The effect of undercooling on the microstructure and tensile properties of hypoeutectic Sn-6.5Zn-xCu Pb-free solders

Hypoeutectic Sn-6.5Zn alloy may be regarded as a better choice than eutectic Sn-9Zn lead-free solder in microelectronics industry. In this study, the properties of hypoeutectic Sn-6.5Zn lead-free solder were modified with minor additions of Cu. SEM investigations reveal that the plain Sn-6.5Zn solder exhibits large number of undesirable acicular structure of angular needle-like Zn particles at the solder matrix. The acicular-shape morphology of Zn was remarkably suppressed after Cu modification. Moreover, a new type of small flower-like gamma-Cu5Zn8 intermetallic compound (IMC) was detected with 0.5 wt.% Cu added specimens. The flower-like morphology of gamma-Cu5Zn8 IMC appears to cause a sharp increase in Young's modulus, yield strength (YS) and ultimate tensile strength (UTS) of Cu modified solder. However, this effectiveness is reduced when 1.5 wt.% Cu addition starts to enhance the growth of coarse dendrite morphology of gamma-Cu5Zn8 phase with enlarged beta-Sn matrix. In addition, a 1.5 wt.% Cu addition was found to induce undesirable effects on the degree of undercooling, melting temperature and pasty range. Constitutive Garofalo model was assembled based on the experimental data of Sn-6.5Zn lead-free solders. (C) 2015 Elsevier Ltd. All rights reserved.