Surface Modification of Epoxy Resin Based PCB Substrates using Argon and Oxygen Plasmas

Abstract

We report the effect of surface treatments of Ar and oxygen plasma on the adhesion quality of the metal-polymer interface. Here, we used three types of thin film metals of different thickness: copper (Cu) as the metal seed coat with thickness of 200 nm; and chromium (Cr) and aluminum (Al) as the film tie coats (thickness varied from 5-15 nm). These thin metal films were sputter-deposited on epoxy-based printed circuit board (PCB). Combination of factors such as the tie coat thickness, ion dose and beam type were evaluated statistically. The surface morphology of the substrates changed substantially with ion dose (beam current density times exposure time), and beam type. Bombardment with reactive Ar and oxygen ion beams resulted in the formation of flake-like particles on the polymer surface and evolved to surface cracks. This was attributed to the chemical reaction and surface erosion by chemical and physical effects of the reactive oxygen and Ar ions. Energy dispersive spectroscopy (EDS) and Fourier transform-infrared (FTIR) spectroscopy results showed that oxygen-treated substrates gave a higher relative percentage of oxygen on the surface compared to the Ar-treated counterpart. The amount of oxygen present in the substrate surface resulted to the formation of more stable metal oxide at the interface of the main metal coat and the polymer substrate, i.e. chromium oxide in between the substrate and Cu. This resulted to higher adhesion strength compared to when untreated substrate is directly deposited with Cu. Plasma treatment with tiecoat metal films of Cr resulted to an increase in adhesion strength to much as 40 times as compared to Cu directly deposited on substrate. Chromium and Al tiecoats also contributed to further improvement in the adhesion strength. Improvement in adhesion strength for the oxygen treated substrates ranged from 1.508 lb-force (90 s treatment time) to 3.983 lb-force (12 min treatment time) and 0.500 to 2.533 lb-force for the Ar treated substrates.