A lot of good points out here. I always remove old solder re-tin motor tabs and motor wire, put the two together and heat with a little solder on the iron. You want to see the entire joint melted not just a partial melt. Blam shiny and just the right amount. Takes some practice but the amount of heat you place, the positioning of your iron tip on that joint and the amount of time you leave that solder iron on there has a lot to do with it also.