Make sure you get ENOUGH HEAT on the metal joints to properly FLOW the solder. In addition to a clean joint that is properly fluxed, the metal must be HOT, otherwise, you will have a cold joint that will fail. A well flowed joint, will be smooth and shiny.....and when the metal is properly hot, it will flow and absorb solder like a sponge absorbing water. A COLD joint, will have messy solder that looks like cake frosting.