We are not using SN100C in our reflow process but I know the type of flux made a big difference with our SAC305 solder. We started out using no-clean flux in our SAC305 paste and were having some issues. We then switched over to a water soluble flux (same SAC305 solder) that is more aggressive. That solved the issues we were seeing. Changing the flux type in your SN100C paste might help.