Sacrificial Passivation of Nanoscale Metal Powders for Transient Liquid Phase Bonding

Abstract

Differential scanning calorimetry (DSC) was used to evaluate the extent of liquid phase formation in particle compacts (compressed dry powders) comprised of organically capped nano-scale Ag particles and Sn. The Ag nanoparticles employed where synthesized with various organic molecules and procedures to produce particles with a range of capping thicknesses and decomposition temperatures (T_d), as measured by thermogravimetric analysis (TGA). A baseline sample containing commercially available un-capped micrometer scale Ag was also investigated for comparison. Results indicate that all of the Sn initially present formed a liquid phase when heated through its melting point when combined with Ag particles exhibiting a comparatively thick cap of low T_d. Slightly smaller fractions of Sn liquid were obtained when the Ag's cap was thin and of a high T_d while particles with thin-low T_d caps exhibited the highest levels of Sn consumption and similar to that observed with the un-capped micron-scale Ag particles. The reduction in the amount of Sn liquid formed is attributed to solid state reaction between the Ag and Sn particles resulting in the formation of a more refractory phase. The extent of the subsequent liquid phase reaction was also evaluated and is demonstrated to not necessarily be adversely effected by the presence of the organics. The significance of this work is the demonstration that organic molecules may be employed to prevent solid state reaction in particle compacts at elevated temperatures, yet allow the subsequent liquid phase reaction proceed uninhibited.