Fast in-situ synchrotron X-ray imaging of the interfacial reaction during self-propagating exothermic reactive bonding

Self-Propagating Exothermic Reactive (SPER) bonding with lead-free solders is potentially attractive for microelectronics assembly due to its highly localised heating and minimal thermal loading of the components and substrates. The transient dynamics of melting, wetting, solidification and defect formation during SPER bonding were observed using in-situ synchrotron X-ray imaging with sub-millisecond temporal resolution and the results were further analysed using electron microscopy and thermal modelling. In-situ imaging revealed the preferential melting of the solder and subsequent wetting of the substrate. Numerous air bubbles were observed to form at the bonding interface. The distribution of these bubbles was found to vary with the thermal conductivity and wettability of the substrates. These bubbles appear to reduce the effectiveness of bonding by promoting the formation of cracks and voids within the solder joint. Our results show that metallisation layers on the bonding substrate can influence the dynamics of melting for the solder materials and thereby directly influence the reliability of SPER interconnects.