Cu/epoxy interfacial adhesion improvement by thiol-based self assembled structures of different chain lengths

This paper aims to study of copper/epoxy interfacial adhesion enhancement with the thiol-based self-assembled treatment introduced by our group. The focus of this paper is to investigate the effects of chain structures as well as chain length on adhesion enhancement. The motivation of this study is to improve the interfacial adhesion of copper/epoxy system and increase the reliability of the electronic packages. The rationale is to use chemical reactive functional groups to link the epoxy and copper leadframe together and with the hydrophobic chain structure it will reduce the moisture ingression into the interface. In order to study the effects of chain length on interfacial adhesion, as well as the impact on moisture reliability, six thiol-based materials were selected with different chain length. Two different chain structures are involved, SAM-A and SAM-B. They are all alky chain structure based which will exhibit the hydrophobic characteristic, and some of them are with minor modifications. To build covalent bond between copper and epoxy, thiol group and hydroxyl group are designed at each end of the chain structure to react with copper leadframe and epoxy resin respectively. Systematic study on the treated surface is carried out. SEM results show no obvious change in the surface morphology of the treated copper surface. This also eliminated the effect by changing surface roughness. X-ray photoelectron spectroscopy (XPS) is applied to proof covalent bonds have been built between the copper leadframe and epoxy underfill and to estimate the coating thickness. To evaluate the interfacial adhesion of the treated surface, copper jigs bonded with epoxy underfill are tested by the tapered double cantilever beams (TDCB) setup. Adhesion is calculated using the interfacial fracture toughness (G_IC). Our preliminary results demonstrate the optimum chain length for SAM-A and SAM-B are 6 and 15 carbons respectively, and adhesion will decrease with further increase- - in the chain length for both of them. G_IC reached a maximum of 187.1±3.3Jm^-2. Compared with the control sample, where G_IC=4.8±0.8Jm^-2, it has an improvement of about 38 fold. This study will help to establish the guidelines for further application of a chain structure adhesion promoter.