The investigation presents new kind of infrared equipment for wide utilization in electronic industry. It is suitable for electronic components soldering to printed circuit boards (PCB) using conventional and lead free solder pastes, different kinds of annealing and others. Conventional reflow furnaces have their own heating profiles. The heating profile of a certain furnace is uniquely determined by its design parameters, namely, it lacks flexibility. The upper and lower limits of heating and cooling temperatures can be adjusted by some degree, but time-dependent temperature control is practically impossible. By application of low inert heaters radiating in the middle IR region and microprocessor control of their operation versatile energy saving installation proper for all kinds of solder pastes is made. This equipment allows realizing precise control (in situ) of the temperature on the PCB during the soldering processes, possibility of individual temperature profile for every printed board during the soldering process in dependence of its size, electronic components density et al.
A two-dimensional phase-field method was developed that allowed us to observe the evolution of the composition field and the microstructure of lamellar eutectics caused by variations in the freezing rate. Solid-phase nucleation, lamella termination, and migration of the three-phase junction were studied to elucidate the role of each in microstructure changes. This evolution is often complex due to the interplay between interface kinetics, the composition field, and the widths of the solid lamellae. When the freezing rate is increased, the supersaturation in front of the lamellae increases, causing formation of deep depressions, followed by nucleation of the other solid phase, instability, and movement of the three-phase junction. If the freezing rate is subsequently held constant, the microstructure eventually becomes stable. With a decrease in freezing rate, instability develops that changes the local growth direction and provokes changes in lamellar width followed by elimination of some lamellae. With freezing rate oscillations, backmelting becomes important, especially for large lamellar spacing, large amplitudes, and low frequencies. Three ranges of oscillation amplitude and oscillation frequency were identified that result in different microstructures.
Abstract-As discussed in our review paper (Wilcox, W. R. and Regel, L. L., Microgravity Quarterly, 1994, 4, 147-156), the in uence of microgravity on eutectic microstructure has been rather erratic and largely unexplained. Directional solidiÿcation in microgravity sometimes coarsened the structure, sometimes made it ÿner, and sometimes, even on the same system, had no measurable e ect. Theoretical models predicted no in uence of the weak buoyancy-driven convection that occurs in the vertical Bridgman technique on earth. Thus, we hypothesized that freezing rate uctuations due to irregular convection might be responsible. For example, with a ÿbrous microstructure an increase in freezing rate must cause new ÿbers to form, either by branching or by nucleation. A decrease in freezing rate would cause ÿbers to terminate by overgrowth of the matrix phase. If the kinetics of ÿber formation di ers from that for ÿber termination, an oscillatory freezing rate would cause the average ÿber spacing to deviate from that at a steady freezing rate. We have been investigating this hypothesis both experimentally and theoretically. Vertical Bridgman experiments were performed on the MnBi-Bi eutectic with freezing rate oscillations caused by periodic electric current pulses passed through the material. With increased current amplitude, more and more grains exhibited irregular microstructures. Of the grains with continued quasi-regular rod structure, the microstructure became ÿner. This result was contrary to that expected from our hypothesis for this system. Numerical modeling also predicted that an oscillatory freezing rate should yield a ÿner microstructure. It was also predicted that freezing interface oscillations should cause the average melt composition at the freezing rate to deviate from the eutectic. This results in the formation of a composition boundary layer of su cient thickness that it would become sensitive to convection. Hence we have arrived at a revised hypothesis. On earth, irregular convection causes freezing rate uctuations that change the interfacial melt composition, leading to a thick composition boundary layer. Convection interacts with this boundary layer to change the interfacial melt composition, thereby altering the response of the system to freezing rate uctuations. ? 2001 Published by Elsevier Science Ltd.
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