Phase Transitions of the CU₆Sn₅ Phase Under Electric Currents

The performance and reliability of interfacial regions in electronic solder joints are critical for the durability of lead-free solder systems. The intermetallic compound <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Cu}_{6}\text{Sn}_{5}$</tex>, which forms at the interface between Sn-based solders and Cu substrates, exists in two crystal structures: the low-temperature monoclinic phase <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\eta^{\prime}-\text{Cu}_{6}\text{Sn}_{5})$</tex> and the high-temperature hexagonal phase <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\eta-\text{Cu}_{6}\text{Sn}_{5})$</tex>. Although the thermal behavior of these phases is well-documented, the effects of electric current on the stability and mechanical properties of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\eta^{\prime}-\text{Cu}_{6}\text{Sn}_{5}$</tex> remain less understood. This study investigates the impact of electric current on the microstructural evolution of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\eta^{\prime}-\text{Cu}_{6}\text{Sn}_{5}$</tex> through high-resolution transmission electron microscopy (HR-TEM) following the synchrotron radiation-based X-ray diffraction (SR-XRD). Single-phase <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\eta^{\prime}-\text{Cu}_{6}\text{Sn}_{5}$</tex> samples were subjected to a current density of 1.5 x 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 5 hours. Microstructural observations from HR-TEM, following SR-XRD, reveal significant insights into the behavior of monoclinic <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\eta^{\prime}-\text{Cu}_{6}\text{Sn}_{5}$</tex> under electric current stressing. These findings offer insights into the relationship between electric current and phase stability in interfacial compounds, which could enhance the reliability of solder joints in advanced electronic systems.