Characterization and growth kinetics of the formation of intermetallic compounds in the liquid state during soldering with lead-free solders

摘要

The research described in this paper developed the experimental techniques necessary for promoting intermetallic layer growth throughout the melting and solidification cycle of the solder and then measuring the thickness of these layers. Most of the literature to date reported on aged soldered joints to investigate in-service intermetallic layer growth. To observe intermetallic compound (IMC) growth during the liquid state of soldering required an experimental setup that quickly raises the temperature of the solder joint followed by fast cooling (comparable to the actual joint conditions). Temperatures at increments of 25 degrees above the soldering temperature and times of 2, 5, and 8 s for growth were carefully controlled. After the experiments, the copper substrate/solder alloy interface was examined using light microscopy, scanning electron microscopy (SEM), and SEM-EDS (energy dispersive spectroscopy). As expected, intermetallic phases such as Cu6Sn5, Cu3Sn, and Ag3Sn were observed, with Cu6Sn5 being the major phase. Statistical analysis was performed for the IMC thickness data obtained from the long profile analysis. Empirical equations capable of predicting the IMC layer thicknesses were determined. Using Arrhenius analysis, the activation energy for intermetallic layer growth was calculated to be approximately 7 kCal/mol. This value is significantly lower than the value of approximately 24 kCal/mol reported in the literature for Cu6Sn5. Even considering the presence of Cu3Sn (counted together with Cu6Sn5) which has about 15 % higher activation energy of formation than Cu6Sn5, the experimentally measured values were still lower. The discrepancy was attributed to the fact that the values reported in the literature were mostly based on solid-state transformations while the measurements and calculations in this work were limited to the intermetallic layer formation from liquid state followed by growth.