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School of Materials Science and Engineering

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    Transition Metal Complexes As Latent Catalyst And Adhesion Promoter In Epoxy Resin
    (Georgia Institute of Technology, 2022-11-09) Li, Jiaxiong
    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 for 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 feed ratio studies on the Co(II) based metal complexes in curing control helped reveal a chemical equilibrium nature of these coordination reactions. The temperature induced coordination paradigm shift in especially the hexafluoroacetylacetonate (6Facac2) chelates were examined 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 revealing metal and ligand dependence in the resin adhesion enhancement. The mechanisms of such 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 favorable interfacial chemical reactions, 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.
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    Epoxy/triazine based high performance molding compound for next generation power electronics packaging
    (Georgia Institute of Technology, 2019-07-26) Li, Jiaxiong
    The power electronics industry has been actively seeking encapsulant materials that can serve in harsher environments. For example, with the power semiconductors leading into SiC era, the higher operation temperature (250 ºC) have proposed great challenges on the packaging materials especially on epoxy molding compound (EMC) technologies, since the temperature exceeds the stability limit of typical epoxy (EP) chemistry. In this thesis, EP/triazine system was selected to develop high temperature stable resin system that can meet the temperature requirement of next generation power electronics packaging. In the first part of the thesis, different approaches were discussed to enhance the high temperature performance of a previously studied cyanate ester (CE)/ biphenyl EP blend which is impaired by the hydrolysis degradation of remaining cyanate groups. Firstly, the effects of different metal catalyst on the CE properties were discussed. Secondly, a triazine containing molecule triglycidyl isocyanurate (TGIC) was employed to increase the triazine content without increasing CE feed ratio to circumstance problem of unreacted cyanate groups. Finally, the high heat resistant novolac type CE was employed to form the NCE/EP blend, and their blends with different feed ratio were systematically evaluated. In the second part, a detailed characterization of a high heat resistant CE/novolac type EP blends and the investigation on their degradation under long-term high temperature storage were summarized. The effects of the CE concentration on the thermomechanical properties of the copolymer were explored, where a tradeoff behavior between the triazine content and crosslink density was accounted for the property change. In addition, the distinguished thermal degradation mechanisms in copolymer with different compositions were identified and illustrated. The knowledge obtained in this work can serve as a reference in the future to formulate EP/triazine based resin system for high temperature applications.