NEW DEVELOPED COPPER ELECTROPLATING PROCESS FOR BMV FILLING WITH THE SPECIAL FEATURE OF A VERY BRIGHT CU DEPOSITION ON FLEX AND FLEX-RIGID PCB APPLICATIONS

摘要

Against the background of increasing technical and commercial demands on PCBs in connection to electronics in automotive, consumer, industrial and especially in mobile devices, flex and rigid-flex applications are playing a central role. Depending on the purpose the flexible printed circuits (FPCs) have to be stable towards any kinds of deflection like bending, twisting or folding - in some cases just once (space saving assembly) in others repetitive (hinged display cable in laptops). The reliability of those parts is the sum of a suitable design, the right selection of material and a proper processing. In addition these factors are also influencing each other which does increase the complexity of this topic. For example the choice of the appropriate Cu foil has a substantial impact on the further processing as well as on costs. While electrodeposited (ED or ECD) Cu foils are cheaper, a higher performance of the resulting devices can be achieved using rolled and annealed (RA) or High Performance Rolled Annealed (HA) Cu foils. Compared to ED Cu foils the special crystal structure of RA and HA Cu foils features crystals of a big size providing a far higher copper elongation and flex ductility. This allows for tighter bending angles as well as a higher numbers of repetitions. Though, this special crystal structure is also known to cause a matt surface after the electrolytic Cu plating step, but FPC makers prefer a shiny surface presumably due to AOI and/or to have a flat copper surface in order to minimize the so called skin effect for high frequency applications. It is known that a rough surface is not beneficial for high frequency signal transmission because of the skin effect [1].