Volume Shrinkage in InPb/Au Solder Joints under Varied Environmental Conditions: A Kinetics Study

The dynamic stability of InPb/Au solder joint volumes under thermal aging has been investigated, exploring the interplay of atmospheric and vacuum conditions. Samples were aged at 25 °C, 85 °C, and 125 °C under atmospheric pressure (103 mbar) and at 125 °C in high vacuum (HV) (10–6 mbar). Using advanced opto-digital microscopy (ODM), high-resolution X-ray microscopy (XRM), and scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), we uncovered significant morphological transformations and pronounced volume shrinkage. These changes were associated with the growth of AuxIny intermetallic compounds together with Pb- and In-rich phases, forming characteristic dark rings at the solder/Au interface. The activation energy (Ea) governing the shrinkage process was precisely determined to lie within 0.38–0.41 eV, consistent with diffusion-controlled migration of In and Pb into Au-rich IMCs, while acceleration factors (AFs) were quantified, enabling long-term behavior prediction. In comparison, SnPb/PtAu and SAC305/ENIG solder joints exhibited negligible volume change and slower IMC growth, highlighting the unique susceptibility of InPb/Au to volumetric variation. These findings reveal the primary influence of temperature and pressure on solder alloy integrity, providing practical insights for reliability assessment in space, aerospace, and other high-reliability electronic applications, where lead is still allowed.