When new to using the hot air method for component removal its a Very good idea to practice on old salvaged PCBS until you get competent with using the hot air gun with out causing any damage. You didn’t notice the solder had reached the molten state because there’s so little of it used to solder components.
The typical lead-free solder has a melting point around 217 degrees C, so you’ll have to get the leads and pads up to that temperature before trying to remove the component. The reason why you need a much higher temperature is because you want to get the solder joints to the melting point as fast as possible. If the hot-air gun is set to a much lower temperature, it’ll take a longer time to reach the melting point. The longer the time to raise the temperature of the leads/pads will increase the overall temperature of the component, possibly past the point of destruction. So the technique is to heat it up fast, remove the part and hot-air gun, then put the part where it can cool off.
Now, if you’re removing a part that’s already fried due to some other reason, no worries then. Just don’t damage the board by overheating the pads and causing them to lift off. If that happens, your headaches are just beginning on this repair.
I am working with SMD for a while and I suggest you use leaded solder to treat the oxidized solder joints with flux before you get to the hot air stage. After solder mixture process, preheat the board close to 200C and continue with hot air station. I use analog hot air stations because I don’t want to turn around and look at the temperature. I know where I have to set the hand dial without looking and I think that is between 7-8 on Hakko 852 station (around 420C). Preheat the chip from distance and can feel when it is enough to start. I count to 5 and viola pull the chip out without any problem. The hard ones are BGA chips and need precision timing and good BGA preheater. Average ICs are rated to 380C/10Sec I think but I am not sure.
