the cover layer due to rapid heat evaporation, so it is necessary to dry and prevent moisture before the FPC hot air leveling. management

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