Three ingredients are required to make a good solder joint: solder, clean metal surfaces to connect and heat. In a selective soldering process all three have a big influence on the final result. The solder properties define not only the hole filling, but have an impact on the appearance and shininess of the solder joint as well.
Heat is required to activate the flux and make the solder flow into the barrels. If there is not enough thermal energy in the assembly the solder will solidify in the barrel during the process and there may not be a sufficient hole fill (defined in the IPC-A-610E chapter 7.3.5).
To improve the hole filling properties and to clean the metal surfaces a flux is applied before the soldering. Critical in this process is that the flux is applied on the right place (cleaning the metal and supporting the solder to penetrate into the barrels) and that the flux is activated. For activation of the flux a specified temperature must be achieved for a certain time. This activation temperature and time are flux specific and should be defined by the flux supplier, since they are the only to be familiar with the ingredients that are used in the flux chemistry.