Abstract
Solder is widely used for electrical interconnections in electronics, and its use involves the melting and subsequent solidification of the solder. This paper reports for the first time the effect of the cooling rate in solidification on the electrical behavior (conduction and dielectric behavior) of the solidified solder. Prior work addressed the effect of the cooling rate on the microstructure and mechanical properties only. The solder is Sn-4Ag (lead free, melting temperature 221 °C). Cooling is from the melt at 230 °C. Comparison of the cooling rates corresponding to air cooling and ice-water quenching indicates that the microstructure is much finer and the resistivity (DC) is higher by 2.4% for the quenching case. However, the permittivity (100 kHz) is the same for the two cooling rates. This means that the microstructural refinement associated with the quenching decreases the mobility without affecting the carrier–atom interaction responsible for the permittivity. The carriers are the valence electrons. Since the polarization associated with the permittivity involves the interaction of a fraction of the valence electrons with the atoms, this further implies that the fraction of carriers that interact with the atoms is not affected by the microstructural refinement and that the decrease in mobility (as indicated by the resistivity increase) does not affect this fraction. The fractional increase in the resistivity due to the quenching (relative to air cooling) during solidification (this work) is small compared to that due to the mild heating of the solid solder to 70 °C (our prior work). The mild heating also does not affect the permittivity.
Published Version
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