Pb-free solders have replaced Pb-containing SnPb solders in the electronic packaging industry due to environmental concerns. Both electromigration (EM) and thermomigration (TM) have serious reliability issues for fine-pitch Pb-free solder bumps in the flip-chip technology used in consumer electronic products. We review the unique features of EM and TM in flip-chip solder bumps, emphasizing the effects of current crowding and Joule heating. In addition, the challenges to a better understanding of EM and TM in Pb-free solders are discussed. For example, the anisotropic nature of Sn microstructure in Pb-free solders can enhance the dissolution rates of Ni and Cu in solders driven by EM and TM.

Laser interferometry is found to be a very convenient technique for studying gradient diffusion in the poly(methyl methacrylate)/methanol (PMMA/MeOH) system. We studied the process in initially dry, supported PMMA films suddenly immersed in liquid MeOH and examined the effects of temperature and sample thermal history. Over the range of conditions studied, the diffusion process was entirely Case II. A simple model was developed to predict the time dependence of the intensity of light reflected from a supported film during Case II. An FFT algorithm made it possible to accurately match the reflected intensity data and the model predictions to determine the key parameters defining the Case II process: The volume fraction of methanol behind the Case II front (

No abstract

No abstract

Aqueous agar gels were directionally frozen in a temperature gradient of 10 ~ cm at rates between 8.8 x 10 -s cm/sec and 8.8 x 10 -4 cm/sec to form highly anisotropic ice-agar composites. Observations of the vitrified agar domains which were localized at the ice grain boundaries were made and the ice domain size was measured as a function of the solidifying conditions. The segregation of the noncrystalline agar was dominated by the vitrification character of the agar gel. Thermal analysis indicated that ice crystallization rate decreased to a negligible level as the water content of the gel decreased to approximately 50 percent by weight agar.