A composite formulation and composite product are disclosed. The composite formulation includes a polymer matrix, tin-containing particles blended within the polymer matrix at a concentration, by weight, of at least 25%, copper-containing particles blended within the polymer matrix at a concentration, by weight, of at least 40%, and one or both of solder flux and density-lowering particles blended into the polymer matrix. The tin-containing particles and the copper-containing particles have one or more intermetallic phases from metal-metal diffusion of the tin-containing particles and the copper-containing particles being blended at a temperature within the intermetallic annealing temperature range for the tin-containing particles and the copper-containing particles.