Sn addition on the tensile properties of high temperature Zn–4Al–3Mg solder alloys

Abstract

The Zn–4Al–3Mg based solder alloy is a promising candidate to replace the conventional Pb–5Sn alloy in high-temperature electronic packaging. In this study, the tensile properties of Zn–4Al–3Mg–xSn alloys (x = 0, 6.8 and 13.2 wt.%) at high temperatures (e.g., 100 °C, and 200 °C) were investigated. It was found that the uniaxial tensile strength (UTS) of Zn–4Al–3Mg–xSn solder alloys all decrease monotonously with the increment of temperature. The elongation ratio at 100 °C is superior to that at room temperature whereas follows a significant drop at 200 °C. The microstructure observations show that a typical brittle fracture of Zn–4Al–3Mg alloy occurs at room temperature and 200 °C under normal tension, whereas a ductile fracture is found at 100 °C. The 6.8 wt.% Sn addition in Zn–4Al–3Mg alloy causes a dramatic decrease of yield strength, and a slight deterioration of the ductility.

Introduction

Owing to their good wettability, high ductility and low shear modulus, the Pb–5Sn and Pb–10Sn solders has been widely used in die attaching of power devices, and still remain the exemptions in RoHS directive [1]. So far there is no reliable high-temperature solder alloy which can replace these high-Pb solders completely. Thus it is crucial to seek for a suitable high-temperature solder which possesses the similar solidus and liquidus as Pb–5Sn or Pb–10Sn, as well as the material properties. Several candidates, such as 80Au–20Sn [2], [3], Bi based alloys [4], [5], Sn–Sb based alloys [6], and Zn based alloys have been reported [7], [8], [9], [10], [11].

The melting temperatures of Tl and Cd are very close to that of Pb (i.e., 330 °C). Unfortunately, both Tl and Cd are toxic as Pb, which excludes them from the potential element candidates. Bi is another potential element, as its melting temperature is about 270 °C. However, it has been found that the presence of Bi will enhance the brittleness of solder alloys dramatically. Due to its low melting temperature (230 °C), Sn also cannot be used as matrix material. It has been reported that Zn based alloy may be a promising candidate for the high-Pb solder replacement. In particular, Shimizu et al. have studied the melting point of a ternary Zn–4Al–3Mg eutectic alloy, and also found that 3% Ga addition in this alloy can be used for die attaching at 320 °C [8]. However, gallium is known to cause liquid metal embrittlement in aluminum by significantly reducing the cohesion between aluminum grains, and thus lead to severe embrittlement failure [9], [11]. Sn addition in Zn–4Al–3Mg alloy may also be suitable as an alternative replacement of high-Pb solders. However, only solidus and liquidus temperatures were presented, and no other material properties associated to the solder reliability were reported [8].

To this end, this paper is devoted to study the effect of Sn addition on the material properties of Zn–4Al–3Mg solder alloy, particularly at high temperatures. The microstructure observations are then performed to reveal the plausible failure modes.