Micromechanical mysteries of Tin based solders

Commercial lead free solders are all essential Tin alloys, and have beta-Sn as the main matrix phase. The mechanics of beta-Sn alloys are still poorly understood due to a complex set of phenomena that come together in the system.  Firstly, beta-Sn has a body centred tetragonal crystal structure which leads to significant anisotropy.  Thermal expansion and elastic stiffness vary from <a> axis to <c> axis leading to stress/strain variations from grain to grain from thermal and mechanical loading. Furthermore, dislocations with different Burger's vectors and slip planes are present but not systematically catalogued resulting in significant anisotropy in the plastic response also.  Indeed, this is to the extent that twinning modes are often required to accommodate plasticity.  And, of course, tin is chosen for solder application because its melting point is low, only ~twice room temperature, so that creep and related thermally activated processes such as grain boundary sliding and migration can occur at room temperature. On top of this in applications Sn is often joining Cu connectors and inter diffusion and reaction products form at the interfaces which drastically alter the mechanical properties, while spontaneous whisker formation can cause shorts and have prevented uptake of these alloys in safety critical applications such as aerospace industries.

This project will use nanoindentation, and micro-mechanical testing (micro-compression, micro-bending) to measure mechanical properties of beta-Sn alloys with aim of determining elastic anisotropy, and rate dependent constitutive laws for different slip systems.  We will also try to unpick the root cause of whisker formation (still very much an open question), and study microstructures and mechanical responses of Cu-Sn alloy interfaces.