Sn-0.7Cu-10Bi Solder Modification Strategy by Cr Addition

Abstract

The application of Sn-0.7Cu-based composite solder in electronic packaging is limited due to its high melting point, poor wettability and low mechanical properties. Herein, we propose a strategy of adding Bi and Cr to improve the solderability of Sn-0.7Cu lead-free solder. The results show that the addition of Bi reduces the melting point of the composite solder. Moreover, the Cr particles adsorb at the interface between solder and substrate, thereby reducing the wetting angle of the composite solder. The wetting angle reaches a minimum value of 25.84° when the content of Cr is 0.2 wt.%. The addition of Bi and Cr changes the microstructure of the composite solder. The nucleation rate of β-Sn rises with the increase of Cr content, thus decreasing the size of β-Sn. Furthermore, the addition of Bi and Cr reduces the thickness of the intermetallic compounds (IMCs). This is due to the adsorption of Bi and Cr at the interface of IMCs hinders the atomic diffusion channels and inhibits the growth of IMCs.

1. Introduction

Electronic components are gradually developing towards miniaturization and densification, which puts forward higher requirements on the melting point, wettability and mechanical properties of solder [1,2,3]. Traditional SnPb solder is banned for the toxicity of Pb [4,5]. Therefore, it is necessary to develop lead-free solders to replace SnPb. At present, the main lead-free solder include SnCu [6,7,8], SnAg [9,10], SnAgCu [11,12], SnZn [13], SnBi [14], SnIn [15], etc. SnAg-based lead-free solders are widely used in electronic packaging due to their good electrical conductivity, wettability and creep resistance. However, the high cost of Ag impedes the development of SnAg-based lead-free solders. Although the melting point of SnZn lead-free solder is the closest to SnPb, the Zn element is very active and could be easily oxidized at high temperature, hence demanding precise management of soldering conditions [16]. The SnCu lead-free solder is widely concerned due to its low cost, high strength and low resistivity [17]. Nonetheless, the disadvantages of SnCu solder such as high melting point, poor creep resistance and low mechanical properties limit its application [18,19].

To date, researchers have mainly improved the melting point, wettability and mechanical properties of lead-free solder through particle strengthening (Al₂O₃, SiO₂, CeO₂) and microalloying (Ni, Co, Zn, Ag, Al) [20,21,22]. Kang et al. [23] reported that the addition of different proportions of Bi and Zn to SnAgCu lead-free solder decreases the electrical conductivity and increases the tensile strength. Chen et al. [24] systematically studied the microstructural evolution and shear properties of SAC105-xBi solder joints, and found that the Bi element can play the role of solid solution strengthening and second phase strengthening in SAC105-xBi solder joints. With the increase of Bi content, the shear strength of solder joints increases and the thickness of intermetallic compounds (IMCs) decreases. Huang et al. [25] investigated the effects of different ratios of Bi on the thermal properties, mechanical properties and corrosion resistance of Sn-2Cu solders. It is found that the addition of 5 wt.% Bi can lower the melting point of the solder from 239.7 °C to 226.6 °C, refine the grain size of Cu₆Sn₅ and reach the maximum tensile strength of solder. Numerous researchers have improved the wettability and mechanical properties of lead-free solder by adding Cr elements [26]. Song et al. [27] added different proportions of Cr to Sn58Bi lead-free solder, and found that the addition of Cr significantly optimized the structure of the Sn58Bi composite solder and reduces the thickness of the IMCs. Son et al. [28] added 0.2 wt.% Cr to SnCu lead-free solder to inhibit the growth of IMCs and improve the shear strength of solder joints. Bang et al. [29] found that the adding Cr to Sn0.7Cu solder effectively inhibited the growth of IMCs layers and the formation of Kirkendall voids, which improved the reliability of the solder.

In this paper, Sn-0.7Cu-10Bi-xCr (x = 0.05, 0.1, 0.2 and 0.3 wt.%) composite solder were successfully prepared. Wetting angle of composite solder was measured by wetting angle tester. The microstructure, melting point and element distribution of the solder alloy were calculated by scanning electron microscopy (SEM), differential scanning calorimeter (DSC) and electron probe micro-analyzer (EPMA) to evaluate the effect of Bi and Cr addition on the melting point, wettability and solderability of Sn-0.7Cu lead-free solder.

2. Materials and Methods

2.1. Preparation of Sn-0.7Cu-10Bi-xCr Composite Solders

Sn (99.5%), Cu (99.99%), Bi (99.99%) and Cr (99.5%) powders with an average size of 40–60 µm were weighed based on different proportion. The weighed metal particles and ethanol were put into a ball mill and milled at 500 r/min for 6 h. Subsequently, the ethanol solution was removed by drying at 60 °C for 2 h in an oven. The obtained Sn-0.7Cu-10Bi-xCr powder was pressed into a 5 g block under pressure of 10 MPa by a tablet press. Thence, the solder block was placed in a tube furnace and heated to 550 °C at a heating rate of 8 °C/min in an Ar atmosphere for 1 h. Finally, Sn-0.7Cu-10Bi-xCr composite solder was obtained. The obtained composite solder (1 cm × 1 cm × 0.2 cm) was cured with epoxy resin and curing agent. The cured composite solder was sanded with 320–2000 mesh SiC sandpaper and then polished with alumina (Al₂O₃) solution of 0.3 µm size to eliminate the scratches on the surface of the composite solder. Afterward, the surface of the composite solder was etched with etching solution (2 vol.% HCl + 5 vol.% HNO₃ + 93 vol.% CH₃OH) to completely expose structure of the composite solder. The microstructure of the composite solder was characterized by scanning electron microscopy (SEM, Quanta 250, FEI, Hillsboro, OR, USA).

2.2. Soldering of Sn-0.7Cu-10Bi-xCr Composite Solders

The surface of the copper plate (2 cm × 2 cm × 0.2 cm) was polished with 320–2000 mesh SiC sandpaper to remove the oxide film, then washed in ethanol several times and dried. The composite solder was cut into similar spheres with a mass of 0.3 g. The surface of the composite solder and copper plate was coated with rosin flux and then soldered in a reflow oven at 250 °C for 5 min. The soldered cross section was cured with epoxy resin and curing agent. The cured cross section was polished with 320–2000 mesh SiC sandpaper and Al₂O₃ solution of 0.3 µm size to eliminate scratches. Following that, the soldered cross section was etched with etching solution, washed several times with ethanol and dried for later use. The microstructure and elemental distribution of the soldered cross section were characterized by scanning electron microscopy and electron probe micro-analyzer (EPMA, EPMA-8050G, Shimadzu, Kyoto, Japan).

2.3. Melting Point Test

The melting temperature of Sn-0.7Cu-10Bi-xCr composite solder was measured by differential scanning calorimetry (DSC, Q2000, TA, New Castle, DE, USA). The composite solder was cut into 5–15 mg pellets and placed in a pure aluminum crucible, and then heated up to 280 °C under N₂ atmosphere at a heating rate of 5 °C/min.

2.4. Wetting Test

The wetting angle of Sn-0.7Cu-10Bi-xCr composite solder was measured by a wetting angle tester (JGW-360A, Chenghui, Chengde, China). The solder joint was cleansed with ethanol and placed on the sample table, and then the camera focus of the wetting angle tester was adjusted to obtain a clear image of the solder joint, after which the wetting angle of the composite solder was calculated.

3. Results and Discussion

3.1. Melting Point of Composite Solders

The melting point of the composite solder is close to the eutectic temperature of SnPb [30], making it adaptable to the existing soldering equipment and soldering process. Besides, the lower melting point of composite solder attenuates damage to electronic components during electronic packaging. Figure 1 shows the DSC curves of the composite solder. The DSC curve has only one peak, id est, an endothermic peak. The onset temperature (T_Onset) and peak temperature (T_Peak) in the Figure 1 are the initial melting temperature and the terminal melting temperature of the composite solder, respectively. As shown in the Figure 1, the initial melting temperature of Sn-0.7Cu is 227.05 °C, which is close to the eutectic temperature of SnCu. The Bi element has higher activity and can lower the melting point of the solder [31]. With the addition of Bi and Cr, the melting point of the composite solder decreases rapidly. The melting points of the composite solder are 212.24 °C, 210.35 °C, 212.89 °C and 211.13 °C when the addition of Cr is 0.05 wt.%, 0.1 wt.%, 0.2 wt.% and 0.3 wt.%, correspondingly, which is about 15 °C lower comparing to that of the Sn-0.7Cu solder. It can be observed from the Figure 1 that the peak temperatures of Sn-0.7Cu-10Bi-xCr are 229.77 °C, 216.55 °C, 213.26 °C, 216.65 °C and 215.59 °C, respectively. The difference between the initial melting temperature and the terminal melting temperature of the composite solder is 2.72 °C, 4.31 °C, 2.91 °C, 3.76 °C and 4.46 °C, respectively, suiting the practical application.

3.2. Wettability of Composite Solders

Excellent wettability of lead-free solder is a sine qua non to ensure the interconnection between electronic components and the substrate, and it is also an important criterion to judge the quality of solder joint [32]. Figure 2 shows the wetting angle of Sn-0.7Cu-10Bi-xCr composite solder on Cu substrate after reflowed at 250 °C for 5 min. As shown in Figure 2, Sn-0.7Cu lead-free solder has the largest wetting angle, which is 29.64°. With the increase of Cr content, the wetting angle of composite solder gradually narrows. When the Cr content was 0.2 wt.%, the wetting angle reached a minimum value of 25.84°, 12.82% lower compared to the wetting angle of Sn-0.7Cu lead-free solder. After that, the wetting angle became larger with the increase of Cr content, but it was still lower than that of Sn-0.7Cu lead-free solder. The results demonstrate that the addition of Cr significantly improves the wettability of the composite solder. The reason why the wetting angle first decreases and then increases is that an appropriate amount of highly active Cr particles adsorbing at the interface between the solder and the substrate reduces the surface tension [33]. Nevertheless, adding excessive amount of Cr particles will form agglomeration. As a result, the activity of Cr particles decreases and thus will not adsorb at the interface between the solder and the substrate. Therefore, the wetting angle increases when the Cr content reaches 0.3 wt.%.

3.3. Microstructure of Composite Solders

The SEM images of Sn-0.7Cu-10Bi-xCr (x = 0, 0.1, 0.2 and 0.3 wt.%) composite solder are shown in Figure 3. As shown in Figure 3a, the microstructure of Sn-0.7Cu lead-free solder has a large number of rod-like and dots-like Cu₆Sn₅ distributed on the grain boundaries of β-Sn. Figure 3b–e exhibit the microstructure changes of the composite solders with different amounts of Bi and Cr added. The white area is the rich-Bi phase, and the gray area is the β-Sn. As shown in Figure 3b, Bi was mainly distributed at the grain boundaries of the β-Sn grains and agglomerated into a clump. With the increase of Cr content, the Bi clumps and β-Sn grains were gradually refined. When the Cr content was 0.2 wt.%, the Bi clump was eliminated and the best refinement of β-Sn grains was obtained. Subsequently, the content of Cr continued to increase, the Bi agglomerated again and the size of the β-Sn grains gradually increased. According to the inhomogeneous nucleation theory, the addition of Cr increases the nucleation rate of β-Sn and thereupon the size of β-Sn grains decreases [34]. However, when the Cr content was excessive, the agglomeration occured between the highly active Cr particles and the non-uniform nucleation effect was weakened thus the size of the β-Sn grains increased.

3.4. Soldering Interface of Composite Solders

The cross-sectional morphology of the composite solder after reflowing at 250 °C for 5 min is pictured in Figure 4 and the image of the thickness of the IMCs is shown in Figure 5. As shown in Figure 4a, the IMCs of Sn-0.7Cu solder are Cu₆Sn₅, which is scallop shape and 5.64 μm thick. The IMCs are in brittle phases, and excessive thickness can easily generate stress concentrations to make the solder joints fail. The scalloped IMCs changed to a planar shape with the addition of Bi and Cr, and the thickness of the IMCs decreased accordingly. As shown in Figure 4b–e, Bi particles aggregated at the interface of the Cu₆Sn₅ IMCs to inhibit the interdiffusion between composite solder and substrate, thereby inhibiting the growth of the IMCs [35]. In addition, the growth of IMCs was inhibited by the addition of Cr particles. The thickness of the IMCs gradually decreased with increasing Cr content. When the Cr content reached 0.2 wt.%, the thickness of the IMCs reached a minimum value of 3.57 μm, 36.70% lower than that of Sn-0.7Cu solder. And the thickness of the IMCs gradually increased when the Cr content continued to increase. The addition of appropriate amount of Cr particles made Cr particles adsorb at the interface or grain boundary of IMCs, which hindered the atomic diffusion channel and inhibited the growth of IMCs [36]. When the excessive amount of Cr was added, the Cr particles agglomerated and the activity decreased so that they cannot adhere to the grain boundaries of the IMCs. Consequently, the thickness of the IMCs increased.

Figure 6 is the EPMA image of the soldering interface of Sn-0.7Cu-10Bi-0.2 wt.% Cr composite solder. The Bi element was widely distributed at the interface of the IMCs, further confirming that the Bi element acts as a barrier layer to inhibit element diffusion between the solder and the Cu substrate. The Cr element was uniformly distributed, which handicapped the atomic diffusion channels and the growth of IMCs.

4. Conclusions

In this paper, the melting point, wettability, microstructure and soldering interface of a new type low-temperature Sn-0.7Cu-10Bi-xCr lead-free solder are studied. The results demonstrate that the melting point of the composite solder decreases by approximately 15 °C after adding 10 wt.% Bi. Over and above, the addition of Cr can improve the wettability of the composite solder. The wetting angle of the composite solder gradually decreases with the increase of Cr content, and the wetting angle of the composite solder reaches the minimum value of 25.84° when 0.2 wt.% Cr is added. The microstructure of Sn-0.7Cu-10Bi-xCr lead-free solder is composed of rich-Bi phase, β-Sn and Cu₆Sn₅. The Bi clumps and β-Sn grains are gradually refined as the of Cr content increases, and the refinement effect is most obvious when the Cr content is 0.2 wt.%. Furthermore, the addition of Bi and Cr inhibits the growth of IMCs. When the Cr content is 0.2 wt.%, the thickness of the IMCs reaches a minimum value of 3.57 μm, 36.70% lower than that of Sn-0.7Cu solder.

Author Contributions

Conceptualization, P.H.; methodology, P.H.; validation, Z.L. and X.Z.; formal analysis, P.H.; investigation, Z.L.; resources, X.Z.; data curation, P.H.; writing—original draft preparation, P.H.; writing—review and editing, Z.L.; visualization, X.Z.; supervision, X.Z.; project administration, P.H. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Figure 1. The DSC curves of Sn-0.7Cu-10Bi-xCr (x = 0.05, 0.1, 0.2 and 0.3 wt.%).

Figure 2. Wetting angle of (a) Sn-0.7Cu, wetting angle of Sn-0.7Cu-10Bi-xCr: (b) 0.05 wt.%, (c) 0.1 wt.%, (d) 0.2 wt.% and (e) 0.3 wt.%.

Figure 3. Microstructure of (a) Sn-0.7Cu, microstructure of Sn-0.7Cu-10Bi-xCr: (b) 0.05 wt.%, (c) 0.1 wt.%, (d) 0.2 wt.% and (e) 0.3 wt.%.

Figure 4. The cross-sectional image of (a) Sn-0.7Cu, the cross-sectional image of Sn-0.7Cu-10Bi-xCr: (b) 0.05 wt.%, (c) 0.1 wt.%, (d) 0.2 wt.% and (e) 0.3 wt.%.

Figure 5. Thickness bar chart of IMC layers at the interface.

Figure 6. EPMA of Sn-0.7Cu-10Bi-0.2 wt.% Cr.

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