Date of Award

Spring 1998

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

First Advisor

Fournelle, Raymond A.

Second Advisor

Blumenthal, Robert N.

Third Advisor

Brebrick, Robert F.


Detailed studies to characterize the microstructural development and mechanical properties of eutectic Sn-Ag solder joints were carried out on samples reflow soldered on copper substrate under various processing conditions. Light (LM) and scanning electron microscopy (SEM) as well as energy dispersive spectroscopy (EDS) were used to study the microstructural evolution, while microhardness and shear strength measurements were used to monitor the changes in the mechanical properties. Some samples were reflow soldered at 30°C above the melting point and then solidified at different cooling rates. Analysis of these samples showed that increasing the cooling rate increased the volume fraction of primary Sn-dendrites, decreased the amount of "formula" intermetallics in the bulk solder, and resulted in finer microstructures with higher hardnesses. Subsequent isothermal annealing of some of these reflow soldered joints at 125° C resulted in an initially fairly rapid decrease in hardness to a given level for each cooling rate studied. However, the cooling rate had little or no effect on the shear strength of the solder joints. Studies of the effect of Cu substrate dissolution on the microstructure of solder joints were carried out using samples reflow soldered isothermally at various temperatures and times. All samples were found to exhibit highly inhomogeneous, non-equilibrium microstructures. Analysis of some of these samples showed that the Cu concentration, the volume fraction of primary Sn-dendrites and "formula" intermetallics in the solder increased with reflow temperature and time as a result of Cu dissolution. In addition, the relative amount of eutectic decreased with increasing time and temperature. The isothermal growth kinetics were analyzed using the Nemst-Brunner equation, and a numerical method based on it was proposed to predict the amount of Cu dissolution into molten solder for non-isothermal reflow conditions.



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