DR. ERIC COTTS

Abstract: We examined the mean time to failure (MTF) of different SnBi-based solder joints as the current density and the temperature were independently varied, in a study that conformed to JEDEC JEP 154 standards. Solder joints with diameters close to 250 micron were fabricated with different low temperature, Sn-Bi near eutectic-based solders. We examined the effect of the addition to the SnBi-based solder of small amounts of ternary and quaternary elements (such as Ag, Sb, Ni and Cu) on failure times and modes. Current densities generally ranged between 1 x 104 and 6 x 104 amperes/cm2, while the temperatures of the solder joints during test ranged from 90oC to 120oC. The temperature and the electrical resistance of the solder joints were monitored directly during the experiments. Thus, the amount of Joule heating of the solder joints was determined. Successful fitting was performed, using Black’s equation, of the variation of the MTF with respect to current density and temperature.The specific form of Black’s equation used in these fits was determined using the selection of a specific failure criterion for the experiment, and quantitative characterizations of the mobility of Bi during current stressing. Different failure criteria included a fixed percentage increase in the electrical resistance of the solder joint (e.g. 10 or 20 %), or a specific increase in the electrical resistance of the solder joint (e.g. 25 mW ). Values of the product of the diffusion coefficient of Bi in Sn, DBi, and of the effective charge, Z*, were measured. These measurements, and the 
careful characterization of Bi accumulation as a function of time during current stressing, were key to formulating a particular form of Black’s Law. The measurements of the temperature and the electrical resistivity of the solder joints as a function of time during the current stressing were also important. Thus, specific failure criterion and form of Black’s law were determined and used to find the MTF for a given solder joint and data set. Corresponding changes in the microstructures of the solder joints were characterized and correlated with lifetimes.

Biography:

Eric J. Cotts received the Ph.D. degree from the University of Illinois, Urbana, in 1983.,He joined the Binghamton University Physics Department in 1987. His academic interests include the structure of liquids and solids, thermal and transport properties of condensed matter, and surfaces and interfaces.