Lead Free melts at a higher temperature. You will need at least 25C above that used for Leaded Solder. 425C is a good temperature. A fixed-temperature normal iron that is designed for Leaded Solder (usually 360-380C) may not give nice results. Also, beware, Lead-Free is very aggressive and devours Soldering Iron Bits - buy some spares. It's not unusual to go thru a Bit per week if you haven't got a Nitrogen feed to the tip.

Tip: Unsoldering Lead-Free (especially on a THP board) is a right pain. It's not kosher, but solder the joint with Leaded Solder first, you'll find it then becomes more free-flowing and unsolders a lot easier.

Best of all, avoid Lead-Free if you're a novice. Poor soldered joints, especially around components that can get hot (eg Power devices), quickly oxidise with a black stain around the joint which is near impossible to resolder.