Knowledge of plating on FPC surface of flexible circuit board

Generally, the PCB made of soft insulating material is called soft PCB or flexible PCB, and rigid flex composite PCB is called rigid flexible PCB. It meets the needs of today's electronic products to high-density and high reliability, miniaturization and lightweight direction, and also meets the stringent economic requirements and the needs of market and technology competition.


1. flexible circuit board FPC plating


(1) the copper conductor surface pretreatment by coating coating process with flexible PCB FPC FPC plating may have adhesive or ink pollution, also will be generated due to high temperature oxidation and color change process, in order to obtain good adhesion force to close the conductor surface pollution and oxide removal, cleaning the surface of the conductor. But some of these pollution conductors and copper binding is very strong, with a weak cleaning agent can not completely removed, so most often used to have a certain strength of the alkaline abrasive and polishing brush is used for processing, coating adhesive are epoxy resin and alkali resistance is poor, which will cause the bond strength decreased, although not obvious visible, but in the FPC plating process, the plating solution will be from the covering the edge of infiltration, serious will overburden stripping. At the end of the welding, the phenomenon of solder people coming under the covering. It can be said that the pre process cleaning process will have a major impact on the basic characteristics of flexible printed circuit board F{C, and the processing conditions should be paid full attention to.


(2) FPC plating thickness of electroplating, metal plating deposition rate has a direct relationship with the electric field strength, and the change of position and relation of electric field intensity in line with the shape of electrodes, wires, the general linewidth is fine, terminal parts of the electrode tip, and the closer the electric field intensity is bigger. The parts of the coating is thick. With the use of flexible printed board, on the same line in many wire width vary greatly the presence of which is more prone to the thickness of the coating is not uniform, in order to prevent the occurrence of such a situation, in line with shunt around cathode absorption current in the electroplating graphics graphics, uneven distribution, maximum guarantee all parts of the uniform coating thickness. Therefore, it is necessary to work hard on the structure of the electrode. Proposed a compromise here, for the standard uniform coating thickness requirements of parts for other parts of the standard strictly, relatively relaxed, such as melting welding plating tin lead wires (welding), the gold layer standards must be high, but for general corrosion with terne plating thickness, the requirements are relatively relaxed.


(3) FPC coating electroplating stains, dirt plating just good, especially the appearance and not what problem, but soon after some surface stains, dirt, discoloration and other phenomena, especially the factory inspection did not find what is strange, but when the user receives a check, found that the appearance of problems. This is due to insufficient drift and the residual plating solution on the surface of the plating, which has been caused by chemical reaction over a period of time. Especially flexible printed board, because of the soft and not very smooth, and its concave place is easy to have various solutions "accumulation" And then change color at the site, in order to prevent the occurrence of this situation, not only to fully drift, but also for adequate drying treatment. A high temperature thermal aging test can be used to confirm whether the drift is sufficient.


2. flexible circuit board FPC electroless plating


Electroless plating is only possible when the line conductor to be plated is isolated and not used as an electrode. General electroless plating bath has a strong chemical action, chemical plating process is a typical example. Electroless gold plating is an alkaline water solution with a very high pH value. The use of such electroplating process, it is easy to produce plating liquid drilling under the cover layer, especially if the film laminating process quality management is lax, low adhesive strength, it is more likely to occur this problem.


The chemical plating of displacement reaction is more likely to lead to the phenomenon that the plating liquid is under the covering layer because of the characteristics of the bath.


3. flexible circuit board FPC hot air leveling


Hal was originally for rigid printed board PCB coated tin and developed technology, because the technology is simple, has also been applied to the flexible printed board FPC. Hot air leveling is put in the tin groove plate directly dipped in the molten, excess solder to blow air. The conditions on the flexible printed board FPC is very harsh, if the flexible printed board FPC do not take any measures to solder immersion, must put the FPC into the center of flexible printed boards made of titanium steel wire mesh, and then immersed in the molten solder, of course, should also be on the surface of flexible printed board FPC for cleaning and flux coating.


Because the hot air leveling harsh conditions is also prone to solder from the end cover into the layer beneath the cover of the phenomenon, especially the covering layer and the copper foil surface bonding strength is low, more susceptible to frequent occurrence of this phenomenon. Because the polyimide film is easy to absorb moisture, the hot air leveling, moisture absorption and moisture evaporation caused by sharp coating blistering or even peeling, so before FPC HASL, must be dried and moisture management.


 

(Source: internal information)

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