Epoxy adhesion strength to ceramic surfaces in commercial optoelectronic assemblies

Optoelectronics manufacturers are under continuous pressure for miniaturisation of optoelectronic modules. One route to further miniaturisation is to reduce the spacing between the optical and optoelectronic components in the optical path adhesively mounted to ceramic carriers. Flow control of the adhesives over the ceramic surface is then imperative. Uncontrolled wetting can lead to an excessive adhesive footprint which interferes in the application of other adhesives for subsequent components. However, insufficient wetting can lead to low strength bonds vulnerable to thermal fatigue and shear failure. This investigation focuses on determining the factors controlling the wetting of adhesives to ceramic surfaces, with the goal of minimising the potential for uncontrolled wetting while maintaining resistance to thermal fatigue and shear forces. In addition positional stability of adhered parts on cure and in-service must not be detrimentally affected. The first step in the investigation was to characterise the surface properties of alumina and aluminium nitride ceramic plates variously processed by commercial suppliers. The surface conditions included lapped, polished, etched and as-fired. The lubricants and abrasives used by the supplier companies for the mechanical surface treatments were not specified. Initial characterisation was performed by XPS, contact angle, SEM and surface texture analysis, amongst others. Commercially available conductive and thermally conductive adhesives were applied to the ceramics and their wetting behaviour linked to the surface properties observed. Finally, single lap joints were prepared with the ceramics and adhesives and were shear tested, and the shear failure modes identified. In this work the surface property measurements, wetting observations and shear failure modes are compared and discussed.