- Dec 18, 2017 -
What we should pay attention during electroplating on the surface of FPC
1. Flexible circuit board electroplating
Flexible printed circuit board FPC after the coating process exposed copper conductor surface may have adhesive or ink pollution, but also because of high-temperature process of oxidation, discoloration. In order to obtain a compact coating with good adhesion, it is necessary to clean the surface of the conductor by removing the contamination and oxidation layer on the surface of the conductor.
However, some of these contaminants are strongly bound to copper conductors and can not be completely removed with weak cleaning agents, so most of them are treated with alkaline abrasives and brushes with a certain strength.Most of the coating adhesives are epoxy resins , and the alkali resistance is poor , which leads to a decrease in the adhesive strength , although it is not visible , but in the FPC plating process , the plating solution may infiltrate from the edges of the cover layer , so that the cover layer can be peeled off .The phenomenon of soldering tin underneath the cover layer occurs during the final welding.
It can be said that the pretreatment cleaning process will have a significant impact on the basic characteristics of flexible PCB, and must be given full attention to the treatment conditions.
(2) FPC plating thickness of electroplating, metal plating deposition rate has a direct relationship with the electric field strength, electric field strength and in line with the shape, and the position relationship of the electrode, generally more fine wire width, terminal parts of the electrode tip, and the closer the electric field intensity is bigger. The parts of the coating was more thick.
In connection with the use of flexible printed boards, the existence of large differences in the width of many conductors in the same line is more likely to lead to uneven coating thickness, in order to prevent the occurrence of this situation. The distributary cathode figure can be attached around the circuit to absorb the uneven current distributed on the electroplating pattern and to ensure the thickness and uniformity of the coating on all parts to the maximum extent.It must be in the electrode structure and mechanism. Put forward 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, and for the general the anticorrosion of the terne plating thickness, the requirements are relatively relaxed.
There is no problem in the state of plating, especially the appearance of FPC electroplating. However, there are some phenomena such as stain, dirt and discoloration on the surface soon after. In particular, the factory inspection did not find anything unusual, but when the user received the inspection, found that there are external problems. This is due to inadequate drift, there is a residual plating bath on the surface of the coating. Caused by a slow chemical reaction over a period of time.
In particular to a flexible printed board , which is flexible and not very smooth , is easy to have various solutions to accumulate in a recess , and then is discolored in that part , so as to prevent the occurrence of such a situation not only sufficiently drifting , but also sufficient drying treatment , so that it is possible to confirm whether the drift is sufficient by the thermal aging test at high temperature .
2. Chemical plating of flexible circuit board
When the line conductor to be electroplated is isolated and can not be used as an electrode, only electroless plating can be carried out. The bath used in general electroless plating has a strong chemical effect. The electroless gold plating process is a typical example. The electroless gold bath is an alkaline aqueous solution with a very high pH value. Especially if the quality management of coating process is not strict and the bonding strength is low, this kind of problem will occur more easily.
Because of the characteristics of the plating bath, the electroless plating of displacement reaction is more likely to be drilled under the overlay, and it is difficult to obtain the ideal electroplating conditions by this process.
3.Flexible circuit board hot air leveling
The hot - air whole plain is a technology developed for coating lead - tin on the PCB of rigid printed board , because this technique is simple and convenient , it is also applied to FPC of flexible printed board .
Hot air leveling is the direct vertical immersion of the plate in the molten lead-tin bath, and the excess solder is blown away by hot air. This condition is very harsh for the flexible printed board FPC. If the flexible PCB FPC can not be immersed in the solder without any measures, the flexible PCB FPC must be clamped in the middle of the wire mesh made of titanium steel, and then immersed in the molten solder. Of course, the flexible printed board FPC surface cleaning and coating flux.
Because the hot air leveling process conditions are harsh and the phenomenon that the solder is drilled from the end part of the cover layer to the covering layer is easy to occur , especially when the bonding strength of the covering layer and the surface of the copper foil is low , the phenomenon is more frequent .