| Summary: | Copper is a widely used conductor in the manufacture of printed wiring boards (PWB). The trends in miniaturization of electronic devices create increasing challenges to all electronic industries. In particular PWB manufacturers face great challenges because the increasing demands in greater performance and device miniaturization pose
enormous difficulties in manufacturing and product reliability. Nanocrystalline and ultrafine grain copper can potentially offer increased reliability and functionality of the PWB due to the increases in strength and achievable wiring density by reduction in grain size.
The first part of this thesis is concerned with the synthesis and characterization of
nanocrystalline and ultra-fine grain-sized copper for potential applications in the PWB
industry. Nanocrystalline copper with different amounts of sulfur impurities (25-
230ppm) and grain sizes (31-49nm) were produced and their hardness, electrical
resistivity and etchability were determined.
To study the thermal stability of nanocrystalline copper, differential scanning
calorimetry and isothermal heat treatments combined with electron microscopy techniques for microstructural analysis were used. Differential scanning calorimetry was
chosen to continuously monitor the grain growth process in the temperature range from
40C to 400C. During isothermal annealing experiments samples were annealed at
23C, 100C and 300C to study various potential thermal issues for these materials in PWB applications such as the long-term room temperature thermal stability as well as for temperature excursions above the operation temperature and peak temperature exposure during the PWB manufacturing process. From all annealing experiments the various grain growth events and the overall stability of these materials were analyzed in terms of driving and dragging forces. Experimental evidence is presented which shows that the overall thermal stability, grain boundary character and texture evolution of copper is greatly related to changes in driving and dragging forces, which in turn, are strongly depended on parameters such as annealing temperature and time, total sulfur impurity content and the distribution of the impurities within the material. It was shown that a simple increase in the sulfur impurity level does not necessarily improve the thermal stability of nanocrystalline copper.
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