Trans. Nonferrous Met. Soc. China 29(2019) 1696-1704

Effects of Ag contents in Sn–xAg lead-free solders on microstructure, corrosion behavior and interfacial reaction with Cu substrate

Phacharaphon TUNTHAWIROON, Kannachai KANLAYASIRI

Department of Industrial Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand

Received 12 November 2018; accepted 25 April 2019

Abstract: The effects of Ag on the microstructure and corrosion behavior of pre-soldering Sn-xAg lead-free solders, and on the formation of intermetallic layer of the solders with Cu substrate were investigated. The Ag contents (x) were 0, 3.0, 3.5, 4.0, and 5.0 wt.%. The Ag content played a role in the morphology of Ag₃Sn phase in the solders. The microstructure analysis showed that the β-Sn phase was surrounded by eutectic networks in the 3.0Ag and 3.5Ag solders and large plate-like Ag₃Sn formed in the 4.0Ag and 5.0Ag solders. Nonetheless, the Ag content slightly impacted the corrosion behavior of the as-cast solders as characterized using potentiodynamic polarization test. After soldering, only a single layer of a Cu₆Sn₅ intermetallic compound formed at the Sn–xAg/Cu interface. By comparison, the Cu₆Sn₅ intermetallic layer of the Ag-doped solders was thinner than that of the 0Ag solder. The fine Ag₃Sn particles in the eutectic networks precipitating in the 3.0Ag and 3.5Ag solders effectively hindered the growth of Cu₆Sn₅ grains compared to large plate-like Ag₃Sn in the 4.0 and 5.0Ag solders.

Key words: Sn-Ag lead-free solders; microstructure; Ag₃Sn intermetallic phase; corrosion behavior; Cu₆Sn₅ intermetallic layer; wettability

1 Introduction

The European Union’s Restriction of Hazardous Substances (RoHS) directive restricts the use of hazardous substances in electronics goods and several other products [1]. Of particular interest is lead (Pb) which tops the list of the banned hazardous substances, despite the low cost, low melting temperature (180 °C), and good wettability [2,3]. The prohibition of lead necessitates electronics packaging manufacturers to explore alternative lead-free solders.

The traditional lead-containing solders are being replaced by the binary and ternary lead-free Sn-based solder systems. The binary solder system is realized by incorporating alloying elements (e.g., Zn, Bi, In, Cu, Ag) into the Sn-based solder to manipulate the solder characteristics for diverse applications [1,2,4-7]. Nevertheless, the addition of alloying elements induces the formation of intermetallic phases and alters the mechanical properties and melting temperatures of the solder.

Specifically, compared to the Sn-Pb system, the Sn-Bi solder with eutectic composition achieves a lower melting temperature but is susceptible to crack due to the inherent brittle nature of Bi [1]. The Sn-Cu solder system exhibits good ductility and conductivity but suffers from high melting temperatures (227 °C for eutectic composition). The Sn-Ag solder possesses good conductivity and good resistance to creep and thermal fatigue, but is costly and has a high melting temperature (221 °C) [8]. Meanwhile, the ternary Sn-Ag-Cu solder system is commonly used due to lower melting temperatures (217-221 °C) as a result of near eutectic composition (i.e. Sn-3.0Ag-0.5Cu). The Sn-Ag-Cu solder system also exhibits good electrical conductivity and high solder joint strength compared with the Sn-Pb, Sn-Cu, and Sn-Ag solder systems [4,9]. However, the ternary system is predisposed to the formation of multiple intermetallic phases and layers during the soldering process, adversely altering the mechanical properties of solder joints [4,7].

The corrosion behavior of the solder also plays a part in the durability and reliability of solder joints [10].

By and large, the solder should be resistant to a corrosive environment, such as moisture and heat [11]. Existing research on the corrosion behavior of solders focused mainly on the solders with near eutectic composition, including Sn-0.7Cu, Sn-3.5Ag, and SAC305 solders [10-14]. Moreover, alloying elements (e.g., Zn, Ce, Al, Ag, In) were incorporated into the solder to improve the corrosion resistance [2,3,13,15,16]. Nonetheless, alloying elements contributed to complex microstructures and subsequent lower mechanical properties. Like the ternary solder system, high contents of alloying elements in the Sn-based solders induced several brittle intermetallic phases that decrease the strength and ductility of both pre-soldering solders and the solder joints [9,17-19].

Based on the previous reports [20,21], the Sn-Ag solder system exhibited better corrosion resistance than the lead-based, Sn-Cu, and SAC solders. Furthermore, a fine intermetallic phase of Ag₃Sn was only formed during soldering, which in turn hindered the growth of brittle intermetallic layer of Cu₆Sn₅ [14,22]. Specifically, near eutectic Sn-3.5Ag (wt.%) lead-free solder is commonly used in the electronic industry due to its lowest melting temperature of 221 °C and uniformly distributed fine-eutectic microstructure, compared Sn-Ag solders with the hypoeutectic and hypereutectic composition [5]. However, during the soldering, variation in cooling rates affected the eutectic reaction of Sn-3.5Ag solder, resulting in non-uniformly distributed fine-eutectic microstructure [23,24]. Moreover, there exists limited research on the relationship between the Ag contents and microstructure as well as corrosion behavior in Sn-xAg solder systems. This research thus focuses on the Sn-xAg solder. Specifically, the effects of different Ag contents on the microstructure and corrosion behavior of the Sn-xAg lead-free solders, and the intermetallic layer of the solders with Cu substrate were investigated. The Ag contents (x), based on the commercially available Sn-Ag lead-free solders, were varied at 0, 3.0, 3.5, 4.0, and 5.0 wt.%. The low-Ag content (1-2 wt.%) Sn-rich solders were deliberately excluded due to the higher sensitivity to the cooling rate compared with the near-eutectic and high-Ag-rich solders [25].

2 Experimental

In this work, the as-cast Sn-xAg lead-free solders were acquired from Ultracore (Thailand), where x is the Ag content at 0 (pure Sn), 3.0, 3.5, 4.0, and 5.0 wt.% (Table 1). Prior to analysis, the solder ingot was sliced into square-shaped thin-sheet specimens of 15 mm× 15 mm×4 mm using a low-speed high-precision cut-off machine (Minitom, Struers). The specimens were ground off by abrasive paper and polished with 1 μm and 0.3 μm alumina suspensions (AP-D suspension, Struers). The polished specimens were then cleaned with ethanol in an ultrasonic cleaner and air-dried.

Table 1 Chemical composition of experimental Sn-xAg solders

In the microstructure analysis, the sonicated specimens were ion-milled using low-energy Ar ion gun with 6 keV for 5 min (IM4000, Hitachi). The as-cast solder matrix and intermetallic compounds of the experimental solders (0Ag, 3.0Ag, 3.5Ag, 4.0Ag, 5.0Ag solders) were characterized by a scanning electron microscope (SEM; SU3500, Hitachi). The chemical composition of intermetallic compounds was determined by field emission scanning electron microscope (FE-SEM; JSM-760F, JEOL) equipped with energy- dispersive X-ray spectroscopy (EDS).

The corrosion behavior of the experimental solders was determined by potentiodynamic polarization in accordance with the ASTM G 102 standard. Prior to the corrosion experiment, the sonicated specimens were retained for 24 h and then rinsed with ethanol. The polarization test was carried out using three-electrode polarization cell, comprising a working electrode (i.e., Sn-xAg solders), counter electrode (platinum wire), and reference electrode (saturated calomel electrode, SCE). A 3.5 wt.% NaCl solution (500 mL) was used as electrolyte (without reuse). In the experiment, the working electrode was immersed in the NaCl solution for 120 s to realize the open circuit potential (OCP). The potential for corrosion test was varied between -1.2 and 0 V at a rate of 0.033 V/s. The experiments were individually carried out in triplicate. The morphology and chemical composition of corrosion products were subsequently characterized by SEM-EDS.

The interfacial reaction on Cu substrate was determined in accordance with the JIS Z3198-3 standard. The Sn-xAg solders and Cu substrate were 6 mm × 2 mm and 15 mm × 15 mm × 0.5 mm, and the soldering condition was 260 °C for 30 s [26]. The solder wettability and the morphology and thickness of the intermetallic layer between Sn-xAg solder and Cu substrate were then determined.

3 Results and discussion

3.1 Microstructure of as-cast Sn-xAg solders

Figure 1 illustrates the SEM and BSE-SEM microstructures of the 0Ag solder (pure Sn), consisting of large-equiaxed grains of 100 μm on average, with the contrast indicating variation in crystallographic orientation (Fig. 1(b)). Figures 2(a-d) depict the SEM images of as-cast 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders, respectively. The microstructure analysis revealed β-Sn phase (dark color area) and fine lamellar eutectic networks of β-Sn and eutectic Ag₃Sn phases (light color area). Specifically, in the 3.0Ag and 3.5Ag solders, the β-Sn phase was surrounded by fine lamellar networks (Figs. 2(a, b)), while large plate-like Ag₃Sn phase was formed in the 4.0Ag and 5.0Ag solders (Figs. 2(c, d)).

Based on the phase diagram of Sn-Ag solder system, the solder microstructure is of nearly full eutectic structure for Sn-Ag solders with near eutectic composition (approximately Sn-3.5Ag) [27]. Nevertheless, the microstructure of the as-cast Sn-3.5Ag solder exhibited a hypoeutectic microstructure with β-Sn phase as the dominant phase. The hypoeutecticity was attributable to the rapid cooling during the solder fabrication, consistent with Refs. [23,28] in which the rapid cooling led to the formation of finely distributed Ag₃Sn (i.e. eutectic Ag₃Sn). Meanwhile, the microstructures of the as-cast 4.0Ag and 5.0Ag solders that are of hypereutectic composition, were of large plate-like Ag₃Sn phase (primary Ag₃Sn) [29].

Table 2 lists the EDS chemical composition of fine and large plate-like Ag₃Sn in the as-cast 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders. The EDS results indicated that the contents of Ag in fine (eutectic) and large plate-like (primary) Ag₃Sn were 63.25-70.68 wt.% and 63.97- 70.38 wt.%, respectively, with Sn accounting for the rest. The EDS analysis revealed a close resemblance between the chemical compositions of eutectic and primary Ag₃Sn.

Fig. 1 Microstructures of ion-milled polishing pure Sn (0Ag solder)

Fig. 2 SEM microstructures of as-cast solders

Table 2 Chemical composition of eutectic and primary Ag₃Sn in as-cast 3.0Ag, 3.5Ag, 4.0Ag and 5.0Ag solders (wt.%)

The area fractions of eutectic and primary Ag₃Sn of the 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders, using ImageJ image processing software, were (10.8±2.6)%, (9.4±0.8)%, (7.8±1.7)%, and (18.3±0.1)%, respectively. Specifically, the elevated Ag content (5.0Ag solder) contributed to the considerably large fraction of Ag₃Sn phase, consistent with Ref. [29], who reported that the large fraction of Ag₃Sn was due to the formation of primary Ag₃Sn which is normally present in Sn-Ag solders with hypereutectic composition. Given that Ag₃Sn is hard and brittle, with the elastic modulus (E) of 90 GPa compared with that of β-Sn phase of 52-68 GPa [6], the Ag content in Sn-xAg solders should thus be taken into account to mitigate the formation of coarse microstructure attributable to the presence of plate-like Ag₃Sn, which in turn adversely alters on the mechanical properties of the solders [29-31].

3.2 Corrosion behavior of as-cast Sn-xAg solders

Table 3 Parameters of corrosion behavior of as-cast Sn-xAg solders in 3.5 wt.% NaCl solution

Fig. 4 SEM images of corrosion products on surface of as-cast solders

Fig. 5 Representative images of film cracking (a) and pitting corrosion (b)

Figure 3 illustrates the representative potentio- dynamic polarization curves of as-cast 0Ag, 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders in 3.5 wt.% NaCl solution. The corrosion behaviors of the solders were similar, consisting of a cathodic and anodic reaction. Table 3 lists the corrosion parameters, including current density (J_corr), corrosion potential (φ_corr), anodic Tafel constant (b_a) and cathodic Tafel constant (b_c) of the as-cast Sn-xAg solders in 3.5 wt.% NaCl solution, derived from Tafel fitting of the polarization curves, polarization resistance (R_p) and the corrosion rate (mm/a), determined in accordance with ASTM G3-89 (2010) and G102-89 (2015) standard [32,33]. Polarization resistances (R_p) of the experimental solders, which are inversely correlated with the rate of corrosion, were insignificantly different, varying between 8.72 and 12.42 kΩ. By comparison, however, R_p of the Ag-doped solders (3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag) was higher than that of the non-Ag solder (0Ag solder), indicating higher corrosion resistance due to the presence of Ag in Sn solders. In other words, the corrosion rate decreased as Ag content increased. Lower corrosion rate in the Ag-doped solders could be attributed to the presence of Ag₃Sn phase which is more thermodynamically stable than Sn-rich phase [10,12]. However, the 5.0Ag solder exhibited higher corrosion rate, probably due to the presence of the large Ag₃Sn fraction, leading to galvanic corrosion and selective oxidation mechanism [12,34]. The galvanic corrosion occurred between the Ag₃Sn and β-Sn phases and selective oxidation of Sn took place in the Ag₃Sn phase. In addition, similarly to R_p, φ_corr was positively shifted as the Ag content increased, suggesting a delayed transition from cathodic to anodic reaction. The cathodic reaction associated with all experimental solders commenced at the corrosion potential of -1.2 V and ended at about -0.7 V. Specifically, in the cathodic reaction, the oxidation and hydrogen evolution reactions were the dominant reactions [11,13]. In the anodic reaction, a passivation was observed with the passivation potential (φ_p) between -0.706 and -0.68 V (Table 3), followed by a rapid increase in J_corr, resulting in the breakdown of passive film and pitting corrosion [3,14].

Fig. 3 Representative potentiodynamic polarization curves of as-cast 0Ag, 3.0Ag, 3.5Ag, 4.0Ag and 5.0Ag solders in 3.5 wt.% NaCl solution

Figures 4(a-e) respectively depict the SEM images of corrosion products on the surface of as-cast 0Ag, 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders. The corrosion products exhibited a similar morphology in which a mixture of platelet-like and needle-like structures with random orientation was observed on the solder surface, consistent with Refs. [2,3,11,14]. Figure 5 depicts the cracking of passive film and pitting corrosion observed on the surface of all experimental solders. Table 4 lists the EDS chemical composition of the corrosion products on the surface of Sn-xAg solders, given variable Ag contents (0, 3.0, 3.5, 4.0, 5.0 wt.%). The corrosion products were mainly Sn, Cl, and O, with relatively small amounts of Ag. Given the presence of Cl and O, Sn reacted with Cl and O to form tin-oxide chloride hydroxide (Sn₃O(OH)₂Cl₂) [11,13], which could be expressed as

3Sn+4OH^--6e+2Cl^-=Sn₃O(OH)₂Cl₂+H₂O         (1)

The presence of Ag in the corrosion product (Table 4) was attributable to selective oxidation as a result of the lower Gibbs free energy of SnO₂ (-515.8 kJ/mol) relative to that of Ag₂O (-11.2 kJ/mol) [12,34,35]. Specifically, due to the lower Gibbs free energy, the oxidation of Sn in the eutectic and primary Ag₃Sn phases preceded that of Ag, which could be expressed as

Ag₃Sn+O₂→SnO₂+3Ag                      (2)

2Ag₃Sn+O₂→2SnO₂+6Ag                    (3)

Table 4 Chemical composition of corrosion products on surface of experimental solders

3.3 Effect of Ag on formation of intermetallic layer

Figure 6 compares the wettability of the experimental solders as a function of spreading factor. The average spreading factors of the 0Ag, 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders on Cu substrate were 94.5%, 94.6%, 93.08%, 94.2%, and 95.06%, respectively. The insignificant difference among the spreading factors indicated that Ag had negligible effects on the wettability of Sn-xAg solders on Cu substrate.

Fig. 6 Spreading factor of 0Ag, 3.0Ag, 3.5Ag, 4.0Ag and 5.0Ag solders on Cu substrate

Figures 7(a)-(e) respectively illustrate the morphologies of the intermetallic layer of the 0Ag, 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders with Cu substrate. A scallop-shaped intermetallic layer, consisting of Sn and Cu, was observed at the interfacial layer between the solders and Cu substrate. The microstructures of the pre-and post-soldering solders were in good agreement, whereby no Ag₃Sn was present in the 0Ag solder joint and the finely distributed Ag₃Sn (eutectic Ag₃Sn) was observed in the 3.0Ag and 3.5Ag joints. Besides, the primary Ag₃Sn (large plate-like Ag₃Sn) and eutectic Ag₃Sn were formed in the 4.0Ag and 5.0Ag joints. The compositions of Cu and Sn were approximately 56 wt.% and 39 wt.% for all the experimental solders, indicating the formation of Cu₆Sn₅, which could be expressed as

6Cu+5Sn→Cu₆Sn₅                              (4)

In the experiment, no Cu₃Sn phase was detected at the interface. The absence of Cu₃Sn phase was probably attributed to the short soldering time, consistent with Ref. [4]. In addition, from the thermodynamic viewpoint, the activation energy of the formation of Cu₃Sn phase is considerably higher than that of Cu₆Sn₅ phase [4,26,36], giving rise to the formation of Cu₆Sn₅ during soldering, as expressed in Eq. (4).

Figure 8 illustrates the cross-sectional EDS map analyses of the 3.5Ag and 5.0Ag solder joints. Specifically, the 3.5Ag solder joint contained only the finely distributed Ag₃Sn (eutectic Ag₃Sn), while both the eutectic and primary Ag₃Sn phases were present in the 5.0Ag solder joint. By comparison, Ag₃Sn was more prevalent than Cu₆Sn₅ in the solder matrix.

Figure 9 compares the intermetallic-layer thickness of the 0Ag, 3.0Ag, 3.5Ag, 4.0Ag, and 5.0Ag solders, with the respective thicknesses of 2.05, 1.14, 1.58, 1.78, and 1.86 μm. By comparison, the intermetallic layer of the Ag-doped solders was thinner than that of the non-Ag solder (0Ag solder). The smaller thickness was attributable to the presence of finely distributed (eutectic) and large plate-like (primary) Ag₃Sn in the Ag-doped solders (Figs. 7 and 8). Specifically, the fine Ag₃Sn particles (eutectic Ag₃Sn) precipitating on the surface of solidified and grain boundaries of Cu₆Sn₅ impeded the growth of Cu₆Sn₅ grains. In addition, the fine Ag₃Sn particles pinned the diffusion channels, preventing Cu atoms diffusing from the Cu substrate and subsequently hindering the formation of Cu₆Sn₅ [37]. The phenomenon is referred to as the shielding effect of Ag₃Sn on the growth of Cu₆Sn₅ grains [22,38,39].

Fig. 7 Morphologies of intermetallic layer of 0Ag (a), 3.0Ag (b), 3.5Ag (c), 4.0Ag (d), 5.0Ag (e) solders with Cu substrate

Fig. 8 Cross-sectional EDS map analysis of 3.5Ag (a) and 5.0Ag (b) solder joint

Fig. 9 Thickness of intermetallic layer (Cu₆Sn₅) of experimental solders

However, the shielding effect was restricted by the presence of large plate-like Ag₃Sn with increase in the Ag content (i.e. 4.0Ag and 5.0Ag solders), resulting in the increased thickness of the intermetallic layer.

Moreover, the competitive growth between Cu₆Sn₅ and Ag₃Sn phases during soldering influenced the shielding effect of Ag₃Sn [37]. Based on the activation energy, the formation of Ag₃Sn requires less energy than that of Cu₆Sn₅ [40,41], contributing to the prevalence of Ag₃Sn with finer and more uniformly distributed microstructure. The ubiquity of fine (eutectic) Ag₃Sn is advantageous to the improved reliability of Sn-xAg solder system and Sn-xAg/Cu joints as well as the enhanced corrosion resistance. In contrast, the presence of large plate-like (primary) Ag₃Sn accelerates the failure of Sn-xAg/Cu joints [29]. Specifically, the tensile and shear strengths of the joints decrease due to the brittleness of Ag₃Sn, which in turn adversely affects the durability and reliability of solder joints [4,29]. To mitigate the formation of large plate-like Ag₃Sn phase, the Ag content in Sn-Ag solders should thus be of hypoeutectic composition between 3.0 and 3.5 wt.%.

4 Conclusions

(1) The morphology of Ag₃Sn phase evolved from the fine particles to large plate-like crystals as the Ag content increased. Specifically, in the 3.0Ag and 3.5Ag solders, β-Sn phase was surrounded by eutectic networks, and large plate-like Ag₃Sn was formed in the 4.0Ag and 5.0Ag solders. In addition, the elevated Ag content contributed to the considerably large fraction of Ag₃Sn phase.

(2) The Ag content slightly affected the corrosion behavior of the as-cast Sn-xAg solders. In addition, the transition from a cathodic to anodic reaction was delayed after the introduction of Ag into Sn-xAg solders resulting in the decreasing of the corrosion rate.

(3) The analysis of the intermetallic layer of the 3.0Ag and 3.5Ag solders revealed the shielding effect of Ag₃Sn on the growth of Cu₆Sn₅ grains, whereby the fine Ag₃Sn particles in the eutectic networks precipitating on the surface of Cu₆Sn₅ obstructed the development of Cu₆Sn₅ grains, and pinned the diffusion channels preventing Cu atoms diffusing from the Cu substrate. However, the shielding effect was blunted by the presence of large plate-like Ag₃Sn in the 4.0Ag and 5.0Ag solders, resulting in the increased thickness of the intermetallic layer.

Acknowledgements

The authors gratefully acknowledge Ms. Sukanya Unarat for her assistance with metallographic specimen preparation for SEM observation. The authors also would like to thank Ms. Suwattana Thongkarm and Mr. Sakul Chotthanasak of Coax group Corporation Ltd., Thailand, for contributing to ion-milling polishing and the SEM analysis. Thanks also to the Center for Advanced Studies in Nanotechnology for Chemical, Food and Agricultural Industries, Kasetsart University Insititute of Advanced Studies (KUIAS) for valuable help in the EDS analysis.

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Sn–xAg无铅焊料中Ag 含量对其显微组织、腐蚀行为和与Cu基体界面反应的影响

Phacharaphon TUNTHAWIROON, Kannachai KANLAYASIRI

Department of Industrial Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand

摘  要：研究不同Ag含量(0、3.0%、3.5%、4.0%和5.0%，质量分数)对预焊Sn-xAg无铅焊料显微组织和腐蚀行为的影响，以及对焊料与铜基体间形成金属间化合物界面层的影响。Ag 含量对焊料中Ag₃Sn相的形貌有一定的影响。显微组织分析结果表明，在3.0Ag和3.5Ag 焊料中，β-Sn相被共晶组织网络所包围；在4.0Ag和5.0Ag 焊料中，形成大的片状Ag₃Sn相。但是，动电位极化曲线测试结果显示，Ag含量对铸态焊料腐蚀行为的影响甚微。与铜基体焊接后，在Sn–xAg/Cu界面处仅形成单层的Cu₆Sn₅金属间化合物。与未掺Ag的焊料相比，掺Ag的焊料与基体的界面中形成的Cu₆Sn₅ 金属间化合物层更薄。与4.0 Ag和5.0Ag焊料中形成的大片状Ag₃Sn相比，在3.0Ag和3.5Ag焊料中析出的共晶网络中细小的Ag₃Sn颗粒沉淀，能更有效地抑制Cu₆Sn₅晶粒的生长。

关键词：Sn-Ag无铅焊料；显微组织；Ag₃Sn金属间化合物相；腐蚀行为；Cu₆Sn₅金属间化合物层；润湿性

(Edited by Xiang-qun LI)

Corresponding author: Phacharaphon TUNTHAWIROON; E-mail: phacharaphon.tu@kmitl.ac.th

DOI: 10.1016/S1003-6326(19)65076-4