In case any posters are wondering, 63/37 solder refers to the combination of alloys used in the solder. This is 63% tin, 37% lead, and almost always a small amount of silver and a few other very minor amounts of other metals and flux. This is call a eutectic solder. This combination of alloys is the LOWEST melting point of tin/lead alloys. Any combination of metals has it's own eutectic point. This solder has no semi solid state. It is either liquid or solid, with no plastic state, a frequent cause of cold solder joints. It melts AND freezes at 183 degrees C. This temperature is ACTUALLY lower than either tin or lead's normal melting points! MAGIC! Not really, but it is the eutectic combination of alloys at work.
I would only use old tin can based rosin flux when soldering connections to track. If trying to solder or desolder surface mount components, look for a special flux designed for that purpose. Mostly, for repair, a product that is a combination of rosin core flux and tiny alloy solder balls is used.
Let's reinforce NEVER use ACID based flux on ANY electronics, even 100 year old stuff!