CALCE Webinar - Pitfalls of Creep Indentation Testing of Sintered Silver Interconnects

David Leisle
Thursday, August 19, 2021 11:00 am US EDT

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Nanoindentation has been popular since the 1970s for querying material behavior at extremely small length scales. In addition to hardness and elastic behavior, recent nanoindentation studies have also focused on inelastic behavior including viscoplastic behavior. This research focuses on using a combination of experimentation and FEA methods to bound the bulk viscoplastic behavior of pressure-less sintered silver, which poses additional challenges because of the heterogeneous morphology. Sintered silver material is seen as an attractive material because of the low sintering temperatures (150 to 300C), both as a die attach in power electronic devices, as well as conductive traces in printed electronic assemblies.

The experimental portion of this study consists of nanoindentation testing on sintered silver test coupons. Constant force tests at room temperature and at elevated temperatures were conducted using a Berkovich indenter and spherical indenters of various diameters. FEA models of the indentation, were used to estimate creep curves, by parametrically fitting the experimental results using an iterative inverse method. There is an element of non-uniqueness in the FEA inverse extraction process, since there are many material constants that have to be estimated from relatively few test conditions. The study has also shown that simpler closed-form models commonly used in the literature to extract creep material properties may underestimate creep resistance of sintered materials by an order of magnitude, because of their nanoscale and microscale heterogeneity. Furthermore, the heterogeneous microstructure may also introduce intrinsic length-scale effects, which result in different apparent creep properties, depending on the radius of the indenter tip. Overall, further work is needed before indentation offers a viable approach for assessing scale-dependent creep properties for heterogeneous materials. This information cannot be obtained from testing of bulk specimens and must necessarily rely on indentation testing with a suitable family of indenter tips.

About the Presenter: David Leslie is currently a PhD candidate in Mechanical Engineering and a graduate research assistant at the Center of Advance Life Cycle Engineering (CALCE), at the University of Maryland, College Park. He is advised by Professor Abhijit Dasgupta, and his dissertation research is in the field of nanoindentation of viscoplastic heterogeneous solids with specific application to pressure-less sintered silver interconnect materials used in high-temperature electronic systems.