Solder Paste vs Solder Flux
This is not the stuff to use to solder electronic boards, this is for plumbing fittings, a flux to be used in addition to stick of solder.

Electronic boards require either 'flux-cored solder' for hand use, or 'solder paste', which is a mixture of small solder balls in a suitable flux, for machine use.

This flux is likely to be too aggressive to leave on a board, and will require cleaning, otherwise it's likely to damage the board. The flux used in flux-cored solder or solder paste is designed to be compatible with the materials on a PCB, and it's usually safe to leave the residues there indefinitely.