Abstract
A significant problem in the microelectronic packaging industry is the presence of moisture-induced failure mechanisms. Moisture is a multi-dimensional concern in packaging, having an adverse effect on package reliability by introducing corrosion, development of hygro-stresses, and degradation of polymers present in the package. Moisture can also accelerate delamination by deteriorating the polymer interfaces within the package. As the interfacial adhesion between the chip, underfill, and substrate decreases, the likelihood of delamination at each encapsulant interface increases. Once the package delaminates, the solder joints in the delaminated area are exposed to high stress concentrations, resulting in a reduction of overall package life.
Moisture can affect interfacial adhesion through two primary mechanisms. The first mechanism is the direct presence of moisture at the interface altering the interfacial integrity of the adhesive joint. The second mechanism is the absorbed moisture in either the adhesive and/or substrate altering the mechanical properties of those materials, which changes the response of the adhesive structure in the presence of an externally applied load. Inevitably, the effect of moisture on the adhesion and fracture of interfaces entails a multi-disciplinary study, and several aspects should be considered. From a global perspective, the primary aspects include moisture transport behavior, changes in bulk material properties from moisture absorption, effect of moisture on interfacial adhesion, and recovery from moisture upon fully drying, although several subsections within each major group occur due to the complexity of the problem.
In this chapter, a systematic and multi-disciplinary study is presented to address the fundamental science of moisture-induced degradation of interfacial adhesion. First, the moisture transport behavior within underfill adhesives is experimentally characterized. The results are incorporated into a finite element model to depict the moisture ingress and interfacial moisture concentration after moisture preconditioning. Second, the effect of moisture on the variation of the adhesive elastic modulus is demonstrated and the physical mechanisms for the change identified. Third, the aggregate effect of moisture on the interfacial fracture toughness 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. Both reversible and irreversible components of the interfacial moisture degradation are evaluated. 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, active nanopore density, saturation concentration, and density of water.
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Ferguson, T.P., Qu, J. (2007). The Effect of Moisture on the Adhesion and Fracture of Interfaces in Microelectronic Packaging. In: Suhir, E., Lee, Y.C., Wong, C.P. (eds) Micro- and Opto-Electronic Materials and Structures: Physics, Mechanics, Design, Reliability, Packaging. Springer, Boston, MA. https://doi.org/10.1007/0-387-32989-7_37
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