Kinetics of the isothermal spreading of tin on the air-passivated copper surface in the absence of a fluxing agent

A specially designed ultrahigh vacuum in situ surface analysis and wetting system has been constructed to study the spreading of liquid metal solders on carefully prepared and well-characterized solid substrates, initial stud ies have been completed for the spreading of pure Sn solder on Cu substrate s in the absence of any fluxing agent, Surface chemical analysis by X-ray p hotoelectron spectroscopy showed the as-received surface consisted of about 8 nm of Cu2O. Solder spreading was performed under 50 Torr of highly purif ied He gas to allow for adequate thermal coupling between the solder and th e substrate. Isothermal spreading experiments were performed on the as-rece ived Cu surface in the temperature range between 262 and 372 degrees C. Mea surable solder spreading was observed on this surface even at temperatures only 30 degrees C above the melting temperature of the purl Sn solder (232 degrees C). The results of the isothermal experiments show that solder spre ading on the as-received Cu surface is an activated process with an activat ion energy of 39.9 +/- 3.1 kcal mol(-1) in the range of 262 to 327 degrees C, and a 1/3-order dependence on spreading time. Between 327 degrees C and 330 degrees C. a discontinuity in the Arrhenius plot is observed. At 330 de grees C: and above, the spreading process is non-activated and shows a 1/5- order dependence on spreading time. It is believed that this discontinuity in spreading rates is a direct result of vacuum-assisted decomposition of t he surface Cu2O, with retention of a subsurface oxide. Similar experiments in an inert atmosphere glove box at atmospheric pressure do not show the sp reading rate discontinuity, although overall spreading rates and kinetics a rt: similar. The overall spreading process is believed to be facilitate ci through an oxide displacement reaction, where the spreading Sn reduces the surface Cu2O to Cu and captures the oxygen to form SnO2 as spreading procee ds. Metallurgical cross-section analysis after spreading on the as-received surface shows the expected formation of Cu-Sn intermetallic zones and a mi crostructure with no gaps at the Cu-Sn interface due to remaining patches o f Cu oxide. (C) 1998 Elsevier Science B.V. All rights reserved.