Compositional effects on the microstructure and vibration fracture properties of Sn–Zn–Bi alloys

Journal of Alloys and Compounds 403 (2005) 191–196

Compositional effects on the microstructure and vibration fracture properties of Sn–Zn–Bi alloys Jenn-Ming Song a,∗ , Truan-Sheng Lui b , Yea-Luen Chang b , Li-Hui Chen b a b

Department of Materials Science and Engineering, National Dong Hwa University, Hualien 974, Taiwan Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 12 April 2005; received in revised form 14 May 2005; accepted 18 May 2005 Available online 14 July 2005

Abstract This study investigated the effect of Bi content on the microstructure and mechanical properties of the Sn–Zn–Bi alloys, especially the vibration fracture behavior. Results show that Sn–Zn–Bi alloys exhibit an alternate normal-irregular eutectic structure due to the inhomogeneous dissolution feature of Bi in Sn matrix. With an increased Bi addition, the vibration properties including damping capacity and vibration fracture resistance became inferior. This can be ascribed to hardening effect caused by Bi dissolution in the Sn matrix. Also, Bi-rich precipitates appeared in high Bi samples led to quick crack propagation. © 2005 Elsevier B.V. All rights reserved. Keywords: Sn–Zn–Bi; Lead-free solder; Resonant vibration; Microstructure

1. Introduction Sn–Zn eutectic alloy has recently been considered as a potential candidate for a lead-free solder material because of its low melting point (198 ◦ C), excellent mechanical properties and low cost [1,2]. However, the Sn–Zn alloy suffers problems of poor wetting, easy oxidation and dross formation [3,4]. Alloying elements of In [5], Bi [6], Al [7] and rare earths (RE, mainly La and Ce) [8] have been chosen to improve the wettability. Among these, the Sn–Zn–Bi alloys are now applied commercially due to its better wetting properties and reduced melting temperature. There are many issues that must be resolved for further practical use of Sn–Zn–Bi solders. Until now, many studies on the microstructural characteristics [9], interfacial reaction [10,11], tensile and creep properties [12], as well as reliability assessment [13] have been performed. Previous reports [14,15] indicated that failure may occur due to vibration when solder joints are assembled, e.g., in vehicles and aircraft, especially when the vibration frequency approaches the ∗

Corresponding author. E-mail address: [email protected] (J.-M. Song).

0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.05.016

resonant vibration of the structure. Interestingly, instead of intermetallics at interface Yang et al. reported that for a PBGA module under resonance, cracks were found propagating at the corner position in the solder ball. Therefore, the vibration fracture resistance should be taken into consideration in alloy design for lead-free solders. This study aimed to investigate the vibration fracture behavior of Sn–Zn–Bi alloys, including crack propagation morphology and the ability to absorb vibration energy. The effect of Bi content on the microstructure and vibration properties is of main concern.

2. Experimental procedures The Sn–Zn–Bi solders investigated were the alloys with compositions of Sn–9 wt% Zn, Sn–8 wt% Zn–3 wt% Bi and Sn–8 wt% Zn–6 wt% Bi (referred to as 0Bi, 3Bi and 6Bi). All the samples were cast using a Y-shaped graphite mold. All the specimens were naturally aged at room temperature to stabilize the microstructure before testing. In order to collect tensile data for reference, rectangular specimens (gauge length section: 20 mm × 5 mm × 2 mm), were also prepared to perform tensile tests (initial strain

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ratio of the length of the main crack to the projected length of this crack along the transverse direction of the specimen. The crack branching factor is the ratio of the length of all branched cracks alongside the main crack to the projected length of the main crack. Each datum was the average of results from five samples. Damping capacity was measured in terms of logarithmic decrement (δ value) which was derived from the deflection amplitude decay of a specimen under free vibration. Logarithmic decrement value is defined as follows [16]: 
1 Ai δ = ln n Ai+n where Ai and Ai+n are the deflection amplitudes of the ith cycle and the (i + n)th cycle separated by n periods of oscillation.

Fig. 1. (a) The vibration apparatus: (1) vibration controller, (2) acceleration sensor, (3) vibration shaker, (4) specimen clamp, (5) specimen, (6) deflection sensor and (7) recorder. (b) The dimension of test specimens (unit: mm).

rate: 7.5 × 10−4 s−1 ). Each datum was the average of three tests. A simple cantilever beam vibration system, as shown schematically in Fig. 1(a), was used for the vibration experiment. The test specimens (Fig. 1(b)), rectangular with dimensions 20 mm × 100 mm × 4 mm and two V-notches near the clamp, were clamped on end to the vibration shaker. The resonant vibration tests were conducted at a fixed vibration force (2.5g, where g denotes the acceleration due to gravity, 9.8 m/s2 ) under resonant vibration conditions. The variation in deflection amplitude against cyclic number was recorded. Each datum was the average of results from more than three samples. To understand the crack propagation due to vibration deformation, crack tortuosity and crack branching were quantified, as illustrated in Fig. 2. Crack tortuosity is defined as the

Fig. 2. Definition of crack tortuosity and branching factor.

3. Results and discussion 3.1. Microstructure and tensile properties Fig. 3 shows the microstructure of the Sn–Zn–Bi alloys. The 0Bi specimens (Fig. 3(a)) exhibit a typical rapidly solidified Sn–Zn eutectic microstructure. Each eutectic cell possessed fine, aligned Zn-rich needles. With Bi addition of 3 wt% (Fig. 3(b)), some coarse eutectic cells with misaligned Zn particles and primary Zn phase were observed. Fig. 3(c) displays that numerous primary Zn needles, Bi-rich phase and a more random eutectic structure can be seen in the 6Bi sample. EPMA elemental analysis of Sn, Zn and Bi of the 3Bi and 6Bi specimens (Fig. 4) reveals the magnified eutectic structures. It is found that broad irregular regions and normal eutectic structure form alternately. Fig. 4(a) shows that in the 3Bi sample the signal of Bi in the Sn phase of the coarse eutectic cells is relatively brighter than that in the normal regions. When the Bi content was raised to 6 wt% (Fig. 4(b)) Bi-rich particles appeared and were distributed uniformly in both the normal and irregular eutectic cells. Since the Bi concentration in the matrix is in the order of magnitude of a few hundredths of that of the Bi-phase particles, the Bi signal of the matrix is almost invisible. Typical tensile stress–strain curves of these Sn–Zn–Bi alloys are shown in Fig. 5. As illustrated, the strength increased in turn from 0Bi, 3Bi to 6Bi, however, the elongation decreased drastically with a higher Bi content. Notably, the slope of the curve within the elastic region was raised with increasing Bi content. The additions of Bi convert the eutectic structure to a partnormal and part-irregular structure. The Bi content of the coarse cells is relatively higher than that of the normal eutectics. Similarly, it has been reported recently that adding Ga in Sn–Zn alloys gave rise to an alternate normal-irregular eutectic structure and extended melting range (temperature range between solidus and liquidus) [17]. During the solidifi-

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Fig. 4. The backscattering electron image and elemental maps of (a) the 3Bi and (b) the 6Bi samples.

Fig. 3. Microstructure of Sn–Zn–Bi alloys: (a) 0Bi, (b) 3Bi and (c) 6Bi (Bi: Bi-rich phase; P-Zn: primary Zn).

cation process, the initially solidified eutectic cells with low solute concentrations formed a normal morphology first and then the residual liquid with a greater solute content solidified to become the irregular eutectic structure at lower temperatures. This inhomogeneous dissolution of low solute elements in Sn–Zn eutectic cells probably accounts for the microstruc-

ture feature of Sn–Zn–Bi alloys mentioned above. In addition, the dissolution of Bi in Sn matrix results in the strengthening effect and thus degraded ductility [9]. When the Bi content reaches a critical value, the Bi-rich phase starts to precipitate as isolated particles. The Sn–Bi phase diagram shows that solubility of Bi in Sn is only 2 wt%. However, similar to a previous report, Bi precipitates were barely observed in the solder alloy with Bi addition of 3 wt%. This is probably due to solute supersaturation caused by the high solidification rate (about 20 ◦ C/s in this study).

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Fig. 5. Stress–strain curves of the specimens.

3.2. Vibration behavior and crack propagation Table 1 displays the resonant frequency and logarithmic decrement of the specimens, showing that resonant frequency remained constant and the damping capacity decreased with a higher Bi addition. As illustrated in Fig. 6(a), the D–N curves (deflection amplitude versus number of vibration cycles) under a constant push force show that the initial deflection increased with increasing Bi content, inversely proportional to the damping capacity. This figure also indicates that the D–N curve could be divided into an initial stage with ascending deflection amplitude, a second stage in which deflection remained constant and a final stage with a descending deflection amplitude. The ascending and constant deflection amplitudes within Stages I and II can be attributed to the effect of strain hardening in competition with that of crack generation and linking within this region [18]. The descending deflection in Stage III is due to the deviation of the actual vibration frequency from the resonant frequency caused by the inward propagation of major cracks [19]. In this study, vibration life is defined as the vibration cycle number at the beginning of Stage III. According to this definition, Fig. 6(b) shows the quantitative results of the initial deflection amplitude and critical number of cycles to failure. It indicates that the vibration life, which decreased with a higher Bi content, was inversely proportional to the initial deflection. Table 1 Vibration properties and crack propagation factors of the samples

Resonant frequency (Hz) Logarithmic decrement (δ) Tortuosity Branching (%)

0Bi

3Bi

6Bi

87 ± 1 0.195 1.12 34.7

87 ± 1 0.173 1.08 17.7

87 ± 1 0.167 1.07 6.2

Fig. 6. (a) D–N curves of the samples under a fixed vibration force of 2.5g and (b) initial deflection amplitude and vibration life estimated from (a).

Vibration deformed structures of the samples after vibration tests are shown in Fig. 7, revealing that the deformation structure was quite different depending on the microstructural features of the solders. A fibrous deformation structure of the Sn-rich phase within Sn–Zn eutectics and cracks initiating from Zn needles could be observed in the 0Bi specimen (Fig. 7(a)). Interestingly, as in the 3Bi sample, Sn fibers could only form within normal eutectic regions (Fig. 7(b)). Moreover, the fibrous type deformation feature was suppressed when the Bi content went up to 6 wt%. Cracks could be found generating from the corner of the Bi-rich islands (Fig. 7(c)). Both the crack propagation morphologies (Fig. 8) and the quantitative data of the crack tortuosity and branching factor (Table 1) show that with a higher Bi content the main crack propagated more straightly and less divergently. Also, it was found that, as in the 6Bi specimen (Fig. 8(c)), several short cracks grew along the main crack and all the cracks tended to pass through the island-like Bi precipitates. The Bi addition gave rise to the dissolution of Bi in the Sn matrix and the precipitation of Bi-rich phase in the case

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Fig. 7. Vibration deformation structures of the (a) 0Bi (the arrows indicate cracks), (b) 3Bi (the arrows indicate Sn fibers) and (c) 6Bi specimens (the arrows indicate cracks).

Fig. 8. Crack growth morphology of the (a) 0Bi, (b) 3Bi and (c) 6Bi specimens (fractured Bi precipitates are indicated by arrows).

of high Bi samples. The dissolution of Bi resulted in a hardening of the Sn matrix, a decrease in dislocation mobility, a lower damping capacity and consequently higher deflection amplitude under resonance. Due to the inhomogeneous distribution of Bi, the irregular coarse eutectic cells with higher Bi concentration seem to possess greater deformation resistance than that of the normal eutectics even though the normal cells are much finer. That is why the fibrous deformation feature only appeared at normal eutectic regions in the 3Bi sample.

As for the 6Bi specimen, the whole Sn matrix supersaturated with Bi greatly hardened and thus the fibrous deformation was stunted. A previous report [20] about the effect of adding Bi on the fatigue of Sn–Ag series solders indicates that the loss of ductility is responsible for the decreased fatigue life of the high Bi alloys. Notably, besides lower ductility and poor damping capacity, Bi precipitates play an important role in crack initiation and growth and contribute to inferior vibration fracture resistance.

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4. Conclusions The addition of Bi significantly changes the microstructure characteristics and vibration properties of the Sn–Zn eutectic alloy. Bi was found to dissolve inhomogeneously and thus caused an alternate normal-irregular eutectic structure feature. Dissolution of Bi in the matrix caused high tensile strength and stiffness. Also, it resulted in low ductility, inferior damping capacity and shorter vibration life. Precipitation of Bi-rich phase could be observed in the 6Bi sample. During vibration, the Bi precipitates are preferred crack initiation sites and the cracks propagate straightly along these particles.

Acknowledgement This work has been supported by the Chinese National Science Council (Contract: NSC 93-2216-E-006-012), for which the authors are grateful.

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