An extended solid solution of Cu-20wt%Sn with lattice parameter 0.3715 nm and grain sizes 7 nm, was obtained by mechanical alloying. A DSC trace of Cu-20wt%Sn showed an exothermic effect at 643 K due to the crystalline grain growth and the formation of an equilibrium solid solution. The reaction of Cu-20wt%Sn with a Ga-In-Sn liquid alloy was studied at room temperature by measuring peak intensities of CuGa2, the intermetallic compound formed during the reaction, as a function of time. The reaction rate increased when crystalline grain size D decreased. Mechanical alloying process used to control the reactivity of nanocrystalline solid solutions as components in low temperature solder pastes.