Ph.D. Dissertation Defense by Timothy Ferguson
Thursday, May 27, 2004
(Dr. Jianmin Qu, Chair)
"Moisture and Interfacial Adhesion in Microelectronic Assemblies "
Moisture poses a significant threat to the reliability of microelectronic assemblies and can be attributed as being one of the principal causes of many premature package failures. Since the vast majority of advanced underfills are epoxy based, they have the propensity to absorb moisture, which can lead to undesirable changes in stress and interfacial adhesion. To ensure the reliability and durability of the electronic packages, the effect of moisture must be understood. In addition to being a moisture sensitive property, the interfacial adhesion is also affected by the elastic mismatch, relative mode mixity, temperature, and the corresponding surface chemistry and topology of the adherends. Therefore, the study of the moisture effect on interfacial adhesion is inevitably a multidisciplinary effort.
In this research, a systematic and multi-disciplinary study was conducted to understand the fundamental science of moisture-induced degradation of interfacial adhesion. The research is comprised of both experimental and modeling components of analysis and consists of four primary components. First, the moisture transport behavior within underfill adhesives is experimentally characterized and incorporated into a finite element model to depict the moisture ingress and interfacial moisture concentration for each respective level of moisture preconditioning. Second, the effect of moisture on the variation of the underfill elastic modulus is demonstrated and the physical mechanisms for the change identified. Third, the aggregate effect of moisture on the interfacial fracture toughness of underfill to both copper and FR-4 board substrates is determined. This includes the primary effect of moisture being physically present at the interface and the secondary effect of moisture changing the elastic modulus of the adhesive when absorbed. Last, the recovery of both the elastic modulus and interfacial fracture toughness from moisture preconditioning is assessed with reversible and irreversible components identified. Using adsorption theory in conjunction with fracture mechanics, an analytical model is developed that predicts the loss in interfacial fracture toughness as a function of moisture content. The model incorporates key parameters relevant to the problem of moisture in epoxy joints identified from the experimental portion of this research, including the interfacial hydrophobicity, epoxy nanopore density, saturation concentration, and density of water.
This research results in a comprehensive understanding of the primary mechanisms
responsible for the interfacial degradation due to the presence of moisture.
The experimental results obtained through this research provide definitive data
for the electronics industry to use in their product design, failure analysis,
and reliability modeling. The predictive model developed in this research provides
a useful tool for developing new adhesives, innovative surface treatment methods,
and effective protection methodologies for enhancing interfacial adhesion