Interfacial microstructures produced between Cu and AlN using a Cu-rich Cu–P–Sn–Ni brazing filler metal as an Ag-free material and a Ti layer as an active metal have been examined. Cu was bonded onto AlN substrates in vacuum for 1 h at temperatures between 650 and 950 °C with 1- and 5-μm-thick Ti layers. In contrast to bonding with a 0.5-μm-thick Ti layer, four different phases containing Ti and O/N were identified during the development of the Cu/AlN interfacial reaction layer: an amorphous P–Ti–O phase, an amorphous Ti–O phase, a rock-salt titanium oxynitride TiOxNy and TiN. The increase in the N concentration in the Ti oxide phase was caused by AlN erosion of the Ti oxide phase in the solid phase promoting the growth of the TiOxNy phase when using the 1-μm-thick Ti film. In contrast to this, the remelting of the Cu phase at high temperature when in contact with AlN using the 5-μm-thick Ti foil promotes the substitution reaction between Ti and AlN, as in the active metal bonding method using Ag.