Investigation of Cu particles size and bonding time on the microstructure and shear property of Cu/In-45Cu/Cu solder joints

The effect of Cu particles size and bonding time on the microstructure and shear property of Cu/In-45Cu/Cu solder joint was studied, and the shedding mechanism of intermetallic compounds (IMCs) in the solder joint during transient liquid phase (TLP) bonding process was investigated. The results showed that the microstructure of Cu/In-45Cu/Cu solder joint was composed of Cu11In9 phase, residual In phase and Cu particles, and the microstructure of solder joints prepared by small Cu particles was dense. The content of IMCs was increased with increasing bonding time, and the Cu/In-45Cu/Cu solder joint was composed of Cu11In9 phase and Cu particles under bonding time 30 min. The Cu2In phase was formed in the solder joint at 60 min, and many cracks appeared at the interface of Cu11In9 and Cu2In phases. The shear strength of Cu/In-45/Cu solder joints with brittle fracture was increased firstly and then decreased with increasing bonding time, and the maximum shear strength of the solder joint was 16.35 MPa at 30 min.


Introduction
With the rapid development of the third generation wide band gap semiconductor materials represented by SiC and GaN, the power devices with high-temperature resistance can be used up to 300°C [1,2]. The chips of core device, components and packages are laminated interconnection. The size of solder joints is decreased to 10 μm using 3D packaging technology, and the weight is reduced by 40-50 times [3]. The 3D components operate with a fast conversion speed with low energy consumption, and the solder joints using TLP technology for high temperature operation can be achieved [4,5].
Lots of research was carried out on the microstructure and mechanical properties of Cu/solder/Cu solder joints, but the previous studies were mainly focused on Sn-based solders. In is widely used for its low melting point, good fatigue resistance and ductility, and In solder is considered as an excellent commercial solder in the field of modern electronic packaging [20]. Tajima [21] investigated the IMCs evolution in the Cu/In-Sn-Ag-Sb/ Cu solder joint, the Ag 3 Sn was replaced by Ag 3 (Sn, In) after adding In, and Ag 2 (In, Sn) was formed by the reaction between In and Ag 3 (Sn, In) with increasing In content. Lin [22] indicated that Cu/In/Ni solder joints with dense microstructure consisted of Ni 3 In 7 and Cu 11 In 9 IMCs, and the Cu 11 In 9 phase divided into Cu 11 In 9 (i) phase at the interface of Cu substrate and Cu 11 In 9 (ii) phase with faceted rod-like morphology in the in situ reaction zone. Yoon [23] pointed out the IMCs in the Au/In/Au solder joints were gradually transformed from AuIn 2 into Au 7 In 3 phases with increasing bonding time, and the time for Au 7 In 3 formation was greatly reduced with increasing bonding pressure. Tian [24] suggested that the IMC in the Cu/In/Cu solder joint was Cu 11 In 9 after the bonding of 40 min, and Cu 2 In phase and Kirkendall voids appeared at the interface of solder joint at 360 min. The shear strength of solder joint reached the maximum value of 13.65 MPa at 40 min and the fracture mode is brittle cleavage fracture.
The researches mainly focused on the IMCs evolution and the effect of reinforced particles on the mechanical properties of solder joints. However, the effect of Cu particles size and bonding time on the microstructure and shear property of Cu/In/Cu solder joints was seldom reported. In the paper, micron-sized Cu particles with different size were added into the low melting point In solder, and the In-45Cu composite solder powders were prepared after mixing. TLP bonding was used to fabricate the Cu/In-45Cu/Cu solder joints. The microstructure of Cu/In-45Cu/Cu solder joints was observed, and its shear strength and the fracture morphologies were studied.

Experimental
The In solder powders with diameter about 1 μm was used as the matrix, and about 45 wt% micron-sized Cu particles with purity of 99.9% and diameter about 1 μm and 45 μm were added into In solder. Three different kinds of Cu particles were prepared, in which the small Cu particles accounted for 0%, 50% and 100% of total Cu particles. The Cu particles and the In particles were mixed with mass ratio of 11:9, and three In-45Cu composite solder powders were obtained. About 11 wt% rosin flux was added in In-45Cu solder powders, and In-45Cu solder paste was obtained after long-time mechanical blending.
The Cu substrates with the size of 10 mm×10 mm×4 mm and 12 mm×12 mm×4 mm were used in the experiment, and the In-45Cu solder paste was placed between the Cu substrates. The schematic diagram of the solder joint preparation was shown in figure 1. The Cu/In-45Cu/Cu sample was placed in the TWB-100 wafer bonding machine with the bonding temperature of 260°C, the bonding pressure of 5 MPa and the bonding time of 3-60 min. The microstructure of the solder joints was observed by ZEISS SUPRA 55 scanning electron microscope (SEM) with energy disperses spectroscopy (EDS), and the void fraction of the solder joints was measured by the ImageJ software. The shear strength was examined by UTMS 5305 electronic universal tester with the shear rate of 0.2 mm min −1 , and the schematic diagram of the shear test was shown in figure 2. The shear fracture morphologies of Cu/In-45Cu/Cu solder joints were observed by SEM.

Microstructure of solder joints with different size Cu particles
The microstructure of Cu/In-45Cu/Cu solder joints with different size Cu particles is shown in figure 3. As shown in figure 3, the microstructure of the solder joint consists of the IMCs around Cu particles, residual In and Cu particles, and the amount of IMCs in solder joint is increased with increasing content of small Cu particles. According to the Cu-In phase diagram, the main compounds are Cu 11 In 9 and Cu 2 In phases at 260°C. The atomic percentages of In and Cu in the white IMCs are 44.02 at% and 55.98 at% according to EDS results (as shown in figure 3(d)), inferring that the compound is Cu 11 In 9 phase.
As shown in table 1, the voids fraction of Cu/In-45Cu/Cu solder joint using small Cu particles is fewer than that of the solder joints using large Cu particles and mixed particles, and it is 16.78%. Combining with figure 3, the microstructure of solder joint using small Cu particles is dense. The schematic illustration of the voids formation in solder joints is shown in figure 4. The gaps among the large Cu particles exist in the solder joints. There is not enough liquid In-rich phase to replenish the original gaps among large Cu particles during bonding process, resulting void formation in the original gaps. As shown in figure 4(b), Cu 11 In 9 phase is easily formed on the Cu particles surface due to the large specific surface area of small Cu particles, and the gaps among small Cu particles is constantly filled by Cu 11 In 9 phase during bonding process. Therefore, the number of voids in the solder joint using small Cu particles is few, and a large amount of Cu 11 In 9 phase is formed (as shown in figure 3(c)). Researches indicate that rapid formation of compounds can lead to poor solder wettability [25,26]. The wettability of In solder deteriorates due to the excessively fast formation of Cu 11 In 9 phase in the solder joint using small Cu particles, and the voids still exist in the solder joint.
Therefore, the size of Cu particles affects the filling influence of liquid phase In in the Cu/In-45Cu/Cu solder joints, and the dense microstructure of Cu/In-45Cu/Cu solder joint using small Cu particles is obtained.

Microstructure of solder joint with different bonding time
The microstructure of Cu/In-45Cu/Cu solder joints using small Cu particles at the bonding time of 3-60 min is shown in figure 5. The microstructure of Cu/In-45Cu/Cu solder joint is composed of a small amount of Cu 11 In 9 phase, In-rich phase, and unreacted Cu particles with the bonding time of 3 min (as shown in figures 5(a), (b)). A large amount of voids exist in the solder joint, because the In-rich phase and Cu particles in the solder joint can not fully react due to the short bonding time, and there are not enough Cu-In phases to fill the gaps of Cu particles in the solder joint. The amount of Cu 11 In 9 phase around Cu particles is increased with increasing bonding time, and the proportion of In-rich and unreacted Cu particles is decreased. The solder joint with dense microstructure consists of Cu 11 In 9 phase and Cu particles at 30 min, and In is consumed substantially (figures 5(g), (h)). The reaction of the In-rich phase and Cu particles is substantially complete in the solder joint due to the appropriate bonding time of 30 min, and Cu-In phases can fill the gaps of Cu particles, causing the dense microstructure with a few voids. In addition, the number of voids is reduced due to the expansion of the IMC phases [27]. The Cu 11 In 9 phase is firstly generated when In reacts with Cu [28][29][30]: In Cu In 11 9 1 11 9 And, Cu 2 In phase is gradually formed with the increasing time [31]: Cu Cu In Cu 7 9 2 11 9 2 Cu 11 In 9 phase is C2/m oblique crystal structure, and Cu 2 In phase is P63/mmc hexagonal structure [32]. The lattice constants of Cu 11 In 9 phase are a=13.027 nm, b=4.406 nm, c=7.460 nm, and β=54.22°, and the lattice constants Cu 2 In phase are a=b=4.471 nm and c=5.384 nm. The density of Cu 11 In 9 and Cu 2 In as the following: Where ρ Cu11In9 is the density of Cu 11 In 9 phase; M Cu , M In are the molar mass of Cu and In; N A is Avogadro constant. The volume change of Cu 11 In 9 +7Cu→9Cu 2 In during the phase transition was theoretically calculated. When 1 mol Cu 11 In 9 reacts with 7 mol Cu to convert 9 mol Cu 2 In, the volume change is as follows: Cu In Cu In Cu Where ΔV is the volume change after Cu 11 In 9 is converted to Cu 2 In; V Cu In 11 9 and V Cu are the volume of Cu 11 In 9 phase and Cu before the reaction, respectively; V Cu In 2 is the volume of Cu 2 In after the reaction. Then, the volume change after the reaction is: Therefore, the volume expands after the Cu 11 In 9 phase changes to Cu 2 In. When the width of Cu/In-45Cu/ In solder joints is a constant, the number of voids is reduced due to the expansion of the IMC phases. As shown in figure 5, the amount of Cu 2 In phase of the solder joint at 30 min is more than the solder joint at 3-10 min, therefore there is only a few of voids in the tightly organized solder joints at 30 min.  A thin IMC layer is formed between the Cu particles and the Cu 11 In 9 when the bonding time reaches to 60 min (as shown in figure 4(j)). The atomic percentages of In and Cu atoms are 32.39 at% and 67.61 at% according to the EDS results (as shown in figure 5(k)), and it is inferred that the thin IMC layer is Cu 2 In. As shown in figure 4(j), some cracks in the solder joints at 60 min, and it is caused by the different thermal expansion coefficients of the Cu 11 In 9 and Cu 2 In phases [33]. The voids fraction of the solder joints at different  bonding time is shown in table 2. As shown in table 2, the void fraction is decreased firstly and then increased with increasing bonding time, and the minimum void fraction of 3.18% is obtained at 30 min. Therefore, the Cu/In-45Cu/In solder joint with dense microstructure and a small amount of voids can be obtained when the bonding time is 30 min. Cu 11 In 9 phase sheds from the surface of the substrate and Cu particles in the Cu/In-45Cu/Cu solder joint according to figures 5(d) and (j). The schematic diagram of compression stress of IMCs on the surface of Cu particles is shown in figure 6(a). The compressive stress σ of IMCs is subjected to liquid In during the growth stage. The IMC sheds from the substrate are caused by the force of σ sinθ along the centerline of the attached Cu particles [34]. The shedding of Cu 11 In 9 on the Cu substrate requires more force (as shown in figure 6(b)), and a small amount of Cu 11 In 9 phase peels off near the Cu substrate.  Cu 2 In phase is formed on the surface of Cu particles, which causes the thermodynamic non-equilibrium state of the initially formed Cu 11 In 9 compound and Cu particles. Cu 11 In 9 phase sheds from the surface of Cu particles, providing the growth space of the Cu 2 In equilibrium phase [35,36]. Figure 7 reveals the relationship between shear strength of Cu/In-45Cu/Cu solder joints and different bonding time. As shown in figure 7, the shear strength of Cu/In-45Cu/Cu solder joints is increased firstly and then decreased with increasing bonding time. The shear strength of Cu/In-45Cu/Cu solder joint is 8.51 MPa at 3 min, and it is increased to the maximum shear strength 16.34 MPa at 30 min. Tian [24] has also obtained similar results, the shear strength of Cu/In/Cu solder joint at 360°C for 40 min is about 13.65 MPa, and the maximum shear strength 16.34 MPa in our experiments is about 19.71% larger than the shear strength of Cu/ In/Cu solder joint. The shear strength of solder joint is guaranteed by constitute phase and voids in the solder joint [37]. The number of the weak region by the residual In and voids in the dense solder joint is relatively low. The stress distribution in the solder joint is relatively uniform. Therefore, the solder joint at 30 min presents superior shear strength. The voids and the residual In/IMC interfaces provides the crack initiation and propagation under the shear stress, the stress concentration on the crack tip promote to the fracture of the solder joint simultaneously. The high content of voids and residual In accelerates the fracture progress (figures 5(a)-(c)) and then the solder joint exhibits the inferior shear strength.

Shear property
The shear fracture morphologies of Cu/In-45Cu/Cu solder joints at different bonding time are shown in figure 8. As shown in figure 8(a), the residual In particles exist in the fracture, and the fracture position of solder joint with brittle fracture mode locates in the solder matrix. The proportion of Cu-In IMCs in the fracture is increased with increasing bonding time, but the proportion of the Cu particles and residual In is decreased. As shown in figures 8(d), (e), a large amounts of Cu-In IMCs and a small amounts of Cu particles exist in the fracture at 30-60 min, and the fracture occurs in the Cu 11 In 9 and Cu 2 In IMCs. The bonding ability of the solder  joint is improved by IMCs gains, but the cracks between Cu 2 In and Cu 11 In 9 IMCs are formed in the solder joint at 60 min ( figure 5(e)).

Conclusions
(1) The microstructure of Cu/In-45Cu/Cu solder joint obtained by different size Cu particles is composed of Cu 11 In 9 phase, residual In and Cu particles, and the microstructure of solder joints prepared by small Cu particles is dense.
(2) The solder joint is composed of Cu 11 In 9 phase and Cu particles at the bonding time of 30 min, and In-rich phase is consumed completely. Cu 2 In phase appears in the solder joint at the bonding time of 60 min, and cracks form between Cu 11 In 9 and Cu 2 In phases.
(3) The compressive stress and the thermodynamic forces cause Cu 11 In 9 phase to shed from Cu particles and Cu substrate surface during the bonding process. The growth space of Cu 2 In phase is provided by the shedding Cu 11 In 9 phase at the bonding time of 60 min.