Chemical and Mechanical Analysis of PCB Surface Treated by Argon Plasma to Enhance Interfacial Adhesion

Plasma treatment of printed circuit board (PCB) is a common step in the electronic packaging processes in order to modify the surface and enhance its adhesion to molding compound. In this paper, PCB surface modifications resulting from plasma treatment were investigated by chemical and mechanical analysis methods. The PCB substrate in consideration was for multichip package, consisting of a core layer sandwiched by solder resist (SR) material, which in turn was composed of epoxy, acryl resins, and several kinds of fillers. Argon was employed as a plasma gas and the number of times that the plasma treatment was applied was chosen as a variable to estimate the effect of repeated exposure to the SR surface during the manufacturing process. The resulting surface conditions were analyzed using roughness measurement, water contact angle measurement, X-ray photoelectron spectroscopy (XPS), nano-indentation, and nano-scratch test method. Comprehensive analyses made it possible to understand the characteristics of the effect of plasma on the PCB surface, i.e., SR material. It was found that the surface roughness after the one-time plasma treatment increased from ~ 62 to ~ 90nm, then decreased more or less to the initial level after three and eight times of repeated treatments. These roughness trends were explained by cleaning out contaminants accumulated in the surface crevices and then preferentially removing or melting away a thin resin layer from the peaks on the surface. The plasma treatment decreased the contact angle significantly and increased work of adhesion. Chemical analysis of these surfaces by XPS showed that the C-C bonds in SR were broken, the population of the polar groups such as O-H, C=O, and COOH increased, and the oxygen content increased. The polar groups increased surface energy, and resulted in increased PCB adhesion with the epoxy molding compound (EMC). Mechanical analysis by nano-indentation and nano-scratch test showed that the surface became soft - - and weak if plasma treatment was too excessive. In this case, a plasma affected zone corresponding to the severe C-C bond breakage was formed near the surface and the adhesion with EMC suffered. Therefore, caution should be exercised in determining the degree of plasma treatment needed.