[QUICK 861DW] what temperature do you use for soldering/desoldering ?

[kripton2035]

Hi everyone,


I'm trying to desoldier a dead mosfet qfn40 from a graphic card.
I'm using my beloved quick hot air station and it's been very difficult to remove it.
I had to use 450°C with 60L/min for 2-3 mins to remove this chip.
I find it quite unusual.
I checked the temperature at the output of the gun, it's correct.


Just wondering, what temperature do you use for this ?
thanks.

[CDaniel]

If what you want to desolder has good thermal conductivity than you have to turn up the temperature ... absolutly normal . And you should use the biggest diameter nozzle adaptor .

[wraper]

You need a preheater under PCBs with a lot of thermal mass such as graphic cards with many copper layers. It's the same as trying soldering on heatsink, you are heating small part while heat is transferred away over large area.

[kripton2035]

I am ok with this thermal mass.
I wonder if going up to 450°C is not too much, I risk burning the chip, or the others next to it ?
would it be better to stay around 400 and use more L/min ?


btw I have a preheater, will try with it next time.

[DaJMasta]

I generally use lower temps than that on mine (though a different hot air station), but it can take some real significant time to remove a power package from a big plane or trace.  Remember that mosfets will often be soldered down to an in-PCB heatsink pad, then of those pins you'll have half a dozen or more on each primary power connection and the trace will be wide enough to accommodate that much, and one side of it is probably going to a very high current power rail (the low voltages use mean it needs to be large high current handling), so you've got a lot of copper around it that needs heating.

The preheater is nice because even if the mosfet is on a thick trace that runs the full size of the board, you can get the whole thing up 50-75C as a starting point, which means you achieve your desired temperature at the spot of interest a lot faster.  I've spent several minutes heating a power device on a big power plane before being able to move it more than once.  If you have something you can use as a jig to hold your hot air gun, you can use it normally for a bit, hold it over the part but a little backed off to maintain the heat application, and then go in and touch the pads with a standard soldering iron at a normal temperature - sometimes the extra little bump you get from the hand iron can free the pad up and get it off a fair bit faster than air alone.  Maybe harder with QFNs, but probably still useful and quite viable for DPAKs and such.

[Fungus]

I wonder if going up to 450°C is not too much, I risk burning the chip, or the others next to it ?

Yes.

would it be better to stay around 400 and use more L/min ?

I use 350°C.

Thermal mass is a bit of a red herring on a decent, thermostatically controlled iron. If the tip temperature drops then the heater is supposed to switch to 100% power until the tip reaches the programmed temperature. If 350°C + patience doesn't do the trick then you're lacking watts and higher numbers on the thermostat won't help.

That said: Turning up the temperature will give you a bigger burst of energy when you first make contact with a joint but once the tip is drained of that initial heat then the only thing you have is the wattage of the iron.

(nb. A bigger tip has more thermal mass so the initial hit is obviously bigger...)

Higher temperatures would only be used for different solder types, eg. lead free.

You need a preheater under PCBs with a lot of thermal mass such as graphic cards with many copper layers. It's the same as trying soldering on heatsink, you are heating small part while heat is transferred away over large area.

This.