Effect of adherend surface morphology on adhesive strength

Abstract

UV-curing acrylic adhesives characterized by relatively high elastic modulus at room temperature and high T g are prone to provide inadequate adhesive strength because of the stress concentration at the interface between the adhesive and the adherend. We used two types of metal adherend (Zn), one with a flat surface structure and the other with a sawtooth surface structure, to verify the effect of the surface morphology of the adherend on the initial adhesiveness and reliability [1]. The results showed that, by employing the sawtooth structure for the adherend's surface morphology, the initial adhesive strength increased significantly regardless of the adhesive type, and that the fracture mode shifted from the dominant interfacial failure mode to the mixed mode of interfacial failure accompanied with cohesive failure. On the other hand, the adhesive strength after reliability tests had different tendencies depending on the adhesive types. It was found that, for adhesives having a higher crosslink density, coupled with the effect of the sawtooth structure, the rate of deterioration accelerated to cause the adhesive strength to decrease significantly, and that the fracture mode transitioned only to the interfacial fracture mode over storage time under reliability test storage conditions of 60 o C/90%RH. Accordingly, to simultaneously achieve higher reliability and increased adhesive strength, it is essential to apply the sawtooth structure for the adherend's surface morphology and use adhesives having a lowered crosslink density. Keywords-flat surface; sawtooth surface; adhesive strength; adhesive reliability Ⅰ.IntroductionAs optoelectronic products have improved their performance over recent years, functional modules that mount optical components and functional devices with high positional accuracy have become more compact and dense. Accordingly, even higher temperature tolerance is required for the sections around heat-producing devices [1][2][3][4]. Hereafter, in order to achieve high positional accuracy in such high temperatures, UV-curing adhesives (abbreviated as "UV adhesives") must have physical properties that demonstrate, when hardened, high glass transition temperature (T g ) as well as high elastic modulus to suppress position aberration against any load. However, UV adhesives characterized by high T g and high elastic modulus are prone to provide inadequate adhesive strength because of the stress concentration at the adhesive interface [5]. For this reason, we focused on the effect of the adherend's surface morphology as a means to enhance the adhesive strength. While there are many reports on such attempts to enhance strength by making the adherend's surface rough with sandpaper or blast processing [6][7], few reports can be found that discuss reliability as well. As explained in this paper, we used adherends with two types of surface morphology, one with a flat surface structure and the other with a sawtooth surface structure, and compared their initial adhesive strength with that after undergoing re...