Poster Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

EBSD measurements of anisotropic residual stress and strain distributions in tin-based solder alloys (#257)

Kazuhiro Nogita 1 , Mark Nave 2 , Selena Smith 1 , Jonathan Read 1 , Yueqin Wu 1 , Stuart D. McDonald 1
  1. Nihon Superior Centre for the Manufacture of Electronic Materials, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD, Australia
  2. Deakin University, Geelong, VIC, Australia

The anisotropic distribution of residual stress and strain in tin-based lead-free solders loaded with a spherical indenter was investigated using cross-correlation-based analysis of electron back-scattered diffraction (EBSD) patterns.  β-tin, with a tetragonal structure (I41/amd, (141), a=5.831Å, c=3.182Å), has significant anisotropy in terms of the coefficient of thermal expansion (CTE) and mechanical properties and is prevalent in many lead-free solders found in electrical interconnects.  The CTE for β-tin in the [100] and [010] directions is 15.4x10-6/˚C, whereas, in the [001] direction it is 30.5x10-6/˚C.  This difference in CTE can potentially cause large stresses at the grain boundaries and hence give rise to thermomechanical fatigue failures during service.  The aim of the work presented here was to establish the usefulness of cross-correlation-based EBSD analysis for characterising the distributions of stress, strain and rotation within the β-tin matrix in order to inform the development of improved solder alloys. Clear differences could be seen between two different alloys in both the magnitude and the anisotropy of the distributions.  The results were compared with those from an un-indented area of specimen to establish their significance above variability due to experimental factors, such as imperfections in specimen preparation and limitations in EBSD pattern quality. This technique is suitable as a key method in research to understand the reliability of micro-electronic interconnects for future electronic packaging applications.