After gathering sufficient microstructural evidence that a practical high-temperature lead-free solder could be produced through liquid-phase diffusion bonding (LPDB) of gas-atomized Cu-Ni powder blended with SN100C (Sn-0.7Cu-0.05Ni, wt.%) commercial solder powder, additional research was completed to enable design of a prototype composite solder paste. By reviewing several experimental alternatives to LPDB for use as replacements and improvements to the currently-used high-Pb solders (which will inevitably be banned by the EU’s RoHS directive), the choice to pursue the prototype solder paste research was verified. One key difference between this research and that of others is the method of pre-alloying Cu with Ni before atomization of the metal powder particles. These Ni additions prevent a detrimental unit cell volume change from occurring in typical Sn-Cu solder alloys by suppressing the hexagonal-to-monoclinic isothermal phase transformation upon cooling during reflow to below 186à °C. Rapid diffusion of the Ni and Cu was explored in order to better design the composite paste’s suggested reflow profile. Research to study and maximize the grain boundary diffusion of Ni and its effect on the resulting intermetallic compounds helped determine the final composition of the composite paste for use in industry.