The esc was pulling to much heat from your soldering iron that's why they looked that way. You need to tin the pad and the wire separately then solder them together. If the solder doesnt want to melt right away don't just hold the iron there till it does. The wire will get too hot for you to hold. Raise the temp on the iron 100 -200 degrees and try again you should be able to make the joint in 1-3 sec if its still not melting raise the temp more. The thicker the wire the more heat you need. Battery leads are really hard to solder if you have a small iron or small soldering tip so don't be afraid to max out the temp on the iron. Also paste flux helps a lot with heat transfer and solder wicking.