Process development and adhesion behavior of electroless copper on liquid crystal polymer (LCP) for electronic packaging application

Liquid crystal polymer (LCP) has potentially a very wide application as substrate material in electronic packaging applications because of its unique advantages. The work in this paper was performed to realize the metallization of LCP for the purpose of board fabrication, and to study the adhesion between deposited copper and LCP. A homogenous electroless plated copper layer on LCP with 4 to 5 mum thickness was achieved, while it increased up to 40 mum with the subsequent electroplating. The timescale of etching, deposit ion rate, and pH value were gradually changing during the plating process and the influences on copper layer quality were investigated. The adhesion force of the copper-LCP layer system was measured by a shear-off-method. Scanning electron microscopy (SEM) was used to check the surface morphology after etching and the interface after shearing on both the backside of the copper layer and the LCP side. The relationship between the shear-off adhesion of copper and the time of chemical etching before plating was examined, and the optimal etching time is discussed. Heat treatment after plating was used, and it was shown that this significantly improved the adhesion strength. Three different acids were used as etchants. The adhesion was found to be different for different acids while the roughness as indicated by SEM was approximately the same. It is therefore proposed that mechanical interlocking is not the only contribution to the adhesion. All the samples have been subjected to 85 degreesC/85% RH conditioning for more than 1000 hours to investigate the influence of humidity on adhesion. Electroplating was used to get a thicker copper layer, but at the same time this process decreased the adhesion between copper and LCP. The microstructure study revealed that strong interlocking existed at the interface of electroless copper and LCP, while these links became weak after the electroplating. It is postulated that this is due to softening and smoothing of the LCP-Cu interface due to the localized current induced heating in the interface region during the electroplating.