Desoldering a fet, what am I doing wrong?

[patrickza]

I need to replace a FET n my BMS. I've got a solder sucker, but it doesn't seem to suck enough of the solder from the hole to release the fet. Also with 3 legs I can't heat them all to pull it while it's molten. Is there a trick I'm missing?

On a related note. How easy would it be to overheat a fet while trying to do this?

 

[Ykick]

I need to replace a FET n my BMS. I've got a solder sucker, but it doesn't seem to suck enough of the solder from the hole to release the fet. Also with 3 legs I can't heat them all to pull it while it's molten. Is there a trick I'm missing?

On a related note. How easy would it be to overheat a fet while trying to do this?

Multi pin components I usually snip 'em out and dragging the soldering iron across the remaining pins usually removes them. Of course, this only applies if you're not trying to save the old component.

 

[corim123]

^Also used this many times with components that readily absorb heat. Or use a hot air rework station. I also dislike solder suckers and prefer to use copper braid if the prior two options aren't ideal.

 

[dnmun]

cut the legs off first, then solder wick down to where there is little to no solder left around the leg and while it is still hot and the solder molten you can whack the pcb upside down to knock the legs out with the impact and then if there is solder left you can try more solder wick and the solder sucker to clear the hole. too much heat and the traces will lift. almost guaranteed.

 

[friendly1uk]

I lift a leg at a time. I put a small screwdriver between two legs like a little crowbar and apply heat to just the one I'm lifting. This way you can apply a fair amount of lifting force as the solder melts. Enough to pull through legs that are almost barbed. Most other methods seem to fall short and require the board to be held in your third hand. Once all the legs are out, you can revisit each hole, melting the solder and blowing the hole clear with a short sharp 'pfff'

Once the legs are done you will find the component no longer puts up a fight. Soon as the solder holding it down starts to flow, it will fall away like it was glued with water. You can't do this while it's legs are holding it there though. Not unless your a spider. In which case you need to melt the solder to put a driver through the hole. Then that screwdriver can be used to lever the component upwards. Even with the legs having to bend to make it possible. This kind of messing about is best saved for when you have no other options though. You need to do the legs first.


Edit: As above, watch for lifting traces. You may be forced to chop legs off and tackle them differently if the board starts to fail.

 

[Ypedal]

dremel with a thin cutting disk works good to cut the legs

 

[patrickza]

Ok, well it's semi fixed. I was taking a spare out of an old BMS that failed many years ago, so I clipped that one out with as long legs as I could get, then clipped the faulty fet leaving behind long legs. The wedged the legs together to make contact. Not the best solution, but it works for now. I'll need to get either a very thin soldering iron or a unclipped fet to make it more permanent but I'm happy. Missed riding that kona, it feels so light and maneuverable compared to the greyborg. Thanks for the tips guys.

 

[corim123]

You need to solder that ASAP! Only take a second and will stop you having to replace it again.