Unsoldering old Mosfets and Soldering in new

[solderman]

I have some mosfets that I would like to replace with new ones. (They are N310AS) I bought a few new ones from Mouser. (The part number on them are NF30AB made by Fairchild)

I would like to replace 4 of them. Not sure if they are going bad or not. One of them gets pretty hot.(at least I know it did before I recapped it, you can see some of the green coating missing from the board around it) The board still works after I recapped it.
I had to recap it, as it was dead.

My questions are, What is the best way to unsolder and remove the old ones? And the correct way to solder in the new ones? What end gets soldered first, the heat sink side or the side with the 2 legs? And what type of iron would be the best to use with what type of solder?

[Rainbow]

Re: Unsoldering old Mosfets and Soldering in new

How hot is hot? What board is that? A photo would be nice.

[Per Hansson]

Re: Unsoldering old Mosfets and Soldering in new

VRM's are supposed to get hot... Most can handle 100°C... However many mainboard manufacturers are then very smart and place caps right next to them... Err, or not so smart...

Though if one of them is becoming allot warmer than the others you might have a problem, question is, how much warmer?

[kc8adu]

Re: Unsoldering old Mosfets and Soldering in new

i heat each leg and lift with a dental hook.
once both are free i use a 60w iron with a screwdriver tip to heat the top of the tab.add a bit of solder if needed.
when it melts i pick it up with hemostats.

[Rainbow]

Re: Unsoldering old Mosfets and Soldering in new

That works fine for big packages (TO-263). I use 75W soldering gun.

For small packages (TO-252), never lift the legs! The legs are very fragile and can easily crack/break.

[AK0R]

Re: Unsoldering old Mosfets and Soldering in new

That works fine for big packages (TO-263). I use 75W soldering gun.

For small packages (TO-252), never lift the legs! The legs are very fragile and can easily crack/break.

If the MOSFET is being replaced, why would that matter? If replacement is being performed due to failure or degradation, the parts aren't going to be re-used on another board, so salvagable condition isn't really important.

[Rainbow]

Re: Unsoldering old Mosfets and Soldering in new

Oh, I forgot to write that this applies when desoldering good parts from bad board. I only use mosfets from non-repairable boards as replacement as it's almost impossible to buy them new here.

[kc8adu]

Re: Unsoldering old Mosfets and Soldering in new

i heat a scrap board from underneath and pick the parts off as the solder melts.i use a cheap heatgun.
harbor freight has one for 9.99
i salvage vrm ic's fets tantalums,ect.

[davmax]

Re: Unsoldering old Mosfets and Soldering in new

i salvage vrm ic's fets tantalums,ect.

You must have a good success rate. Surely there are increased risks in recycling. This is an interesting isssue.

[Rainbow]

Re: Unsoldering old Mosfets and Soldering in new

Reusing the components from totally bad boards is the cheapest way to fix other boards. And it can be also the best way if you get exact replacement if it's otherwise unavailable.
For me, the success rate is very high. You need to be careful when desoldering and then test the component that it's not totally bad (diode test on a multimeter is good way to test various components including mosfets).

[Smithrll46]

Re: Unsoldering old Mosfets and Soldering in new

What is the recommended technique for soldering the MOSFET back onto the motherboard? How you solder the flat side to the backplane?

[Rainbow]

Re: Unsoldering old Mosfets and Soldering in new

I use old (older than me ) 75W soldering gun. I heat up the board a bit, then add solder. While still heating, put MOSFET there using tweezers. The MOSFET is cold, so the solder will harden without a proper joint - so it still needs heat. When the MOSFET gets hot enough, the solder under it will melt again - then adjust its position using tweezers and remove the soldering gun, careful not to move the MOSFET out of its place. Leave it cool down and then solder the other two pins.

[davmax]

Re: Unsoldering old Mosfets and Soldering in new

Replacing MOSFETs is not too simple. First thermal shock should be avoided see this link:

http://www.st.com/stonline/products/literature/an/5037.pdf

Low temperature solder should be used. See details of this link:

http://www.howardelectronics.com/chipquik/smdrepar.html

It would seem that a solder loaded paste is a good way to go.

[arneson]

Re: Unsoldering old Mosfets and Soldering in new

I also have an old soldering iron. It looks like a cattle prod and gets very hot.
The tip is huge so it's only good for the big ugly joints, like a PSU main.
If I'm fast I can remove and resolder the FET's with it.

[davmax]

Re: Unsoldering old Mosfets and Soldering in new

Yes you need to be fast. Preheating and high power soldering iron say 80W is required. But recommend solder paste when installing new MOSFET. 135F solder is not recommended for these devices, it would let go under normal conditions. Flaming obvious for some.