Abstract
High resolution serial block-face scanning electron microscopy has been performed on sintered micro-silver pastes that are used as lead replacement joints for die bonding. The size and spatial distributions of the porosity before and after ageing were determined by quantification of the segmented 3D images. The elastic modulus was determined by an image-based finite element model and validated by results obtained from a dynamical resonance method. In agreement with contemporary analytical models on the elastic behaviour of porous materials, the elastic modulus was found to be a function of pore fraction only. Ageing the specimens does not alter the density or Young's modulus.
Highlights
Sintered micro-silver pastes are good candidates to replace leadbased alloys for die bonding of power electronics due to their excellent electrical properties and high melting temperature as described by Li et al (2013) who investigated the creep properties of these materials
Other mechanical properties of sintered microsilver pastes can be found in the literature where Siow (2012) has provided a review of the work done in determining elastic modulus, strength and factors affecting the bonding strength of such joints
With respect to the elastic behaviour, Panin et al (2005) concluded that an increase in grain size due to annealing had no effect on the elastic properties as determined by nano-indentation
Summary
Sintered micro-silver pastes are good candidates to replace leadbased alloys for die bonding of power electronics due to their excellent electrical properties and high melting temperature as described by Li et al (2013) who investigated the creep properties of these materials. The effect of the pore distribution on the elastic properties of sintered micro-silver pastes during ageing is one of the key parameters for modelling the elastic behaviour of the entire electronic system in the operating conditions. This study represents a first step towards addressing this issue by using imaging to determine the porous structures of both as-sintered material and aged material and relating it to the elastic properties. Several analytical models have been proposed that link porosity with the elastic moduli of porous materials. Roberts and Garboczi (2000) used such an approach to model the elasticity of porous ceramics and showed their results to be consistent with the analytical model proposed by Bert (1985) who developed another empirical model that is very close in form to Eq (2) for spherical pore geometries. The authors use SBFSEM datasets to produce FE models of the sintered pastes that are validated against experimentally determined values for Young’s modulus using a dynamic resonance method (DRM) as described by Gadaud et al (2009) who derived a formalism for interpreting the torsional vibrations to determine elastic and shear moduli
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