Bulk and surface passivating properties of low frequency direct PECVD SiNx:H were investigated on 0.5-1 Ωcm p-type mc-Si material. It is observed that bulk passivation is preserved for different SiNx:H layers, deposited in a wide range of gas flow ratio and power. This allows optimisation of the layer composition towards rear surface passivation of mc-Si, while maintaining the bulk passivation. Lifetime measurements showed that a thermal treatment of SiNx:H leads to a lower minority carrier trapping density in the mc-Si bulk. Also a strong effect of the surface condition on the surface passivation was found. Different surface conditions have a strong impact on blister formation, which are possibly related to the diffusion mechanism of hydrogen from SiNx:H into the mc-Si bulk.