First-principles calculations on physical properties of Ni3Snx binary system intermetallic compounds and Ni/Ni3Sn interfaces in Nickel-Tin TLPS bonding layer
Graphical abstract
Introduction
In recent years, the new generation of semiconductor (SiC, GaN) devices with the service temperature higher than 500 °C has been more and more used in electronic industry, which urgently requests that the electronic packaging technique shows the feature of ‘‘low-temperature bonding and high-temperature service’’ [[1], [2], [3]].
In this demand, an innovative Nickel-Tin transient liquid phase sintering (TLPS) bonding for packaging the high-temperature service power devices has been developed by our team, in which, the 30Ni-70Sn mixed powder was designed as the reactive system [4,5]. During the bonding processing, the low melting point element Sn will be heated to be melting liquid phase at the low temperature (340 °C) and react with the high melting point element Ni rapidly, after be insulated for 240 min, the element Sn has been consumed and transforms to Ni-Sn intermetallic compounds completely. For the microstructure of the bonding layer, the residual Ni particles were surrounded by Ni3Sn intermetallic compounds, combined with a small amount of Ni3Sn2 and Ni3Sn4 interspersed, the high melting point components leads to the bonding layer has a heat-resistant temperature higher than 790 °C [4].
Actually, besides the temperature resistance, the strength of the bonding layer plays a more important role on influencing the range of application. Although the shear strength of Ni-Sn TLPS bonding layer can reaches 28.5 MPa [5], which is higher than others published results, such as Sn-Cu, Sn-Ag and Ag-In systems [[6], [7], [8], [9]], it still has a lot of room for improvement. The destructive experiments indicated that, during the shearing processing, the cracks were derived from the Ni/Ni3Sn interface or the Ni3Snx intermetallic compounds and then leads to the broken of the bonding layer [5]. Therefore, it is significant to investigate the physical properties of Ni3Snx intermetallic compounds as well as the interfacial properties of Ni/Ni3Sn interface, which is the essential prerequisite for optimizing the microstructure and avoiding the formation of crack. However, up to now, the corresponding researches have not been completed because that the Ni3Snx intermetallic compounds and the Ni/Ni3Sn interface in the bonding layer is too microscopic to be distinguished and examined by experiments.
First-principles calculation based on density functional theory (DFT) is a novel method for micro-investigation at the atomic scale, which has been widely used in modern science, especially in the research on physical properties and surface/interface properties of solid phases. Yang [10] investigated the physical properties of Cu-Ti intermetallic compounds in Ag-Cu-Ti filler metal by first-principles calculation and found that CuTi shows the strongest anti-cracking performance because of the larger elasticity modulus and the smaller brittleness. Du [11] researched the mechanical properties of Ni-Zr intermetallic compounds, and believed that Ni5Zr is the most stiffness phase and NiZr2 is the most ductile phase among these binary Ni-Zr intermetallic compounds. Yin [12] studied the fracture behavior of TiN/VN interface from first principles and found that TiN/VN(111) system has a smaller tensile strength than VN (111) surface, demonstrating that TiN/VN(111) shows the weaker anti-cracking performance. Dreyer [13] researched the brittle fracture toughness of GaN and AlN from first-principles surface-energy calculations, and believed that fracture toughness is similar for GaN and AlN, and that the nonpolar planes have significantly smaller fracture toughness than the c plane.
In this work, the physical properties of Ni3Snx intermetallic compounds as well as the interfacial properties of Ni/Ni3Sn interface were investigated for studying the crack initiation behavior and mechanism in the Nickel-Tin TLPS bonding layer. The result is not only beneficial to optimize the microstructure and improve the shear strength of the novel bonding layer, but also shows the important guiding significance on micro-fracture behavior investigation.
Section snippets
Methods
DFT (density functional theory) with ultrasoft pseudopotentials was employed in the CASTEP code (Cambridge Sequential Total Energy Pack-age) to carry out the calculations of energies and electronic structures, in which, the pseudopotential was applied for describing the electron-ion interaction [14]. LDA (local density approximation) with the Ceperley-Alder-Perdew- Zunger (CAPZ) functional and GGA (generalized gradient approximation) with the PBE (Perdew-Burke-Ernzerhof) functional were
Structural properties
Fig. 1 shows the standard-pressure crystal structures of Ni, Ni3Sn, Ni3Sn2 and Ni3Sn4. From it, Ni and Ni3Sn both show the cubic structures with FM-3M space groups, and per cell consists of 4 atoms. While for Ni3Sn2 and Ni3Sn4, they appear the orthorhombic structure with PNMA space group and the monoclinic structure with C2/M space group respectively, in which, per Ni3Sn2 cell consists of 20 atoms and per Ni3Sn4 cell consists of 14 atoms.
The optimized crystal structure parameters for Ni, Ni3Sn,
Interface selection
Ni/Ni3Sn interface, which is also considered as the cracks initiation region, contains a large number of possible combinations, such as Ni (100)/Ni3Sn (100) interface, Ni (120)/Ni3Sn (110) interface, Ni (111)/Ni3Sn (110) interface and so on. Because that is impossible to calculate all of them, it is very important to choose the most representative Ni/Ni3Sn interface preferentially.
Braffet's [40] mismatched lattice theory indicates that the solidification interface usually be effected by the
Discussion
From the analysis results about the physical properties, crystal structures and bonding characteristics for Ni and Ni3Snx intermetallic compounds, it can be found that the fracture-relevant properties of the crystals, such as elasticity moduli B, G and E, ductility or brittleness evaluation criteria G/B ratio and universal anisotropic index AU are effected by crystal structure and bonding characteristic.
Elasticity moduli B, G and E, which express the anti-deformation ability of the crystal in
Conclusion
In this work, the physical properties of Ni3Snx binary system intermetallic compounds and interfacial properties of Ni/Ni3Sn interface in Nickel-Tin TLPS bonding layer were investigated by first-principles calculations. The results indicate that for Ni and Ni3Snx intermetallic compounds in Nickel-Tin TLPS bonding layer, elasticity modulus B, G and E as well as universal anisotropic index AU are both controlled predominantly by the crystal structure, while G/B ratio is influenced by the bonding
Acknowledgments
The authors would like to express their gratitude for projects funded by the National Natural Science Foundation of China (No. 51474026 and No. 51605027) and the General Armament Department of China (61409230503).
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