Ni-B film of $1{mu}m$ thickness was electrolessly deposited on an electroplated Cu bus electrode. The film, which encapsulates the Cu bus electrodes, prevents Cu oxidation and serves as a diffusion barrier against Cu contamination of the transparent dielectric layer in a plasma display during the firing process at $580^{circ}C$. The microstructure of the as-deposited barrier film was amorphous phase and crystallized to Ni and $Ni_3B$ after annealing at $300^{circ}C$. The good barrier properties observed here can be explained by $Ni_3B$ precipitates at the grain boundaries acting as a fast diffusion path via pre-annealing at $300^{circ}C$ before the firing process at $580^{circ}C$.