Molecular design of thiol based nano network for copper-epoxy interfacial adhesion

With small size, multifunctional and the requirement of high speed in consumer electronics, thermal dissipation becomes a major concern in package designs. Copper substrate, which is widely used as thermal and electrical material, is prone to delaminate when adhered to an epoxy compound. Electronic packages delaminate under critical stress. In solving this problem, thiol, which forms covalent bond specifically with copper, is used as a coupling agent. This thesis aims at providing a molecular design scheme of thiol molecule as adhesion promoter for the Cu-epoxy interface. The chemical structure of thiol candidates with 1) different end group (bond to epoxy) and 2) different chain structure (determines the surface density) are investigated with the help of Molecular Dynamic (MD) simulation, and their adhesion strengths are evaluated by button shear test. Aromatic amino thiol is finally chosen as the candidate in this study. While adsorbing the thiol onto the substrate, deposition procedures in terms of solution concentration and immersion time have been studied. The interfacial fracture strength of the treated sample is measured in fracture test with taper double cantilever beam (TDCB) specimens. More than thirty fold improvement in GIC is recorded with the specimen treated using 5mM solution for 10 hours. The fracture strength is effectively comparable to that of the bulk epoxy. To investigate the adhesion mechanism of the thiol coupling layer, the surface characteristics of the treated sample as well as that of the fracture surface have been studied. The surface composition, chemical bonds, morphology and surface density of the adsorbed thiol have been determined. The result shows that with insufficient thiol adsorption, improvement is limited. With the optimal surface density, the adhesion is enhanced twenty fold with formation of nano-fibril network. A further toughening of the interface is recorded with mixed mode failure which improves the fracture strength to thirty fold. The possible adhesion mechanism is extensive plastic deformation occurs with simultaneous debonding and epoxy yielding during delamination of the interface