A "cold" solder joint is generally grainy, which is a sign that the solder didn’t melt and wet properly, so you may not have a good metallurgic bond. The connection will most likely break loose with any amount of physical or thermal stress. Be warned that graininess is a visual cue for lead-based solders. Lead-free solders often look grainy even when there is a good metallurgic bond.
The quickest way to fix a cold solder joint is to simply use a solder iron to remelt the solder. The flux will have been fully activated in the original soldering process, so additional flux can be added using a CircuitWorks Flux Pen.
Lightly press the Flux Pen tip to the solder joint until the tip saturates enough for flux to start to flow.
Rub saturated Flux Pen tip over the solder joint.
Apply the heated solder tip to the solder joint and remelt.
If necessary, clean flux residues with either a swab saturated with a solvent cleaner like isopropyl alcohol (IPA) or a CircuitWorks Flux Remover Pen.