Phase Determination in SLID Bonding

Solid-liquid interdiffusion (SLID) bonding is a technique based on intermetallic compounds (IMCs), enabling a thermal stability at temperatures far surpassing the bonding temperature. The technique has been developed as a die attach and interconnection technology for high-temperature applications, but is also excellent for fine-pitch bonding, and for obtaining bonds with thin layers of well-defined metallurgy. Determining the phases of IMC in a SLID bond is crucial in order to understand and predict the properties of the bond. The re-melting temperature of the bond is defined by the IMCs present, and thus directly defines the high-temperature range the SLID bond can survive. Furthermore, the phases present in a SLID bond determines whether the bond is at thermal equilibrium, or if reactions to form new IMCs are expected over the lifetime of the SLID bond (at the actual application temperature). Also, material properties such as electrical conductivity and elastic modulus will depend on which phase is present in a SLID bond. The two most common SLID systems are treated in this paper: Cu-Sn has a relatively simple phase diagram, with two IMCs. The possible phases in a Cu-Sn SLID bond are easily identified by Energy-Dispersive X-ray Spectroscopy (EDX) in the Scanning Electron Microscope (SEM), and they are easily differentiated in optical microscopy as well as in SEM microscopy. Routine investigations of spatial distribution of the various phases can thus be performed by microscopy. Au-Sn has a more complex phase diagram. Au and Au-Sn IMCs are easily distinguished in optical microscopy, but not so easily in SEM. The different IMCs are not discernable neither by microscopy nor by EDX. By using Electron Backscatter Diffraction (EBSD) in combination with electron microscopy and EDX, we demonstrate phase identification and the spatial distribution of phases in a complex Au-Sn SLID bond.