Novel Thin Copper Transfer Films for Fine Line Formation on PCB Substrates

Printed circuit boards (PCB) using Ajinomoto build-up films (ABF) as insulating layers are widely used as package substrates. Along with the increasing demand for downsizing electronic devices with advanced functions, package substrates are strongly required to be miniaturized with high-density circuit wirings.The manufacturing method for multilayer PCBs using ABF is a semi-additive process including the lamination and curing steps of ABF resin composition, laser via formation and then desmear process accompanied with roughening the ABF surface prior to electroless copper plating as a seed layer to electroplate a thicker copper layer. The high adhesion between the plated copper and ABF insulating layer depends mainly on an anchor effect; however a smooth surface is desired for fine line formation, for example, of less than L/S = 15/15 microns.Recently, we've developed very thin copper transfer films consisting of a supporter film, an alkaline-soluble resin layer, and a thin copper layer (150–1500 nm thick) deposited by vacuum evaporation or sputtering. In this paper, we report on the applications of very thin copper transfer films for both build-up PCB and core materials. With regard to build-up applications, the combination of a new ABF that has a low coefficient of thermal expansion and a high Young's modulus is demonstrated as a high-tech resin coated copper (RCC) film for fine line formation. Furthermore, the combination of very thin copper transfer film and ABF-prepregs, which are glass-cloth materials impregnated with ABF resin compositions, are proposed for use as core materials, especially for CSP substrates.These new applications can achieve the high adhesion between the copper and insulating layers without anchor effects and that mechanism is presumed from the analysis of its interface using XPS. In addition, reliability tests were performed under the stress conditions of reflow tests and HAST.