Elsevier

Materials & Design

Volume 80, 5 September 2015, Pages 152-162
Materials & Design

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

https://doi.org/10.1016/j.matdes.2015.05.016Get rights and content

Highlights

  • Small amounts of Cu have been added into Sn–6.5Zn solder alloy.

  • A new type of flower-like γ-Cu5Zn8 IMC was detected with Cu addition.

  • The shape and size of flower-like IMCs control the overall Sn–6.5Zn properties.

  • Optimal percentage of Cu was 0.5% Cu to enhance the overall properties.

  • 1.5% Cu addition induced undesirable effects on Sn–6.5Zn properties.

Abstract

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 γ-Cu5Zn8 intermetallic compound (IMC) was detected with 0.5 wt.% Cu added specimens. The flower-like morphology of γ-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 γ-Cu5Zn8 phase with enlarged β-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.

Introduction

Low temperature solders have been extensively used in microelectronics packaging industry for many decades [1]. The growing focus on high performance and miniaturization has generated an urgent need to develop high-performance lead-free solder alloy for package quality and reliability [2], [3]. This creates the demanding issue of producing reliable solder alloys with better mechanical properties and low cost. Therefore, understanding both the solders and processing aspects are crucial for manufacture sound electronic products without any serious problems. Among the novel lead-free solders, the eutectic Sn–9Zn system has been considered as one of the most attractive lead-free solders owing to its excellent mechanical properties, low melting temperature (198 °C), significant benefit on cost and other comprehensive performances [4]. Even though these alloys can be applied to a wide variety of applications, the poor wetting characteristics and poor oxidation resistance, which may be attributed to the higher surface tension and oxidation sensitivity of Zn, still limit the adoption of these alloys to certain applications [5]. The adequate characterization of mechanical behavior of solder alloys is one of the key issues to address, in order to gain deformation data on small solder joints. One innovative, potentially viable and economically affordable approach, to enhance the oxidation resistance and improve the mechanical properties of eutectic Sn–Zn solder is the addition of an appropriate secondary phase for the formation of Zn-based IMCs [6], [7]. Another approach is the reduction of Zn content in alloy matrix [8]. According to Sn–Zn equilibrium phase diagram [8], reducing Zn content near the hypoeutectic Sn–6.5Zn composition will remain the equilibrium eutectic melting point at the same level. Notably, some researchers have started to focus on the above weakness points, which are still the main challenging issue for the packaging industry. For instance, Wei et al. [9] carried out the thermal tests for hypoeutectic Sn–6.5Zn and eutectic Sn–9Zn alloys. It was found that the Sn–6.5Zn can behave in the same way as the eutectic Sn–9Zn during melting, although it has remarkably better wettability to Cu than Sn–9Zn solder. They also proposed that Sn–6.5Zn can be used as a lead-free solder, while mostly keeping its benefit of generating the same eutectic temperature of Sn–9Zn alloy. Apart from its favorable melting temperature and wettability, the microstructures and mechanical properties of hypoeutectic Sn–4 wt.% Zn alloy, hypereutectic Sn–12 wt.% Zn alloy and eutectic Sn–9 wt.% Zn alloy were examined by Garcia et al. [6]. The microstructure of eutectic Sn–9 wt.% Zn alloy induced higher mechanical strength than those of Sn–4 wt.% Zn and Sn–12 wt.% Zn alloys owing to the formation of globular Zn-rich phase in the former. However, requirements such as high mechanical strength and creep resistance are of prime importance for the selection of alternative solder alloys. In a recent study [10], [11], the as-cast microstructure of Ni and Sb-containing Sn–6.5Zn solder was characterized. The resulting thermal behavior and mechanical strength were also determined. It was established that the enhanced solid solution effect of Sb and the flower-like shaped (Ni, Zn)3Sn4 IMC phase produced by Ni addition play an important role on the melting temperature, undercooling and the mechanical strength of new solders. The amount of undercooling is reduced, while the melting temperature and pasty range remained at the hypoeutectic Sn–6.5Zn level. Microstructural analysis also revealed that the enhanced solid solution effect of Sb and the flower shaped (Ni, Zn)3Sn4 IMC phase are beneficially effective in reducing the creep rate of Ni and Sb-containing Sn–6.5Zn solders [11]. The creep resistance of Sn–6.5Zn solder was enhanced to about ∼270% and ∼182% with the addition of with Ni and Sb, respectively. In the present study, the influence of small amounts of Cu on the microstructural evolution and thermal behavior of Sn–6.5Zn solder were investigated. In addition, the mechanical properties of these Sn–6.5Zn–xCu alloys were measured depending on the content of Cu additions.

Section snippets

Experimental procedures

Commercially pure elements of Sn (99.9%), Zn (99.99%) and Cu (99.99%) were used to elaborate the proposed solder alloys. Composition of solder alloys were Sn–6.5Zn, Sn–6.5Zn–0.5 wt.% Cu and Sn–6.5Zn–1.5 wt.% Cu alloys. The process of melting was carried out in a vacuum arc furnace under protection of high purity argon atmosphere at 800 °C for about 1 h. In order to get a homogeneous composition within the ingots, the alloy samples were re-melted three times to produce rod-like specimen with a

Thermal behavior

The melting temperature and undercooling processes of solder alloys are a crucial factors that have to be taken into account in order to maintain high electronic packaging quality. To simulate the cooling rates of ball-grid array (BGA), the thermal behavior of three solder alloys was quantified using DSC analysis at 10 °C/min. The results are shown in Fig. 2 and summarized in Table 2. For Sn–6.5Zn and Sn–6.5Zn–0.5Cu solders, only one endothermal peak (Tm) emerged at 200.5 °C and 200.6 °C,

Conclusions

In this study, effects of Cu addition on microstructure, thermal behavior and tensile properties of Sn–6.5Zn based alloys have been investigated. The results are summarized as follows:

  • (1)

    Addition of 0.5% Cu into plain Sn–6.5Zn solder leads to formation of new type of dark flower-like γ-Cu5Zn8 IMC and reducing the amount of large needle-shaped Zn phase, while the addition of 1.5%Cu leads to creation of coarse dendrite-like morphology of γ-Cu5Zn8 phase.

  • (2)

    The amount of Cu addition has been found to

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