Direct Electroless Copper Plating on Polyimide for FPCB Applications

Abstract

As electronic devices are getting smaller and lighter, the density of copper lines on flexible printed circuit board (FPCB) is getting higher. Conventionally, subtractive method was used for copper line on a flexible films, however, as the line pitch is getting smaller, the lateral etching of copper cause serious problem. To replace the subtractive method, semi-additive method was used for fine pitch copper line fabrication. In semi additive process, sputtered layer for the electroplating copper was required. The feasibility of electroless plating to replace high cost sputtered copper seed layer was investigated. Electroless depositions of copper were conducted on different substrate to find optimum conditions of electroless copper plating. To find optimum conditions, the effects and selectivity of activation method on several substrates were also investigated. The adhesion strength between polyimide and copper was improved by treating the polyimide surface with butylamines. Pretreatment prior to electroless plating is very sensitive and surface dependent. Surface morphologies were investigated with FESEM.