International Journal of Mechanical Sciences 161–162 (2019) 105020 Contents lists available at ScienceDirect International Journal of Mechanical Sciences journal homepage: www.elsevier.com/locate/ijmecsci Correlation of microstructure and constitutive behaviour of sintered silver particles via nanoindentation Xu Long a,∗ , Bo Hu b , Yihui Feng c , Chao Chang d , Mingyu Li b,∗ a School of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, Xi’an 710072, China b Harbin Institute of Technology Shenzhen Graduate School, Materials Science and Engineering, Shenzhen Key Laboratory of Advanced Materials, Shenzhen 518055, China c State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China d School of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China a r t i c l e i n f o Keywords: Silver nanoparticles Silver microparticles Nanoindentation Microstructure Constitutive behaviour a b s t r a c t During the sintering process, a three-dimensional network-like porous structure forms as the consequences of atom diffusion and neck formation among silver particles. In this paper, two representative types of pressureless sin- tered silver particles, i.e., silver nanoparticles (AgNP) and silver microparticles (AgMP), are investigated to reveal the intrinsic correlation between microstructure and constitutive behaviour. The heterogeneous microstructure is found to result in distinguishing differences in terms of morphology and thermal stability. The difference of sintering mechanisms between these two types of silver pastes are observed by a scanning electron microscope and discussed. Regarding thermal stability, the effect of particle size and shape on the sintering temperature and mass loss are evaluated using the thermogravimetric analysis and the differential scanning calorimetry. The microstructural effect on the mechanical behaviour of both sintered AgNP and AgMP samples is measured by per- forming nanoindentations with a Berkovich indenter penetrated up to 2000 nm with different indentation strain rates. During indentation, Young’s modulus and hardness are measured as the functions of penetration depth. By solving dimensionless equations, the parameters of a power-law constitutive model are obtained analytically based on the integrated work done in the loading stage and the contact stiffness in the unloading stage in the nanoindentation experiments. The correlation between microstructure and constitutive behaviour is elucidated by the comparison of sintered AgNP and AgMP but further investigations related to the size and shape effects of particles are expected. 1. Introduction Die-attach materials in semiconductor devices serve as thermal- mechanical connections. With the development of wide-bandgap semi- conductors such as electronic devices based on the SiC technology, tradi- tional die-attach materials including lead-free solder alloys are difficult to satisfy the demand of high heat dissipation as well as high temper- ature reliability [1–3]. Due to the characteristics of low-temperature sintering and high-temperature working, the paste with silver particles is becoming one of the potential die-attach materials of electronic pack- aging structure of high-density microsystems [4]. As the strain and the induced stress in the die-attach layer usually occur because of the mis- match of thermal expansion coefficients (CTE) between different mate- rial layers, the constitutive behaviour of the sintered silver paste has to be well described with the help of experimental and numerical results prior to reliable incorporations into extensive applications. In the ex- ∗ Corresponding authors. E-mail addresses: xulong@nwpu.edu.cn (X. Long), myli@hit.edu.cn (M. Li). isting publications, a unified visco-plastic Anand model was applied to study the tensile behavior and ratcheting behavior of sintered silver in- terconnects [1,5]. Creep behavior of sintered silver lap-shear joints was investigated and a constitutive relation for the steady-state creep was established by Li et al. [6]. In order to focus on the intrinsic local prop- erties by ruling out the size effect of specimens, the strain rate sensitivity with indentation strain rates and penetration depths are investigated by a novel technique of multiple strain-rate jumps, and the stress exponent is also determined to indicate the deformation mechanism of creep be- havior by Long et al. [7]. However, technical challenges still exist due to the lack of constitutive models for the thermal-mechanical reliability evaluation of electronic packaging structures. The sintered silver paste is usually of porous morphology, which makes its mechanical behavior more complicated. After reviewing the existing studies, three aspects are found to be closely associated with the porosity to indicate the intrinsic porous morphology of sintered silver https://doi.org/10.1016/j.ijmecsci.2019.105020 Received 20 November 2018; Received in revised form 6 July 2019; Accepted 11 July 2019 Available online 11 July 2019 0020-7403/© 2019 Elsevier Ltd. All rights reserved.