ROSIN core solder comes in many various grades from totally useless, to capable of bonding corroded wires, and everything in between. Using a weak flux (usually says it's "eco") will not give you a good bond, nor will using "lead-free" solder since the original solder on the card is lead-based. A good rule of thumb for soldering fluxes.....if it smokes and smells bad when it hits the iron and leaves behind a reddish-brown "goo", it's the right stuff to use on these cards. Just clean the card well with rubbing or denatured alcohol when finished to get rid of the leftover rosin.