Committee Members:
Prof. C. P. Wong, Advisor, MSE
Prof. Meilin Liu, MSE
Prof. Zhiqun Lin, MSE
Prof. Seung Soon Jang, MSE
Prof. Vanessa Smet, ME
Transition Metal Complexes as Latent Catalyst and Adhesion Promoter in Epoxy Resin
Abstract:
Epoxy based materials are widely used in electronic packaging, serving as key enablers for many structures and in various aspects determining the process efficiency and package reliability. As a thermosetting polymer, epoxy curing control towards designed temperature response and thermal profile are desirable to fulfill the needs of specific applications such as no-flow underfill in advanced flip-chip packages. As such, controllable latent catalysts have been pursued for decades. Epoxy-copper interfaces are commonly found at encapsulant, substrate and printed circuit board applications where the contacts of epoxy composites are made with lead frame, mentalizations and bond wires. The delamination and crack of epoxy-copper interfaces is one of the major failure mechanisms of a package. Traditional approaches for improving the epoxy/copper adhesion include pre-treatment of substrate with physical/chemical etching and applying coupling agents. Adhesion promoter additives in the epoxy resin would be further appreciated for saving cost and process time, as well as accessing numerous novel structures. In addition, the covalent bond or hydrogen bond formation assisted by coupling agents are susceptible to hydrolysis degradation under moisture aging. Coordination bonds between transition metal species and organic ligands with O or N doners on the other hand are more stable against moisture, in the meantime benefitting the crosslinks at interface without being consumed by the bulk. Targeting at these issues, this dissertation explores in-formulation metal complex based chemistry for latent catalyst and adhesion promoter in epoxy resins.
This dissertation systematically studies the effects of introducing a series of transition metal chelates on the curing kinetics and copper-adhesion performances of epoxy/anhydride resin systems. First row transition metal (Co(II), Ni(II), Cu(II), Zn(II)) chelate-based modifiers bearing different β-diketone ligands were used as model compounds to differentiate metal and ligand effects. The first part of the dissertation introduces the controllable curing kinetics of epoxy resin using metal chelate additives. The interaction between metal β-diketonate with Lewis base phosphine catalyst manifested distinguished and useful thermal latent cure characteristics. It was found that other than the species of metal cation, inductive effects of the diketone ligands played a crucial role in determining the metal-phosphine interaction and thus the catalytic response of the resin. In-depth studies on the Co(II) based metal complexes on the curing control helped reveal a chemical equilibrium nature of these coordination reactions. The temperature induced paradigm shift in especially the hexafluoroacetylacetonate (6Facac2) chelates were examined in detail in the second part, and the underlying ligand mediated metal-base interaction strength upon heat treatment was analyzed in detail through structural characterizations and calculations. The third part of the dissertation presents the effects of the metal complexes on the adhesion strength of epoxy-copper joints and the resistance to moisture aging. The parametric studies on transition metal complexes with different metal and ligand types provided trend plots of the adhesion promoters. The mechanisms of the adhesion improvements were investigated through extensive chemical characterizations of the fractured surfaces, along with associated understandings of both copper and epoxy behaviors when incorporating transition metal chelate species. Both the organic bonding composition, which is related to the cure kinetics regulation discussed in the first section, and the metal-polymer coordination effects were determined to be responsible for adhesion enhancement in the metal complex doped resin systems.