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Karel Matous



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Microstructure Statistics--Property Relations of Silver Particle--Based Interconnects


A. Gillman1, M.J.G.H. Roelofs2, K. Matous1, V.G. Kouznetsova2,
O. van der Sluis2,3,  and M.P.F.H.L. van Maris2

1
Department of Aerospace and Mechanical Engineering,
University of Notre Dame,
Notre Dame, IN, 46556, USA.

2
Department of Mechanical Engineering,
Eindhoven University of Technology,
Eindhoven, The Netherlands.

3Philips Research Laboratories,
High Tech Campus 4, 5656 AE,
Eindhoven, The Netherlands.


Abstract


      This paper presents a novel approach for establishing microstructure statistics-property relations for a silver particle-based thermal interface material (TIM). Several sintered silver TIMs have been prepared under different processing conditions, generating samples with distinct microstructures. The 3D microstructure is revealed and visualized using the combination of Focused Ion Beam (FIB) milling and Scanning Electron Microscopy (SEM) imaging. Representative synthetic model microstructures have been generated based on Gaussian random field models, having well defined analytical description. The statistical characteristics of the samples and the synthetic models are shown to have a good correspondence, indicating that the linear effective properties of these complex materials can be predicted based on analytical estimates available for the synthetic models. This is verified by computing the effective elastic and thermal material properties using the computational homogenization approach based on the finite element models of the real samples. The computational homogenization, providing the reference solution, and the higher-order statistical estimates for the synthetic models are in very good agreement. These results can be used in the development of new silver particle-based materials, whereby the expensive and time consuming effective material property characterization can be replaced by efficient estimation based on the synthetic random field models.
        

Conclusions


    In this paper, a methodology for establishing microstructure statistics-property relations has been presented and applied to a sintered silver particle-based interconnect material for high power electronic applications. The novel original feature of this approach is the combination of the statistical morphological measures of real and synthetic microstructures with the higher order statistical micromechanics and direct finite element computational homogenization. The main steps and conclusions of this contribution can be summarized as follows.

  • Three sintered silver samples were produced under different processing conditions, i.e. sintering temperatures of 230C (sample S1), 280C (sample S2) and 330C (sample S3). The complex 3D interconnected microstructures of these samples were revealed by the SEM-FIB technique, i.e. through the reconstruction of 3D structure based on Scanning Electron (SEM) microscopy images of sample surfaces revealed by sequential Focused Ion Beam (FIB) milling.

  • The statistical analysis of the microstructural features has revealed that increasing sintering temperature leads to higher volume fraction of the solid phase (lower porosity). More interestingly, the mean ligament thickness and the width of ligament thickness distribution have increased between the samples S1 and S2 and was statistically almost equivalent between the samples S2 and S3.

  • In order to better understand the microstructure statistics-property relations for these complex morphologies, synthetic microstructures have been generated based on Gaussian random field models, having a well defined analytical description. The statistical characteristics of the real samples and the synthetic models have been analyzed and shown to have a good correspondence.

  • The effective thermal conductivity and Young's modulus of the considered sintered silver materials have been computed using the computational homogenization approach based on the finite element models of the 3D microstructures. The results show that the effective thermal conductivity of this 3D interconnected microstructures, in the considered range of volume fractions, is mostly determined by the volume fraction, with the microstructural variations playing a secondary role. The Young's modulus, on the other hand, is more sensitive to the local microstructural features.

  • The computed effective properties have been compared to the predictions based on the analytical higher-order statistical micromechanics estimates for the synthetic microstructures, demonstrating very good agreement. This shows that the linear effective properties of these materials can be predicted based on the analytical estimates for synthetic morphologies.

Acknowledgment

    The authors are grateful to Dr. Sebastian Fritzsche from Heraeus for providing the materials for this study. This work was partially supported by the NANOTHERM project co-funded by the European Commission under the "Information and Communication Technologies", Seven Framework Program, Grant Agreement No 318117.


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2017 Notre  Dame  and Dr. Karel Matous