(A) study on the new manufacturing process for flexible printed circuit board (PCB) and the effects of copper oxide on mechanical reliability of Cu/PI interface

Abstract

Flexible substrate has been paid attention to for its capability of mass production with low cost since it can be applied to reel to reel process. In general, flexible substrate is composed of polyimide/copper(Cu) double layers. Polyimide/Cu is known to have good electrical properties such as high conductivity, low dielectric constant. Conductor lines of $5 \mu m$ -pitch on polyimide in this study were successfully fabricated without non-uniform pattern shape by electroplating copper and coating polyimide on patterned stainless steel. $5 \mu m$ pitch line is impossible for conventional etching method and semi-additive method. This process seems to be more complex than conventional etching method and semi-additive method but these processes(etching method and semi-additive method) must be changed to other process suitable for fine line metallization. New method in this study has good advantages that it includes the comparable process with conventional PCB processes (lithography process, electroplating process, thermal lamination process) and conventional PCB materials (polyimide, dry film, stainless steel etc.). For multi-layer flexible substrate, via holes were drilled by UV laser and filled with electroplating copper and tin. And then, PI layers with conductor line and vias were laminated at once with careful alignment between layers. Solid state reaction between tin and copper during lamination formed the intermetallic compounds of $Cu_6Sn_5$ and $Cu_3Sn$ and achieved a complete multi-layer flexible substrate. There is tin cap layer between copper via and copper pad. This help to bond copper via to copper line because tin has so low melting point that solid state diffusion is possible at lower temperature. At this point, thickness of tin layer is critical factor to get good reliability. 2-layered Sn consist of $η-Cu_6Sn_5$ and remaining pure Sn were formed between Cu/Sn via and pad, and upper and lower sides were connected as well as the result under the c...