Sn addition on the tensile properties of high temperature Zn–4Al–3Mg solder alloys
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.
Section snippets
Materials and experimental procedures
The Zn–4Al–3Mg solder alloy was fabricated by using Zn, Al and Mg ingots (purity >99.9%). The alloy was melted at 700 °C under the CO2 protective atmosphere and kept for 4 h to achieve the homogeneous characteristics. A certain amount of ZnCl2 (0.1–0.15% of total ingot weight) was then pressed into the molten alloy for the oxide reduction. After 5–10 min stabilization, the molten alloy was poured into stainless steel mold to obtain the Zn–4Al–3Mg alloy. Similarly, the Zn–4Al–3Mg–xSn alloys were
Melting characteristics
Fig. 2 illustrates the measured DTA curves of Zn–4Al–3Mg–xSn alloys during a typical heating/cooling cycle. Based on the ICTA (International Confederation for Thermal Analysis) standards, it can be found from Fig. 2a that the melting point TM of Zn–4Al–3Mg alloy is 340.8 °C, which is consistent with the existing result [8]. And the onset of nucleation point TN of Zn–4Al–3Mg alloy is 330.1 °C, which leads to the undercooling of 10.7 °C for Zn–4Al–3Mg alloy according to Mueller’s approach [12].
Conclusions
The Sn addition effect on the tensile property of Zn–4Al–3Mg alloy at high temperatures was studied. The following conclusions can be drawn:
- (1)
The Sn addition can reduce the solidus and undercooling of Zn–4Al–3Mg alloy to some extent. Larger Sn addition (e.g., 13.2 wt.%) does not result in further reduction of solidus and undercooling.
- (2)
The Zn–4Al–3Mg alloy maintains the strength at 200 °C, which is still comparable with Pb–5Sn high-temperature solder at room temperature. However, the high yield
References (16)
- et al.
High temperature creep and hardness of eutectic 80Au/20Sn solder
J Alloy Compd
(2008) - et al.
Mechanical properties versus temperature relation of individual phases in Sn–3.0Ag–0.5Cu lead-free solder alloy
Microelectron Reliab
(2009) - et al.
Experimental investigation and thermodynamic calculation of the Al–Mg–Zn system
Thermochim Acta
(1998) - et al.
Physics and materials challenges for lead-free solders
J Appl Phys
(2003) - et al.
Interfacial reactions of Si attachment with Zn–Sn and Au–20Sn high temperature lead-free solders on Cu substrate
J Electron Mater
(2009) - et al.
Development of Bi-base high-temperature Pb-free solders with second-phase dispersion: thermodynamics calculation, microstructure, and interfacial reaction
J Electron Mater
(2006) - et al.
Interfacial reactions between Bi–Ag high-temperature solders and metallic substrates
J Electron Mater
(2006) - et al.
High-temperature lead-free SnSb solders: wetting reactions on Cu foils and phase-in Cu–Cr thin films
J Mater Res
(1999)