We study the stellar specific angular momentum of the disc components for a sample of ten field/group unbarred lenticular (S0) galaxies from the CALIFA survey. The location of these discs in the stellar j-M plane allows us to obtain clues on the physical processes that lead to the formation of S0s from spirals. We found that eight of our S0 discs have a distribution in the j-M plane that is fully compatible with that of spiral discs, while only two have values of j lower than the spirals. These two outliers show signs of recent merging. Our results suggest that merger and interaction processes are not the dominant mechanisms in S0 formation in low-density environments. Instead, S0s appear to be the result of secular processes and the fading of spiral galaxies after the shutdown of star formation.