Revealing Interfacial Structures between Silica and Epoxy Adhesive via Spectroscopy and Morphological and Adhesion Tests

Abstract

Adhesives and glass are commonly used in electronic packaging. The macroscopic and microscopic (molecular-level) structures at the interface between glass and epoxy adhesives have attracted a great deal of interest in the field of surface analysis. In many previous studies, sum frequency generation vibration spectroscopy (SFG-VS) with a total internal reflection (TIR) silica prism was used to probe these types of interfaces. However, for adhesives with relatively high refractive indexes, the requirement of a TIR geometry is not matched, and the origin of SFG-VS signals from the interface involving the prism and adhesive is complicated. Many efforts have been made to resolve this issue. This study provides an approach to analyze the linear and nonlinear optical transmission and emission in a silica−epoxy adhesive−silica sandwich sample with different adhesive thicknesses (i.e., 1.5, 4.5, and 13.5 μm). The origin of SFG-VS signals and structural information in the adhesive were revealed from spectroscopy, morphological, and adhesion tests. It was observed that an insufficient adhesive thickness leads to void structures in the adhesive layer, generating a relatively high SFG-VS signal at the adhesive−air interface. However, samples with a filled adhesive layer emit much weaker SFG signals. Lap shear tests, energy-dispersive X-ray imaging, scanning electron microscopy, and SFG-VS with the introduction of liquids into the sample were used to confirm conclusions obtained from linear and nonlinear optical measurements. This study provides a straightforward experimental approach to analyze the relatively complicated interfaces involving polymers.