How to resolder a mosfet?

[Rub87]

Hi,

2 weeks after I removed the 120mm blowing on my NF7-s mosfets, one got to hot and unsoldered itself, I was only running 1.95v @ 2.8GHz..

The mosfets were sinked, and normally they don't ran so hot, but I was running distributed.net for 2 weeks and, yeah, the mosfet with the smallest heatsink unsoldered itself resulting in a 12v rail shortcut on the mainboard. I removed the mainboard from the case, unsoldered the 2 pins with were still connected to the board, then the shortcut in the board was gone..

Now I have a board with only 5 mosfets anymore :/

As I used a little to much AS adhesive and a little to less AS3 I think it will not be possible to remove the heatsinks from the mosfets..

Now I want to solder the one that had fallen of back on .. But how you get the board and the mosfets hot enough so the solder will aplly to both sides?

Some pics..

[iddqd]

Soldering a MOFET directly on the board like it was robotically soldered before is extremely difficult. Hell, even unsoldering one is a royal pain in the ass, even if I use a torch. My success rate for unsoldering is about 50%. Anyway, what I propose is you find some thick wires, and solder those to the mobo, and then the MOSFET to the wires a little above the mobo. That should make it a bit easier. The voltage would get a bit dirtier, but not enough to make a difference, I suppose. You could also get a new MOSFET with uncut legs that you can solder to the board upright, instead of flat on the back like the ones they have already trimmed.

Also, that section where the MOSFET once was looks severely burned. I would have doubts if the baord would work even after you somehow managed to solder the MOSFET back on.

[dippyskoodlez]

yea.. you never know how durable these boards are sometimes though :D replaced about 12 exploded caps off of a p3 board, and it works good as new. probably better :D

I'd try soldering it back on... I dunno if the back of it is grounded or not. o.O

[iddqd]

Well, capacitors don't have flat SMD-style bases, like the MOSFETs do, unless you mean SMD capacitors, but those can't explode. Either way, those are WAY easier to solder/desolder/whatever

[dippyskoodlez]

Well, capacitors don't have flat SMD-style bases, like the MOSFETs do, unless you mean SMD capacitors, but those can't explode. Either way, those are WAY easier to solder/desolder/whatever


the big round caps.. the tops were leaking... ;) yes.. definatly a lot easier to replace than a mosfet.

[Entity_Razer]

isn't it impossible to solder them on "from a distance" ,with the wires like explained above? I mean, won't that cause either fluctiating signals (the legs are equally long for a reason ;) ) or giving bad signals.

Either way I suggest you use a thick wire, strap it around your soldering iron, sharpen the wires point and solder with that.

looks like a tricky spot to solder something :(

[Rub87]

Yes, I was also thinking of fluctuation..

The drain and source wire will cause a lot of magnetic euhm, don't know the english word, and these wel generate a current in the gate wire and cause A lot of fluctuation I think, I also had this problem with my Vdimm supllie, the gate ware was around 20cm and had a lot of fluctution, shortned it to 3cm almost completly solved this problem...

[1337mr.hanky]

i would solder some wires at the ends of the mosfet, and than solder these wires at the mobo where the mos used to be, and connect the mosfets heatspreader at your case with some thermat prut between it

[iddqd]

You don't care about the signal, since the large MOSFETs are used to regulate DC current only, and it doesn't really matter, since that particular current carries no information. Just current. Yes, you would get slightly dirtier voltages, but with short wirse (<10cm) it wouldn't make a big difference. And you could go as short as 2-3cm. Besides, I beleive a dirty voltage is better than nothing at all.

isn't it impossible to solder them on "from a distance" ,with the wires like explained above? I mean, won't that cause either fluctiating signals (the legs are equally long for a reason ;) ) or giving bad signals.

Either way I suggest you use a thick wire, strap it around your soldering iron, sharpen the wires point and solder with that.

looks like a tricky spot to solder something :(

No, no, no, this won't work. Instead of precision soldering tiny SMD parts, you're trying to solder something flat to the board that has a huge surface area to boot. The only way to do this is put some solder on the base, put it in place and heat it quickly with a torch. After that, it's all luck. Hell, I haven't had much lock desoldering these things either.

You could also get a small ramsink, and solder it to the base of the MOSFET, and then solder a wire to the ramsink. That would increase the thermal performance a lot.

[persivore]

You need to keep the connetions on the gate of the MOSFET as short as possible to stop noise being induced onto the gate voltage and causing poor regulation. The gate can be very sensitive to any form of noise.
For soldering/desoldering MOSFETs to boards, I usually use a soldering iron rated to at least 40w. You could also try using a gas powered soldering iron, which usually get much hotter than normal solering irons, or you could even try using a heat gun.

[celemine1Gig]

You need to keep the connetions on the gate of the MOSFET as short as possible to stop noise being induced onto the gate voltage and causing poor regulation. The gate can be very sensitive to any form of noise.
For soldering/desoldering MOSFETs to boards, I usually use a soldering iron rated to at least 40w. You could also try using a gas powered soldering iron, which usually get much hotter than normal solering irons, or you could even try using a heat gun.


I fully agree. Those mosfets can stand a lot, so don't worry. Get a good soldering iron or gun (I'd say ~80W) and try your luck.

[iddqd]

I fully agree. Those mosfets can stand a lot, so don't worry. Get a good soldering iron or gun (I'd say ~80W) and try your luck.
Yes they can, but the mobo can't. You could twist the wires to cancel out noise.

[freecableguy]

The fact that the MOSFET got hot enough to unsolder itself should tell you something about the application in which it is being used. I recommend you either cease present work or get a NEW board.

[Rub87]

Yes the mosfet got so hot because I removed the fan and we had very high temperatures the past 2 weeks over hete in Belgium..

[Rub87]

Hi,

FINALLY it happened, and I resoldered that biatch, I didn't test the board yet, but the mosfet seems pretty good..

So I just sanded aaway the moset of the dirt on the board and mosfet, then used my 48w soldering iron to put some solder on the spot were the MOSFET should go, did the same on the MOSFET itself..

Then I used a paint heat gun thing en just made the mosfets that hot and pressing in down to the mobo for some time until the solder on the board was also melting and afterwards I removed the redunant solder with .. "copper wire that sucks up de solder when you haet it and press agains the solder" and that's it..

And yeah I made a Al plate to protect the other components such as the parallel port and stuff from melting due the heat..

pics (http://users.pandora.be/rub87/rest/mosfet) :banana:

edit: I forgot to mention that I removed the caps for better acces to the spot..

[celemine1Gig]

Apart from using WAY too much solder (but as you wrote, you cleaned that up afterwards, if I fully undestood you), it looks like it could work in case the mosfet is still alive. So good luck when trying the board again.

[join nbk]

huum
i had a friend who did this to the same board, ill see what he said exactly

[Kasparz]

I resolder one of my 9600se mosfet.
Old mosfet i remove with 25w iron and screwdriver :D :rofl:
And then solder new one.
Card works perfectly today.

[NickS]

LOL. Nice job resoldering it, but the heatsink looks like it started to bow (melt LOL) under the heat!

I remember one guy saying he was benching an old AXP system (heavily OC'ed), and he looked at his case (through the window) for a sec, and saw a mosfet start to slide off LMFAO. What would you do in that case? I'd prolly run over and rip the surge protecter out of my wall..

[slavik]

there is no stopping a true geek ... :P

next up: motherboard manufacturers sued for not providing good cooling to power regulating components ...

[MaxxxRacer]

intersting.

if anyone cares the way they are soldered on in the first place is that wherever there will be something to be soldered (surface mount, not stuff that goes through the board) they put a little bit of low temp solder that has some resin in it to make the components stick to the board before the final soldering. T his is done with a template to get the solder exactly where they want it. this solder is actually like a paste at room temp due to the non-hardened resin.

The next step is a robot placing all the components on the board.

after that it goes into an oven at 200C i think. cant remember the exact temp. the entire board is put in (minus the stuff with pins that goes through the board) and the solde melts, fusing all the components to the board.

[crodan85]

after that it goes into an oven at 200C i think. cant remember the exact temp. the entire board is put in (minus the stuff with pins that goes through the board) and the solde melts, fusing all the components to the board.

Im not sure about wave soldering but there are 4 stages in Infrared reflow soldering.

1. Preheat: the board is preheated to 150c in 60 seconds

2. Flux Activation:150c-180c 120 seconds

3. Reflow: 180c-215c at 1 to 3c a second

4. Cooling controlled cooling less than 6c a second

bear in mind that all soldering pastes have different temp profiles.

[Pik4chu]

intersting.

if anyone cares the way they are soldered on in the first place is that wherever there will be something to be soldered (surface mount, not stuff that goes through the board) they put a little bit of low temp solder that has some resin in it to make the components stick to the board before the final soldering. T his is done with a template to get the solder exactly where they want it. this solder is actually like a paste at room temp due to the non-hardened resin.

The next step is a robot placing all the components on the board.

after that it goes into an oven at 200C i think. cant remember the exact temp. the entire board is put in (minus the stuff with pins that goes through the board) and the solde melts, fusing all the components to the board.
Thats correct! the low temp solder is put on the board almost like silk screening is done for ink onto shirts for example. Its quite cool to watch the whole process btw, especially the robotic arm placing components. It does it incredibly quickly and rather frightening accuracy. heh

The thru-hole components are soldered using what is almost like a solder "bath" where the solder is almost sprayed ontot he under side of the board, thanks to the flux in only the right areas the solder adheres cleanly tot he pins and the holes w/o shorting anything out.

Unfortunately most of this stuff is irrelevent due to the inability to reproduce it by the average person :rolleyes: