Acid based flux is a quick recipe for green rotted conductors resulting in a failed connection, and as suggested previously; for use on plumbing & sheet metal work, and joint should be washed or neutralized.
Silver solder, something I have been using recently installing plumbing during construction of our new digs, exhibits rather different melting characteristics from 50-50 lead-tin solder; it passes from solid to fluid in a rather narrow temperature range that appears to be higher than the 50-50 solder. I have found it less user friendly and don’t practically like it, and suspect that it is harder to use on electronics with low watt irons where higher heat is necessary to get it to flow. Longer heat-up time, exposes components to excessive heat, and a heat sink may be necessary for safe soldering on anything other than standard 10-18 gage conductors used in automotive harnesses.
I still like the old EPA unfriendly; 60-40 rosin core solder. It flows at a lower temperature than 50-50, and much lower than silver blends, has a longer plastic stage before melting, making for an easily controlled joint.
For soldering conductors such as found in our old car harnesses, I prefer a two heat range Weller 100-140 watt soldering gun. It heats up fast, and with a drop of molten solder on the tip, heat is quickly transferred to the conductor splice because of the larger contact area created with the drop reducing transient heat traveling up under the insulation. As soon as the solder flows shinny, remove the hot tip, don’t let it linger exposing the conductors to any extra heat. On electronics, a low watt pencil iron is sufficient, but one must take care not to over heat the component, use of a heat sink such as needle nosed pliers maybe good insurance.
Not related to automotive soldering info department:
I construct Tiffany reproduction stained glass lamp shades using his copper foil method as opposed to lead came. On these projects, liquid acid flux is used to clean and etch the copper for soldering. When building a shade, the project is first tacked, and rough soldered together with 50/50 solder, and than a decretive solder bead is built up over the 50/50 using 60/40 solder which melts as a lower temperature than the 50/50. This tacking & beading can take many hours on a large lamp requiring an adjustable heat iron to prevent over heating the 50/50 base. When completed, the lamp shade must be completely scrubbed inside & out with a flux neutralizer to prevent destroying joints, and producing festering unsightly white stuff (I suspect is some kind of lead oxide).
I have about 15 hours of soldering out of 90 construction hours into this 1650 piece lamp hung over our kitchen table. Each piece of glass is cut from a sheet of art glass (automotive tie-in: I use automatic transmission fluid to lubricate the cutting wheel), their edges are than ground square using a wet diamond glass grinder, copper foiled (a little over a 100 yards in this project), than placed & secured on a three dimensional pattern to be soldered into five repeating sections which are than joined to form the shade. This shade utilizes about ten one pound rolls of solder.
Flowering Lotus: 18" dia. by 14" tall, requiring 11 square feet of glass.