Improvement on the mechanical properties of eutectic Sn58Bi alloy with porous Cu addition during isothermal aging

Figure 2 shows the microstructure of three kinds of the as-soldered and aged solder joints. As shown in figure 2(a1), the SnBi alloy presents a eutectic lamellar structure composed of β-Sn and Bi-rich phases, in which the dark and white regions represent β-Sn and Bi-rich phases respectively in the solder bulk. Meanwhile, it is clearly that amounts of Bi particles distribute on the β-Sn dendrites. With the addition of 110 ppi porous Cu, the SnBi solder bulk presents some interesting regions which are enclosed by Cu frames, and its microstructure is significantly refined in such regions. It can be inferred that the growth of dual phases is inhibited by Cu frames in these regions. In contrast, the similar regions are not detected in the SnBi@500P-Cu/Cu solder joint as shown in figure 2(c1). The distribution of copper frames is dense and uniform in the SnBi@500P-Cu solder bulk. Therefore, the solder bulk shows more homogeneous microstructure in the varieties of the regions of the SnBi@500P-Cu/Cu composite solder joint. Moreover, Cu₆Sn₅ IMC layers form between the solder bulk and Cu frames in the two solder@P-Cu/Cu composite solder joints.

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The microstructure of all kinds of the solder joints shows an increasing tendency during the isothermal aging process. As revealed by Belyakov [16] et al, Bi atoms can precipitate out of β-Sn phase under the aging condition. These Bi particles with high Gibbs energy attain a state of chemical equilibrium by reducing the number of grain boundaries, which accelerates the diffusion of Sn and Bi atoms. Thus, it has a positive influence on the growth of dual phases. Moreover, it can be seen from figures that the phenomenon of microstructure coarsening is obvious after merely aging 7 days, while the growth rate of the dual phases decreases with the extension of aging time. As shown in the figures of solder joints after aging for 14, 21 days, it is clear that the quantity of Bi particles precipitated from β-Sn phase decreases, indicating that the growth of β-Sn and Bi-rich phases requires higher thermal driving force to maintain. Therefore, the degree of microstructure coarsening of solder joints after aging for 14 and 21 days is lower than that for merely 7 days.

As shown in figures 2(b2)–(b4), not only the dual phases of regions enclosed by Cu frames coarsen in the SnBi@110P-Cu/Cu solder joint, but also the thickness of the Cu₆Sn₅ IMC layer between Cu frames and solder bulk increases as the aging time extends. And the proportion of Bi-rich in such regions gradually increases during the aging process. It is noted that almost merely Bi-rich phase and IMC layer remain after aging for 21 days. This result can be confirmed by the element mapping of the SnBi@110P-Cu/Cu composite solder joint in figure 3. Compared with the SnBi@110P-Cu/Cu solder joint, the IMC layer on the surface of Cu frames also changes to be thicker in the SnBi@500P-Cu/Cu solder joint after aging. Even the adjacent Cu frames are bonded by the IMC layers in the solder bulk after aging for 21 days. In addition, a new IMC layer occurs between the Cu frames and Cu₆Sn₅ IMC in the SnBi@P-Cu/Cu composite solder bulk. The EDS is carried out to identify the composition of the new IMC layer as Cu₃Sn.

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The morphology of IMC layer at the interface near Cu substrate of the three kinds of solder joints is presented in figure 4. As shown in figure 4(a1), the as-soldered SnBi/Cu solder joint shows a thin scallop-shaped interfacial IMC layer. As a member of researchers reported, the composition of the IMC layer was confirmed as Cu₆Sn₅ [6, 11, 17]. Figure 5 exhibited the relationships between IMC growth and the square root of the aging time. With the addition of porous Cu, it can be seen from the figure that the IMC layer at the soldering interface is slightly higher than that of the SnBi/Cu solder joint. Our previous study found that the increase of Cu atoms at the soldering interface caused by porous Cu was the main factor for the rise of interfacial IMC layer [15].

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In the SEM image of the aged solder joints, it can be seen clearly that the thickness of IMC layers in all kinds of solder joints displays the upward tendency with the prolonged aging time. Among these, the SnBi/Cu solder joint exhibits the maximum average thickness of IMC layer after aging at 100 °C for 21 days. During the aging process, the growth of the interfacial IMC layer of the solder joint mainly follows the following empirical diffusion formula:

$$\begin{eqnarray}&&X={X}_{0}+\sqrt{Dt}\end{eqnarray} \tag{ 1 }$$

where X is the total thickness of interfacial IMC layers at various aging time, X₀ is the thickness of as-soldered solder joint, t is the isothermal aging time and D is the diffusion coefficient. According to the figure 5, the IMC layers growth (X − X₀) presents the linear relationship with the square root of aging times (days^1/2). The growth rate of IMC layers of SnBi/Cu, SnBi@110P-Cu/Cu and SnBi@500P-Cu/Cu solder joints is 0.61506 μm/days^1/2, 0.48725 μm/days^1/2, 0.55385 μm/days^1/2, respectively. Consequently, the aged SnBi/Cu solder joint shows the highest thickness of the interfacial IMC layer due to the greatest growth rate of IMC layers, while the growth rate of IMC in the SnBi@P-Cu/Cu composite solder joints is lower than that of SnBi/Cu solder joint due to the depletion of Sn element in the solder bulk by porous Cu frames. On the other hand, the morphology of IMC layers at the soldering interface of all solder joints gradually transforms from scallop-shaped to flat-shaped under the isothermal aging condition. As revealed by Wang [18] et al, the diffusion rate of Cu and Sn atoms at the valley of the Cu₆Sn₅ IMC is higher than those at other positions. Therefore, the IMC layers of the solder joints change to be smoother after aging for a long time. Moreover, Cu₃Sn IMC layer forms at the Cu₆Sn₅/Cu as revealed by figure 6(e).

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Another important finding is performed in figures 4(a2)–(a4), in which amounts of voids appear in the Cu₆Sn₅ IMC layer of the aged SnBi/Cu solder joint. According to the previous study [19, 20], the formation of these voids is mainly attributed to the Kirkendall effect. The diffusion rate of Cu atoms at the soldering interface is higher than that of Sn atoms during the aging process. Thus, atomic vacancies exist at the interface between Cu substrate and solder bulk, finally forming larger voids with the extension of aging time. In contrast, the number of voids at the soldering interface in the SnBi@P-Cu/Cu composite solder joints is lower than that of the SnBi/Cu solder joint. As shown in figure 6, it can be inferred that the addition of porous Cu leads to the increase of Cu atoms at the interface, which can improve the atomic vacancies caused by the diffusion of Cu atoms at the Cu substrate. Therefore, the addition of porous Cu can suppress the formation of voids at the soldering interface.

The shear test was carried out with the shear height of 150 μm and the shear rate of 0.1 mm s⁻¹. Figure 7 represents the shear strength of all solder joints before and after aging. It is obviously that the shear strength of all solder joints shows a decreasing tendency during the aging process. Especially in the SnBi/Cu solder joint, its average shear strength degrades sharply from 52.72 MPa to 36.73 MPa after aging at 100 °C for 21 days, which may ascribe to the thicker IMC layer and a certain amount of voids at the soldering interface. In contrast, it can be found in figure that the shear strength of the SnBi/Cu solder joint is significantly enhanced with the porous Cu addition. Among these, the SnBi@500P-Cu/Cu composite solder joint exhibits the maximum shear strength before and after aging.

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In order to find out the evolution of fracture mechanism during the aging condition, fracture morphologies of three kinds of solder joints are presented in figures 8, 9 and 12. Figure 8 shows the fracture morphologies of the SnBi/Cu solder joint with different aging time. It can be seen in figures 8(a1) and (a2) that the fracture of the as-soldered SnBi/Cu solder joint mainly appear in the solder bulk. And no obvious deformation of the solder is found in the fracture, indicating that the fracture mode is a typical brittle fracture in the SnBi/Cu solder joint. As shown in figures 8(b1)–(d2), the fracture position of the solder joint gradually transforms from the inside solder to the solder/Cu₆Sn₅ interface with the extension of aging time. And the overall fracture morphologies of the solder joint change to be flatter during the aging process. However, a large amount of voids are observed in the factures of the aged solder joints. In especial, the solder joint aged for 21 days presents larger voids and EDS result reveals that Cu₆Sn₅ IMCs exist in these voids. As mentioned in the microstructural analyses, the formation of the voids may be attributed to the Kirkendall effect. The stress will mainly concentrate at these voids during the shear test, which contributes to the failure in the solder joint. Therefore, it can be inferred that the voids at the soldering interface is considered as the main factor to degrade the shear strength of the SnBi/Cu solder joint. Additionally, the thicker interfacial IMC layer and coarsening microstructure also have a negative influence on the shear performance.

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Compared with the as-soldered SnBi/Cu solder joint, the fracture position in SnBi @P-Cu/Cu is closer to the soldering interface due to the thicker interfacial IMC layers. As shown in figures 9 and 12, the fractures of the SnBi@P-Cu/Cu composite solder joints have a similar rule with SnBi/Cu as the aging time prolongs. Nevertheless, it can be found that the fractures occur within the solder bulks when the solder joints are aged for 21 days. The bulge of the solder bulk is observed in figures 9(d1) and 12(d1). And in their magnified fracture, residual Cu frames exist at the solder bulk. According to the element mapping and EDS result of SnBi@110P-Cu/Cu as shown in figures 10 and 11, Cu₆Sn₅ IMCs are found on the surface of Cu frames. Additionally, in the magnification of figures 9(d2) and 12(d2), it is clear that amounts of cracks form in the Cu₆Sn₅ IMCs, revealing that the cracks firstly appear at the Cu₆Sn₅ IMC layer between the Cu frames and the solder bulk during the process of shear test. This result can be attributed to that the thicker Cu₆Sn₅ between the Cu frames and the solder bulk contributes to the stress concentration when the test force is loading. Meanwhile, the porous Cu frames distribute in the solder bulks with a continuous network structure. Therefore, the cracks form at the IMC layers near the Cu frames and propagate along them, and finally fails. On the other hand, it is worth noting that the number and size of voids within the fracture of the aged SnBi@P-Cu/Cu is lower than that of the SnBi/Cu solder joint, which conforms to the previous investigation of interfacial IMC layer.

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Based on the investigation of the microstructure and fracture morphology of the solder joints, several factors that the shear performance of the SnBi/Cu solder joint is remarkably enhanced after adding porous Cu into solder joint are summarized. Firstly, the dispersed Cu frames can improve the strength of the SnBi solder matrix due to the higher mechanical properties of Cu alloy. Secondly, while the cracks initially form within the Cu₆Sn₅ between the Cu frames and solder in the aged SnBi@P-Cu/Cu solder joint, porous Cu distributes in the solder bulk with a reticular structure, indicating that the Cu frames can inhibit the propagation of the cracks when they appear within Cu₆Sn₅ IMCs. Consequently, the shear strength of the solder joints is enhanced after aging for a long time. Furthermore, the Cu atoms dissolved by the porous Cu diffused into the solder bulk can contribute to the improvement of the shear performance. On the other hand, the Cu frames are bonded with the Cu₆Sn₅ IMC layers in SnBi@500P-Cu/Cu after aging due to the increasing of the interfacial layer thickness, which leads to that the shear strength of the SnBi@500P-Cu/Cu solder is higher than that of SnBi@110P-Cu/Cu.

Nanoindentation test is implemented with a force of 20 mN and a loading speed of 5 mN/s. Due to that the SnBi@110P-Cu solder bulk is divided into two regions, the hardness of different regions in the solder bulk is obtained, respectively. As shown in figure 13, the sign a represents the region enclosed by Cu frames, and b is the other region in the SnBi@110P-Cu composite solder bulk. The hardness of the solder bulk in the SnBi/Cu, SnBi@110P-Cu/Cu and SnBi@500P-Cu/Cu solder joints before and after aging is presented in figure 13. The hardness of region a in the SnBi@110P-Cu solder bulk displays the maximum value because of the finer microstructure before aging. While region b shows the similar hardness of the solder bulk with the SnBi/Cu solder joint, the hardness of the SnBi@500P-Cu solder bulk is between the region a and b of the SnBi@110P-Cu solder bulk. The hardness in all of the solder joints shows an anabatic trend as aging time prolongs. As discussed in microstructure, it can be concluded that the coarsening β-Sn and Bi-rich phases are the main factors resulting in the rise of hardness. As shown in figure 2(b4), the especial microstructure, which is composed of Bi-rich and IMC layer in the SnBi@110P-Cu/Cu solder joint, contributes to that the hardness of region a is higher than that of the others after aging 21 days. In contrast, the hardness of the SnBi@500P-Cu solder bulk is closer to that of the SnBi/Cu solder due to the similar microstructure after aging for a long time.

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