Nanoindentation testing was used to obtain mechanical properties on realistic Sn-Ag solder joints made with (i) eutectic Sn-3.5Ag solder and (ii) in-situ Cu6Sn5 particle reinforced, eutectic Sn-Ag matrix composite solder. The composite solder joint contained ˜20 volume percent of intentionally-added Cu6Sn5 intermnetallic particles which were ∼5 microns in size. The Cu6Sn5 particles were dispersed in-situ in eutectic Sn-3.5Ag matrix alloy as a reinforcement phase to stabilize the microstructure by acting as a non-coarsening microconstituent phase. Mechanical properties characterization and deformation behavior were assessed on the bulk microstructure and on microconstituents in the actual solder joints. Mechanical property data obtained on constituents included hardness, elastic modulus, strain rate, and creep behavior. The pushing and rotation of the reinforcement particles that often occurred when contacted by the indenter provided a means for evaluating the interfacial shear strength of reinforcement particles in the solder matrix.