Viscosity and Surface Tension of Liquid Sn-Cu Lead-Free Solders

Abstract

The viscosity of Sn-0.7wt.%Cu and Sn-2wt.%Cu lead-free solders was measured using a torsional oscillation viscometer from 503 K to 773 K. For both melts the viscosity decreased with increasing temperature, and the values dropped abruptly at certain temperature ranges, i.e., 553 K to 573 K for Sn-0.7Cu and 573 K to 603 K for Sn-2Cu. Correspondingly, two segments, a low-temperature zone and a high-temperature zone, were found on both g-T and ln g-1/T plots, where g is the viscosity and T is the absolute temperature. The activation energy e and the unit volume of flow m m were obtained by applying the parameters of linear fitting for each temperature zone. The surface tension of the solders was calculated using the measured viscosity values, and the results were checked by measuring the wetting angles of metallurgical reactions between the solders and the Cu substrates. Both viscosity and surface tension are closely related to the liquid structure of the solder alloy.

Structure and properties of Sn-Cu lead-free solders in electronics packaging

“…During soldering, the viscosity of solder alloy directly affects gap filling and weldability. The viscosity of Sn-0.7Cu solder alloy decreases with the increase of temperature [110]. But with the increase of temperature, the liquid structure of the melt could be changed, and the surface tension of the solder first decreases to a certain value and then increases slightly.…”

“…As shown in Figure 16 [110], the surface tension of liquid Sn-Cu solders at different temperatures was calculated, based on the above two formulas. The surface tension of the two kinds of solder is similar.…”

Effect of Trace Cu on Microstructure, Spreadability and Oxidation Resistance Property of Sn-<i>x</i>Cu Solders

“…The less surface tension, the better ow property, and the solder would be easier to spread. The relationship between surface tension σ of liquid solder and the temperature T is as given by equation follows: [7][8][9]…”

“…Where, m is the molar mass of liquid, ρ is the liquid density, k is the empirical constant and T c is the critical temperature with surface tension being zero. The relationship between the melt viscosity η and the temperature T of molten metal can be generally described by the Arrhenius equation: [7][8][9]25)…”

“…In recent years, many domestic and overseas scholars have done a comprehensive research on wettability, interface compounds, [3][4][5] surface tension, 6) melt structure and viscosity, 7,8) electromigration 9) of eutectic alloy Sn-0.7Cu. Since Sn-0.7Cu lead-free solder is poor in wettability and spreading property for metal substrate and easily oxidized in application, 10) some elements such as Pd, Cr, Ca, 11) Ag, In, 12) Al, 13,14) Bi, 15,16) Ni [17][18][19] and nanoparticle, 20,21) had been added into Sn-0.7Cu solders to improve its comprehensive performance, which achieved good research results.…”