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Knowledge of FPC surface plating on flexible circuit board

Sep 03, 2018

Generally, the PCB made of soft insulating substrate is called flexible PCB or flexible PCB, and the rigid flexible PCB is called rigid flexible PCB.

It meets the need of high density, high reliability, miniaturization and lightweight development of electronic products, and also meets the strict economic requirements and the need of market and technology competition.

 


Basic knowledge of FPC flexible circuit board


Soft PCB is generally classified according to the number of layers and structure of the conductor:


1.1 single-sided flexible PCB

Single-sided flexible PCB with only one layer of conductor, the surface may be covered or not covered.

 

The insulating base material used varies with the application of the product.

 


Commonly used insulation materials are polyester, polyimide, polytetrafluoroethylene, soft epoxy - glass cloth.

 


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


1) one side connection without covering layer

The conductor figure of this kind of flexible PCB is on the insulating substrate, and there is no covering layer on the wire surface.

 

Like the usual single-sided rigid PCB.

 


This kind of product is the cheapest kind, often be used at not crucial and have environmental protection application occasion.

 


The interconnection is realized by soldering, welding or pressure welding.

It was used in early telephones.

 

2) one side connected with covering layer


Compared with the previous type, this type only has an extra layer of coverage on the surface of the wire according to the customer's requirements.

 

The solder pan should be exposed when covering. Simply do not cover the end area.

 


Where precision is required, the form of clearance hole can be used.

 


It is the most widely used one - sided flexible PCB and widely used in automobile instrument and electronic instrument.

 


3) no overburden double joint


This kind of connection disc interface can be connected on the front and back of the wire.

 


To this end, a channel hole is opened on the insulating substrate at the welding disc, which can be made by punching, etching or other mechanical methods at the required location of the insulating substrate.

 

It is used for both sides of mounting elements, devices and occasions requiring soldering. There is no insulating base material in the weld pad area at the access point.

 


4) double-sided connection with covering layer


This class differs from the previous class in that it has a covering layer on the surface.

However, the overburden has access holes, and it is also allowed to be terminated on both sides, and the overburden remains.

 


This type of flexible PCB is made of two layers of insulation and one layer of metal conductor.

 


It is used where a covering layer is needed to be insulated from the surrounding devices, and the ends need to be insulated from each other, and both sides need to be connected.

 


1.2 double-sided flexible PCB

Double-sided flexible PCB with two conductors.

 


This kind of double-sided flexible PCB has the same application and advantages as single-sided flexible PCB.

 


It can be divided into a non-metallized hole and no covering layer by having, having and having no covering layer;

B no metallized holes and covered;

C with metallized holes and no covering;

D has metallized holes and a covering layer.

Double-sided flexible PCB without overburden is rarely used.

 


1.3 multi-layer flexible PCB

Soft multilayer PCB can be made into flexible multilayer PCB by using multi-layer pressing technology as rigid multilayer PCB.

 


The simplest multi-layer flexible PCB is a three-layer flexible PCB formed by two copper shielding layers on both sides of a single-sided PCB.

 


This three-layer flexible PCB is equivalent to coaxial or shielding conductors in electrical characteristics.

 


The most commonly used multilayer flexible PCB structure is the four-layer structure, which USES the metallization hole to realize interconnection between layers.

 


The advantage of multilayer flexible PCB is that the substrate film is light and has excellent electrical characteristics, such as low dielectric constant.

The multi-layer flexible PCB made of polyimide film is about 1/3 lighter than rigid epoxy glass cloth multi-layer PCB, but it loses the excellent flexibility of single-side and double-side flexible PCB. Most of such products do not require flexibility.

 


1. Flexible circuit board FPC plating


(1) flexible PCB FPC FPC electroplating before processing after cladding coating exposed surface of the copper conductor may have adhesive or ink pollution, also there will be generated due to the high temperature process of oxidation, discoloration, to obtain good tight adhesion coating must remove pollution and oxide layer on the surface of the conductor, the conductor surface clean.

But some of these pollution is combined with copper conductor is very strong, with weak detergent cannot completely remove, so that most often used with a certain intensity of alkaline abrasives and brush and used for processing, most layer adhesives is a kind of epoxy resin and alkali resistant performance is poor, so as to cause a decline in bonding strength, though not visible, but in FPC electroplating process, plating solution is likely from the edge of the cover, serious when overburden stripping.

During the final soldering, the solder drillers appear under the overburden.

It can be said that the pretreatment cleaning process will have a significant impact on the basic characteristics of flexible printing plate F{C.

 


(2) the thickness of the FPC electroplating electroplating, plating metal deposition rate is directly related to the intensity of electric field and electric field intensity and with the shape of the line graph, the position relationship between electrodes, the general wire line width of the thinner, terminal parts of the terminal is pointed, the greater the closer the distance of the electric field intensity and electrode, the parts of the coating is thick.

In with the flexible PCB related use, in the same line width difference many conductors in great conditions exist it's easier to produce coating thickness is not uniform, in order to prevent the happening of this kind of situation, can be in line attached diversion around the cathode graphics, absorb graphics on uneven current distribution in electroplating, coating thickness on the maximum guarantee all parts evenly.

So you have to work on the structure of the electrode.

Proposed a compromise here, for the part of the high standard strict coating thickness uniformity, for the other parts of the standard relatively relaxed, for example, the fusion welding lead, tin plating, metal wire by (welding) of gold plated layer standard is higher, and for general corrosion of lead, tin plating, the plating thickness required relatively relaxed.

 


(3) FPC electroplating stains and dirt have just been electroplated on the coating state, especially there is no problem with the appearance. However, some surface stains, dirt, discoloration and other phenomena appear soon after. In particular, no abnormal appearance is found in the factory inspection.

This is due to insufficient drift, coating surface on the residual liquid, after a period of slow chemical reaction caused.

In particular, flexible printing plate, due to soft and not very smooth, its concave is easy to have various solutions "accumulated?

In order to prevent the occurrence of this situation, not only sufficient drift, but also sufficient dry treatment should be carried out.

The high temperature thermal aging test can confirm whether the drift is sufficient.

 


2. Flexible circuit board FPC electroless plating

When the conductor of a line to be electroplated is isolated and cannot be used as an electrode, only chemical plating can be carried out.

Generally, the plating solution used for electroless plating has strong chemical effect.

The chemical plating solution is an alkaline solution with a very high pH.

This kind of electroplating process can easily occur under the coating layer, especially if the lamination process quality management is not strict and the bonding strength is low.

 


The chemical plating of replacement reaction is more likely to occur under the covering layer due to the characteristics of the plating solution.

 


3. FPC hot air leveling for flexible circuit board

Hot air plain is a technology developed to coat lead and tin on rigid PCB.

Hot air leveling is the direct vertical immersion of the plate into the molten lead tin tank, the excess solder with hot air.

This condition is too rigid for flexible PCB FPC, if the flexible PCB FPC do not take any measures cannot be immersed in the solder, must take flexible PCB FPC sandwiched between titanium and steel wire mesh, again into the molten solder, in advance, of course, also want to clean the surface of the flexible PCB FPC processing and coating flux.

 


Due to the harsh hot air leveling process, the phenomenon of solder drilling from the end of the overburden to below the overburden is also easy to occur, especially when the overburden and copper foil surface adhesion strength is low, which is more likely to occur frequently.

As polyimide film is easy to absorb moisture, the moisture absorbed by hot air leveling process will cause the overburden to bubble or even peel due to rapid evaporation. Therefore, dry treatment and moisture-proof management must be carried out before FPC hot air leveling.


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