Ching-Feng Yu

IMC growth reaction and its effects on solder joint thermal cycling reliability of 3D chip stacking packaging

Abstract The study aims at assessing the growth reaction of the Ni3Sn4 intermetallic compound (IMC) during bonding process and its dependences on the thermal-cycling reliability of the Cu/Ni/SnAg micro-joints of an advanced 3D chip stacking package under accelerated thermal cycling (ATC) loading. The growth reaction of the IMC during bonding process is also predicted through experiment and classical diffusion theory, and the relation between the IMC thickness and bonding process temperature and time is derived according to the predicted activation energy of the chemical reaction between Sn and Ni by experiment. Moreover, the micro-joint reliability prediction is made using finite element (FE) analysis incorporated with an empirical Coffin–Manson fatigue life prediction model and also ATC experimental test. To facilitate the FE modeling, the temperature-dependent thermoelastic properties of both single crystal and polycrystalline Ni3Sn4 IMC are characterized through molecular dynamics simulation and the Voigt–Reuss bound and Voigt–Reuss–Hill approximation. Results show that monoclinic single crystal Ni3Sn4 reveals a high elastic anisotropy or direction dependence of elasticity. The diffusion reaction of Sn and Ni exhibits that a longer bonding process time and a higher bonding temperature could not only increase the IMC thickness but also vary its surface morphology. In addition, the thermal–mechanical performance of the micro-joints is strongly affected by the geometry and material of IMC layer, where IMC with a thicker thickness, a less Young’s modulus, a smaller CTE and even a more rounded surface morphology can better the reliability.

Strain- and strain-rate-dependent mechanical properties and behaviors of Cu3Sn compound using molecular dynamics simulation

This work aims at investigating the mechanical properties and behaviors of orthorhombic Cu3Sn crystals at room temperature through molecular dynamics (MD) simulation. The focuses are placed on the tensile stress–strain behaviors and properties of the Cu3Sn single crystal and also their dependence on applied strain and strain rate. An attempt to characterize the deformation evolution of the Cu3Sn nanostructure during the stress–strain test is also made. In addition, the elastic properties of bulk polycrystalline Cu3Sn are estimated, as a function of strain rate and applied strain, by using the monocrystal results. The effectiveness of the MD model is demonstrated through comparison with the nanoindentation results and also published theoretical and experimental data. The calculated orthotropic elastic and shear moduli and Poisson’s ratio of Cu3Sn single crystal reveal not only high anisotropy, but also the great effects of applied strain and strain rate only as the strain rate exceeds a threshold value of about 0.072% ps−1. Specifically, raising the strain rate increases the orthotropic elastic properties and also the ultimate tensile and shear strengths of the nanocrystal, whereas increasing the applied strain reduces them.

Crystal size and direction dependence of the elastic properties of Cu3Sn through molecular dynamics simulation and nanoindentation testing

Abstract The study aims at exploring the elastic properties of orthorhombic Cu 3 Sn crystals through a proposed molecular dynamics (MD) simulation model based on the modified embedded atom method (MEAM) and nanoindentation testing. The focuses of the study are placed on their dependence on the crystal size and direction. The electronic nature of single crystal Cu 3 Sn is also examined by using first-principles calculations based on density function theory (DFT). According to continuum mechanics, the elastic stiffness coefficients of the single crystal Cu 3 Sn are derived from the calculated energy, and used in the generalized Hook’s law in compliance form to compute the associated elastic constants. The simulated elastic properties are compared with the results of the published first-principles calculations. For comparison with the present nanoindentation finding and the other published experimental data, the effective elastic properties of the polycrystalline Cu 3 Sn together with their size dependence are also derived using the Voigt–Reuss bounds and Voigt–Reuss–Hill average based on the calculated single crystal data. The simulation results show that the orthorhombic Cu 3 Sn crystals exhibit a high elastic anisotropy, which has been also confirmed by the electronic structure analysis, and also a strong size and direction dependence of elasticity. In addition, the calculated effective elastic properties of the polycrystalline Cu 3 Sn agree well with the present nanoindentation results and the published theoretical/experimental data.

Structural, mechanical, thermodynamic and electronic properties of AgIn2 and Ag3In intermetallic compounds: ab initio investigation

The structural, mechanical, thermodynamic and electronic properties of two Ag–In phase crystals, i.e., AgIn2 and Ag3In intermetallic compounds (IMCs), are explored using ab initio calculations within the generalized gradient approximation. The optimized lattice constants of AgIn2 and Ag3In crystals are first investigated in the study. Next, the elastic constants of the two single crystal structures as well as their associated polycrystalline elastic properties, such as bulk modulus, Youngs modulus, shear modulus and Poissons ratio, are predicted through Voigt–Reuss–Hill approximation. The mechanical characteristics of these two crystals, such as ductile–brittle characteristic and elastic anisotropy, are further assessed by way of the calculation of the Cauchy pressures, Zener anisotropy factor and directional Youngs modulus. Additionally, the temperature-dependence of Debye temperature and heat capacity are obtained according to a quasi-harmonic Debye model, and their band structures and density of states profiles are evaluated through analysis of electronic characteristics. The calculation results show that these two IMC crystals are not only an elastically anisotropic, low stiff and very ductile material but also a conductor. The elastic anisotropy, mechanical property, Debye temperature and heat capacity of Ag3In all surpass those of AgIn2, and also, Ag3In tends to be much stiffer than AgIn2. Furthermore, the heat capacity of these two crystals strictly follows with the well-known T3-law at temperature below the Debye temperature and would reach the Dulong–Petit limit at temperature above the Debye temperature.

Size, temperature, and strain-rate dependence on tensile mechanical behaviors of ni 3 sn 4 intermetallic compound using molecular dynamics simulation

This study focuses on exploring the mechanical properties and nonlinear stress-strain behaviors of monoclinic Ni3Sn4 single crystals under uniaxial tensile test and also their size, temperature, and strain-rate dependence through constant temperature molecular dynamics (MD) simulation using Berendsen thermostat.The deformation evolution of the Ni3Sn4 atomic nanostructure during the tensile test is observed. In addition, the tensile yield strains of various Ni3Sn4 single crystals at different strain rates and temperatures are characterized through unloading process. At last, by way of linear regression analysis, the corresponding normal elastic stiffness constants are approximated and then compared with the literature theoretical data. The radial distribution function analysis shows that Ni3Sn4 single crystal in a one-dimensional nanowire configuration would become a highly disordered structure after thermal equilibration, thereby possessing amorphous-like mechanical behaviors and properties. The initial elastic deformation of Ni3Sn4 single crystal is governed by the reconfiguration of surface atoms, and its deformation evolution after further uniaxial tensile straining is characterized by Ni=Sn bond straightening, bond breakage, inner atomic distortion, cross-section shrinking, and rupture. The calculated normal elastic constants of Ni3Sn4 single crystal are found to be consistent with the literature theoretical data.

On the mechanical properties of Cu 3 Sn intermetallic compound through molecular dynamics simulation and nanoindentation testing

Cu3Sn crystal is a well-known intermetallic compound (IMC), which is often observed at the interface of Sn solder and Cu metallization. It is generally recognized as the major cause of the failure of solder bumps and electrodes in microelectronics industry. The aim of the study is to investigate the elastic mechanical properties of orthorhombic Cu3Sn crystal by way of molecular dynamics (MD) simulation and dynamic nanoindentation testing. In the MD simulations, the force field between atoms is modeled with the modified embedded atom method (MEAM). Based on the continuum mechanics assumption, the elastic stiffnesses of the Cu3Sn can be derived from the calculated energy, and then, used in the generalized Hooks law in compliance form to calculate the associated mechanical properties. Further, using the Voigt-Reuss bounds and Voigt-Reuss-Hill approximation these mechanical properties are averaged for facilitating the comparison with experimental data. Besides, the size-dependent effects on the mechanical properties of the crystal are also assessed. The numerical results show that average bulk modulus, Youngs modulus, shear modulus and Poissons ratio of the orthorhombic Cu3Sn crystal are 128.9 GPa, 132.7 GPa, 49.9 GPa and 0.328, and most importantly, they agree very well with our nanoindentation testing results and those published theoretical/experimental data in literature.

First-principles density function calculations of physical properties of triclinic Cu7In3

First principles density functional theory calculations within the generalized gradient approximation are performed to comprehensively study the structural, elastic, electronic and thermodynamic properties of triclinic single and polycrystalline Cu₇In₃. The polycrystalline elastic properties are predicted using the Voigt-Reuss-Hill approximation and the thermodynamic properties are evaluated based on the quasi-harmonic Debye model. Their temperature, hydrostatic pressure or crystal orientation dependences are also addressed, and the predicted physical properties are compared with the literature experimental and theoretical data and also with those of three other Cu-In compounds, i.e., CuIn, Cu₂In and Cu₁₁In₉. The present calculations show that in addition to being a much better conductor compared to Cu₂In and Cu₁₁In₉, Cu₇In₃ crystal reveals weak elastic anisotropy, high ductility and low stiffness, and tends to become more elastically isotropic at very high hydrostatic pressure. Moreover, the Cu₇In₃ holds the largest high-temperature heat capacity among the four Cu-In compounds.

First-principles calculations of structural and elastic properties of hexagonal Cu 2 In intermetallic compound

In recent years, Pb-Sn solders have been commonly applied in the microelectronic packaging industry over last decades due to their remarkable properties, such as good wetting, low melting temperature and low cost. Nevertheless, because of the toxic Pb element in the solders, significant environmental and health issues are created. Due to the high environmental awareness, green electronics products are presently advocated and proclaimed through legislation [1]. It has been extensively reported that multi-component Pb-free solders are potential to be the substitute of the Pb-Sn solders for developing green products. In recent years, extensive thermodynamic database of some metal elements, such as Ag, Cu, In, Sn, and Zn, has been established by many studies in replace of Pb to synthesize a novel Pb-free solder alloy [2]. The In-based solders could be one of the favorable candidates due to their good wetting, thermal fatigue durability, high ductility and appropriate melting point. However, there are some significant technical challenges needed to be resolved prior to their full and successful implementation and application. For example, Cu has been widely utilized in the under bump metallurgy (UBM) of chip and substrate metallization for solder bonding in the microelectronics industry. Because of the active chemical diffusion characteristics, it tends to diffuse into Sn-In based solder during assembly process and isothermal aging testing to form several intermetallic compounds (IMCs) at the interfaces between the solder and UBM, such as Cu2In. The IMCs can induce a great influence on the structural stiffness and material strengths of solder joints, which are essential to the reliability performance of the electronic packaging. In literature, extensive focuses have been placed on the interfacial formation and evolution of the Cu2In IMC by many researches. It should be noted that the success of application of the Pb-free solder in the advanced interconnect technology strongly relies on the full comprehension of the mechanical properties of the IMC. Unfortunately, only limited studies have been attempted to explore the essential subject in literature. Thus, the main goal of the study attempts to provide a more complete and comparative investigation of the structural and elastic properties of the Cu2In IMC through first-principles calculation by density functional theory (DFT) [3] within the generalized gradient approximation (GGA) [4] based on pseudopotential method. It is believed that through the investigation of its elastic properties, one can have a better understanding of its relation to the thermal-mechanical reliability of solder joints.

Influence of IMC surface geometry and material properties on micro-bump reliability of 3D Chip-on-Chip interconnect technology

This study aims at investigating the growth reaction of the Ni3Sn4 IMC during thermocompression bonding process, the anisotropic elastic constants of the IMC, and the effects of the material properties and surface geometry or morphology on the interconnect reliability of a three-dimensional (3D) Chip-on-Chip (CoC) interconnect technology with Cu/Ni/SnAg micro-bumps subject to accelerated thermal cycling (ATC) loading. The research starts from the investigation of the growth reaction of the Ni3Sn4 IMC during thermocompression bonding process through experiment and classical diffusion theory. The relationship between the Ni3Sn4 IMC thickness and bonding temperature/time is derived based on the predicted activation energy of the chemical reaction of the IMC layer by experiment. Next, the elastic stiffness coefficients of single crystal monoclinic Ni3Sn4 are calculated through molecular dynamics (MD) simulation using the polymer consistent force field (PCFF). The degree of anisotropy in the Ni3Sn4 crystal system is also confirmed by the electronic structure of single crystal Ni3Sn4 using first-principles calculation based on density function theory (DFT). For comparison with the published experimental data and also use in the subsequent reliability analysis, the effective elastic properties of polycrystalline Ni3Sn4 are derived using the Voigt-Reuss bound and Voigt-Reuss Hill average based on the calculated elastic stiffness coefficients. At last, 2D plane strain finite element (FE) analysis together with an empirical Coffin-Manson fatigue life prediction model are performed to predict the interconnect reliability of the 3D CoC interconnect technology. The computed results are compared with the ATC experimental data to demonstrate the effectiveness of these two FE models. The dependence of the interconnect reliability on the thickness, material properties and surface geometry or morphology of the Ni3Sn4 IMC is addressed.