Effect of UV/ozone treatment on surface tension and adhesion in electronic packaging

Surface tension of material surfaces and interfaces is an important parameter that affects wetting and adhesion. Surface tension can be divided into three components: Lifshitz-van der Waals component, acid component, and base component. In this study, the three-liquid-probe method was used to investigate the surface tension and its three components of various surfaces of electronic packaging materials: benzocyclobutene (BCB) passivation, FR-4 board, polyimide board, and alumina board. When UV/ozone was employed to treat the surfaces, the surface tension increased, and the base component increased the most. The change in surface tension due to UV/O-3 treatment decayed with time after the treatment. The difference in surface tension between untreated and treated surface became smaller with the increase of time after UV/O-3 treatment. Different substrates showed different rate of decay in surface tension change. Among the surfaces studied, BCB passivation showed the fastest decay after treatment, while alumina showed the slowest decay. The contact angles of several liquid underfill materials on BCB passivation and their surface tension before and after curing were also measured. It was found that the wetting was not the controlling factor in adhesion of the system investigated.