Study on surface tension and adhesion in electronic packaging

Abstract

Different materials are used in electronic packaging, and thus there are many interfaces in an electronic package, and the adhesion at these interfaces is critical to the reliability of the assembly. Surface tension and its three components (Lifshitz-van der Waals component, acid component, and base component) are important parameters that affect the adhesion. In order to improve the wetting and thus possibly adhesion of underfill material with substrate and passivation of flip-chip device, some techniques can be employed to modify the surfaces. It is necessary to understand the surface behavior before and after treatment, and its effect on adhesion. In this study, the three-liquid-probe method was used to investigate the surface tension and its three components of various surfaces: benzocyclobutene (BCB) passivation, FR-4 board, polyimide board, and alumina board. The surface tension was changed with treatment by UV/ozone, which was indicated by the contact angle measurement. The base component increased most after UV/O/sub 3/ treatment. It was also shown that the change in contact angle decayed as time elapsed after treatment. Different substrates showed different decay rate. Among the surfaces studied, BCB passivation showed the fastest decay rate after treatment; while, alumina showed the slowest decay rate. The contact angles of several liquid underfill materials on BCB passivation and their surface tension before and after curing were also measured; the wetting was not the controlling factor in adhesion of the system investigated.