Development of a Spherical Micro-Indentation Test Methodology for Creep Analysis and On-set of Tertiary Creep for Solder Alloys

Abstract

Development of a spherical micro-indentation test methodology for creep analysis and onset of tertiary creep for solder alloys Dumbi Azubuike Otunyo The continuous miniaturization of electronic devices and systems demands more understanding of the creep deformation and stress relaxation of solder alloys in order to improve the structural integrity of electronic devices and systems. Creep is characterized by a slow, time-dependent deformation of the material under constant stress or force. These solder alloys are often subjected to time-dependent deformation during service conditions, hence strain-time relationships obtained from creep experiments are used to analyze the deformation mechanism of mechanical structures and electronic interconnects under stress controlled conditions. Recently, instrumented indentation methods were proposed to study creep deformation mechanism as well as determining creep parameters. Instrumented indentation is advantageous when the material is only available as small test pieces or the area of interest is limited to a small area or feature. Soft materials such as lead alloys present difficulties when machining samples for use in traditional material property evaluation tests such as tensile tests. Therefore instrumented indentation can provide a simple testing methodology for investigating creep mechanical properties of solder alloys. In this research, material time-dependent behavior of Sn3.5Ag, Sn37Pb and Sn5Sb solder alloys were studied by a unique depth sensing micro-indentation method at room temperature. Stress exponent values were determined through a constant loading process utilizing a spherical micro indentation method, where the strain rate is extracted from the indentation rate. The measured stress exponent values are in good agreement with conventional creep experiments. Utilizing a multiple loading and partial unloading micro-indentation testing procedure, time-dependent stiffness changes of these materials could be measured. This continuous stiffness responses during a creep test can be correlated to test materials micro structural changes during creep, therefore making it capable to predict onset of tertiary creep failure. Test results show a correlation between measured continuous stiffness response and creep damage with the capability to predict the onset of tertiary creep of these solder alloys. ACKNOWLEDGMENTS I would like to thank my advisor Dr. Bruce Kang for giving me such a wonderful opportunity to work with him on such an exciting topic. I would also like to thank my thesis committee members, Dr Edward Sabolsky, Dr. Steven Woodruff, Dr. Xuyean Song and Dr. Jacky C. Prucz for their help and technical support I would also like to thank my friends, family and the Benjamin M. Statler College of Engineering for their support during my tenure as a graduate student at the West Virginia University.