The effect of roughness and pattern of the core material to adhesion in making build-up layers

In very fine-line printed circuit board applications, the problem of adhesion of the copper lines on the substrate material has been essential. When, in the future, the line width and spacing will be less than 30 μm, very thin layers for etching the initial tracks will be used. To get thin layers of metal, we used sputtering in seeding. There are several metals and metal combination possibilities for sputtering. In previous tests, we have noticed that sputtered chromium gives a very good adhesion to copper in epoxy reinforced core laminate when the roughness of the laminate is Ra 0.6 μm and Rz 6.0 μm. However, when the surface of the substrate is too shiny ( Ra 0.2 μm, Rz 2 μm), not even chromium gives proper adhesion. In order to get a good additive copper layer, it is recommended also to sputter a copper layer on the chromium and after that to grow, by an additive method, a thicker layer of copper on the laminate. In this paper we have studied the surface roughness of the substrate material and tested the adhesion strength of the metal layer on these surfaces. It has been shown that the surface must be even to be able to create fine lines, but if the surface is too shiny, an acceptable adhesion is not achieved