Flexible Printed Circuit(FPC) Surface Finishing Technology
1.Flex Printed Circuit(FPC) Plating
It may have adhesive or ink pollution on exposed copper conductor surface after FPC coating process when FPC plating pre-processing. There will be oxidation,discoloration due to high temperature process.we must remove the contamination and oxide layer of conductor surface in order to obtain a good adhesion of close coating,so that the conductor surface clean.But some of these pollution and copper conductor is very strong combination,it can not be completely removed with a weak cleaning agent,so we need use a certain strength of alkaline abrasive and throw brush to dispose.Covering adhesives are mostly epoxy resin and alkali resistance is poor,this will result in a decrease in bond strength.Although not obvious,the plating solution may penetrate from the edge of the cover,severe peeling of the cover.the solder drill into the coating below phenomenon.It can be said that the pre-treatment cleaning process will have a significant impact on the basic characteristics of the flexible printed circuit board FPC.Must pay full attention to the processing conditions.
When FPC thickness plating,the deposition rate of electroplated metal is directly related to the strength of electric field,the strength of the electric field follows the shape of the line pattern,electrode positional to change, In general the wire width is more thinner,the terminal of the terminals is more sharper,the closer distance to the electrode is,the electric field strength is more strong,the plating thickness is bigger.In use of flexible printed boards,there are different widths of the wires in the same FPC lead to the coating thickness is non-homogeneity. In order to prevent it happening,can attach a shunt cathode pattern around the line,absorbs uneven current distributed on the plating pattern. Great guarantee the plating homogeneity of whole areas. So it must work hard in the structure of the electrode. Here is a compromise, strict control for the high requirement of plating, and relax control for the standard requirement.
FPC Plating stains, dirt just plated with a good plating state,Especially the appearance is no problem, But soon some surface stains, dirt,discoloration and other phenomena.Especially did not find any defect when factory inspection,found the appearance problem when user received and make inspection.This is due to insufficient rafting,there is residual plating on the surface of the coating,after a period of time slowly caused by chemical reactions.Especially the flexible printed circuit board, due to the soft and not very flat, it easy have a variety of solutions accumulate in concave areas. And then occur reaction and discoloration in that areas. It need fully drift in order to prevent this situation happening,but also to be fully dried.It can use high temperature thermal aging test to confirm whether it’s drifting sufficient.
2.Flex Printed Circuit(FPC) Electroless
When the line conductor need be electroplated is isolated and can not be used as an electrode,it can only be electroless plating.General chemical plating used in the bath have a strong chemical effect,chemical gold plating process is a typical example.Electroless gold plating is a very high alkaline aqueous solution.use this plating process, it’s easy occur plating solution flow into under coating layer,especially if the quality of the coating process didn’t be controlled well,bonding strength is low,it’s more prone to happen this problem.
The electroless plating of the replacement reaction is due to the characteristics of plating solution.It’s easier to drill into under coating layer. Electroplating with this process is difficult to get the ideal plating conditions
3.Flex printed circuit(FPC) Hot Air Leveling
Hot air leveling was originally developed for rigid printed circuit board PCB coating and developed technology,it also applied in flexible printed circuit board FPC surface finishing as this technology is simple.Hot air leveling is direct immerse whole panel into the molten lead tin bath and blow the excess soldering.This condition is very harsh for flexible PCB.If the flexible printed circuit board does not take any protection can not immerse in the soldering.Prior cleaning and coating flux flexible printed circuit board,fold the flexible printed circuit board into titanium steel wire mesh and then immerse in molten soldering.Due to the hot air leveling process have same risk that solder drill under coating layer.Especially when the adhesive strength is low between cover layer and copper foil, it’s easier to happen this phenomenon frequently.As the polyimide film is easy absorb moisture.the moisture absorption water will evaporate as rapid heat lead to cover layer bubble and peeling.So it must be dry and moisture-proof management before FPC hot air leveling.
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