Modification of a strong (high Cu) Sn-Ag-Cu (SAC) solder alloy with selected alloy additions (X=Co, Fe, Zn, Ni, Si, Mn, and Ge), substituting for Cu, has proven successful for retention of solder joints (between Cu substrates) with consistent strength and ductility after thermal aging at 150C for up to 1000 h. Such improvements are critical to Pb-free assembly of portable electronics and electrical devices with potential for drop impact failure of interconnects. SEM revealed a variety of as-soldered joint microstructures that exhibit a range of shear strengths from asymmetric four-point bend (AFPB) testing. After extensive aging, electron microprobe results indicate that intermetallic layer enrichment, perhaps by Cu substitution, is typical, except for the Ge addition. Use of SAC + X in Cu solder joints appears to change interdiffusion of Sn and Cu through Sn-Cu intermetallic compound (IMC) layers and to modify IMC growth. These effects inhibited void formation at the Cu substrate/Cu3Sn interface and prevented embrittlement on thermal aging, detected initially as premature AFPB failure. Izod impact test data, post-AFPB microstructures, and diffusion modeling have added understanding of operative mechanisms and helped to rank preferred choices. Supported by ISU Research Foundation and USDOE-BES (W-7405-Eng-82).