The way to get a proper solder joint is to heat the parts to the point they will melt the solder, not the iron. If the iron melts the solder before the parts are hot enough to do it you will get a cold joint.
Always clean and tin the parts this will insure you do not get a cold joint.
I use a pin torch myself if possible as this always works better than an iron for me. of course this will not work close to flamable parts. PUFF.....SMOKE............FIRE FIRE FIRE

VOICE OF EXPERIENCE.

Miloh.