Structural, mechanical, thermo-physical and electronic properties of η′-(CuNi)6Sn5 intermetallic compounds: First-principle calculations

https://doi.org/10.1016/j.molstruc.2016.01.059Get rights and content

Highlights

  • All η′-(CuNi)6Sn5 intermetallic compounds were researched.

  • The influence of Ni atom doping position was considered and discussed.

  • Structural, mechanical, thermo-physical and electronic properties were all investigated.

  • Uneven distribution of Young's modulus at (001) planes is the main reason for anisotropy.

  • Ni s and Ni p states can replace Cu s and Cu p states to hybridize with Sn s states at −7.98 eV.

Abstract

First-principle calculations have been performed to investigate the structural, mechanical, thermo-physical and electronic properties of η′-(CuNi)6Sn5 intermetallic compounds. The results indicated that, the doped Ni atom can not only enhance the stability of the η′-Cu6Sn5, but also improve the mechanical and thermo-physical properties, which are more dependent on the Ni atom doping number than the doping position. In all the η′-(CuNi)6Sn5, Cu3Ni3Sn5 (Cu1+Cu3 site) shows the best stability, the most excellent deformation resistance and the highest hardness. The Cu6Sn5, Cu3Ni3Sn5, Cu4Ni2Sn5, Cu1Ni5Sn5 and Ni6Sn5 are ductile while the Cu5Ni1Sn5 and Cu4Ni2Sn5 are brittle. The anisotropies of η′-(CuNi)6Sn5 are all mainly due to the uneven distribution of Young's modulus at (001) planes, moreover, the anisotropy of Cu1Ni5Sn5 (Cu1+Cu2+Cu4 site) is the strongest while that of Ni6Sn5 is the weakest. The calculated Debye temperature and heat capacity showed that Cu4Ni2Sn5 (Cu2 site) possesses the best thermal conductivityD = 356.9 K) and Cu2Ni4Sn5 (Cu1+Cu2 site) possesses the largest heat capacity. From the electronic property analysis results, the Ni s and Ni p states can replace the Cu s and Cu p states to hybridize with Sn s states at −7.98 eV. Moreover, with the increasing number of the doped Ni atom, the hybridization between Cu d states at different positions is receded, while that between Ni d states is enhanced gradually.

Introduction

In microelectronic packaging, η′-Cu6Sn5 is a familiar intermetallic compound (IMC), which usually forms at the interface between the Sn-based solder and the Cu substrate, and plays an important role to influence the reliability of the solder joints [1], [2], [3]. In fact, the failure behaviors of solder joints are often determined by the IMCs through their following intrinsic properties: thermal conductivity, ductile/brittle transition, elastic and creep properties [4].

In order to improve the reliability of the Sn-based solder/Cu substrate solder joints, the η′-Cu6Sn5 IMCs have been optimized by doping alloy elements [5], [6]. It is found that Ni doping η′-(CuNi)6Sn5 IMCs have a great deal of performance enhancement compared with η′-Cu6Sn5 IMC, and is conducive to improve the reliability of the solder joints [7], [8]. However, although some macro- or micro-scale experimental investigations were carried out, how the doped Ni atom improve the properties of η′-Cu6Sn5 IMC is still not fully understood.

First-principle calculation based on density functional theory (DFT) and approximation conditions is a novel method used to investigate the effects of dopants on the properties of IMCs, and the results were widely recognized [9], [10], [11]. Xu [9] researched the electronic structures, mechanical properties and thermodynamic properties of the Co-based Co3X (X = Ti, Ta, W, V and Al) intermetallic compounds by first-principle calculations, and found that obtained elastic parameters of Co3X compounds hold a linearly increasing trend as the melting point of the metallic element of X rises. Shao [10] investigated the influences of Zn doping on the relative stability of Cu6Sn5 using first-principle calculations, and indicated that the stability of Cu6Sn5 could be improved by Zn doping due to the hybridization between Zn-d states and Sn-s states. Jiang [11] performed to predict the site preference of Pt, Hf, Cr and Ir in L12 Ni3Al, and believed that Pt always prefers Ni sites, while Cr and Hf always prefer Al sites in Ni3Al. The site preference of Ir was found to be strongly composition-dependent: Ir prefers Ni sites in Al-rich and Al sites in Ni-rich Ni3Al, and shows no site preference in stoichiometric Ni3Al.

In the present work, the structural, mechanical, thermo-physical and electronic properties of all the η′-(CuNi)6Sn5 IMCs were systematically investigated using a first-principles density functional plane-wave ultrasoft pseudopotential method. This work can not only supply the theoretical guidance for optimizing the η′-(CuNi)6Sn5 IMCs and improving the reliability of the Sn/Cu solder joints, which is revealing the remarkable engineering value, but also clarify the influence mechanism for doped Ni atom improving the mechanical and thermal–physical properties of η′-Cu6Sn5 IMCs at the range of atomic and electronic scales, which is showing the important scientific significance.

Section snippets

Calculational details

η′-Cu6Sn5 shows the orthorhombic structure (space group C12/C1) with the cell parameters of a = 10.665 Å, b = 7.055 Å, c = 9.535 Å and β = 99.02°, as given in Fig. 1. From Fig. 1, Cu atoms occupy four different sites in η′-Cu6Sn5, in which, two Cu atoms occupy one 8f sites (Cu1), two Cu atoms occupy another 8f sites (Cu2), the remaining two Cu atoms occupy the 4a and 4e sites (Cu3 and Cu4), respectively. Therefore, considering the difference of the Cu atoms sites, the Ni doping η′-(CuNi)6Sn5

Structural property

In order to assess the accuracy of different potential computational approaches, in particular of the effectiveness of the pseudopotentials used, the structural properties of η′-Cu6Sn5, which conclude lattice constants, angle β and volume, were calculated by LDA-CAPZ and GGA-PBE, and the results were compared with others published calculated and experimental data, as listed in Table 1 [19], [20], [21], [22].

From Table 1, the calculated structural properties of η′-Cu6Sn5 obtained in this work

Conclusion

First-principle calculations have been performed to investigate the structural, mechanical, thermo-physical and electronic properties of η′-(CuNi)6Sn5 intermetallic compounds. The results indicated that, the doped Ni atom can not only enhance the stability of the η′-Cu6Sn5, but also improve the mechanical and thermo-physical properties, which are more dependent on the Ni atom doping number than the doping position. The calculated B, G, E and 10Hv of Cu3Ni3Sn5 (Cu1+Cu3 site) are all the largest,

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