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Old 29th January 2003, 06:04 AM   #1
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Default Desoldering surface mount semiconductors

I just fried a PC motherboard (pain)... the meltdown took a couple of transistors (two n-channel MOSFETS in parallel... ST B40NE03L-20 if anyone wants to spare me finding some replacements) and a couple of electrolytic caps. Now, the thing is it seems like a not too complicated fix to perform, but i can't unsolder the darn things. They use the "d2pak" packing, where the drain is the flat side of the transistor and it's soldered directly to the board. Any ideas?
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Old 29th January 2003, 07:36 AM   #2
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Hi Lisandro_P

With a good soldering iron this shouldn't be a problem. It is possible that the transistors are glued on the board. Don't know if they do it with the bigger ones. So you will have to pull a bit when the tin is melting. We also use desoldering wick to take off the old tin. Tip: never reuse SMT's.

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Old 29th January 2003, 08:02 AM   #3
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Default Re: Desoldering surface mount semiconductors

Quote:
Originally posted by Lisandro_P
I just fried a PC motherboard (pain)... the meltdown took a couple of transistors (two n-channel MOSFETS in parallel... ST B40NE03L-20 if anyone wants to spare me finding some replacements) and a couple of electrolytic caps. Now, the thing is it seems like a not too complicated fix to perform, but i can't unsolder the darn things. They use the "d2pak" packing, where the drain is the flat side of the transistor and it's soldered directly to the board. Any ideas?
Keep it simple - just burn with the soldering iron. Eventually the transistor gets loose. If you happened to have two soldering irons it's easier. Both the pcb and the transistor can take rather much heat.
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Old 29th January 2003, 08:13 AM   #4
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Default Re: Desoldering surface mount semiconductors

Quote:
Originally posted by Lisandro_P
I just fried a PC motherboard (pain)... the meltdown took a couple of transistors (two n-channel MOSFETS in parallel... ST B40NE03L-20 if anyone wants to spare me finding some replacements) and a couple of electrolytic caps.
Did you kill part of an onboard switching power supply? If so, don't forget to check the diodes as well (unless you've already done this, in which case...)

Quote:

Now, the thing is it seems like a not too complicated fix to perform, but i can't unsolder the darn things. They use the "d2pak" packing, where the drain is the flat side of the transistor and it's soldered directly to the board. Any ideas?
For those devices, I've had success with hot air, liberal use of flux, and sometimes a little extra solder.
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Old 29th January 2003, 06:04 PM   #5
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Default Re: Re: Desoldering surface mount semiconductors

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Originally posted by schmad
Did you kill part of an onboard switching power supply? If so, don't forget to check the diodes as well (unless you've already done this, in which case...)[/B]
Yeah, so it seems. I think i'll replace both devices with a single MOSFET rated for the combined power of both. Do you know what does these kind of onboard psu's feed? I'm just hoping the meltdown didn't took the CPU with it (crossing fingers).

BTW i managed to pull one out (just stuck the soldering iron to the exposed metal and pulled a bit with a screwdriver), but the other one's so tricky...
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Old 29th January 2003, 07:53 PM   #6
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To desolder: first the legs, heat them, apply a bit of fresh solder and then lift them when the solder is melted with tweezers (that you put under the leg, they break off sometimes). Then heat the tab, also applying a bit of fresh solder. Clean up with the solder wick (make sure you get every bit) and then clean with isopropyl. When soldering the new one, first the tab, then the legs and keep the tab flat on the pcb, heat well so that the solder can flow under the tab (use flux).

Now a fried smps is not simple to repair! I spend this afternoon trying to get one running again and it keeps blowing.

Replace only by the same fets! A bigger one may seem nice, but the gate capacitance may be too large. This will lower the efficiency (if it does not affect stability), making everything even hotter. The capacitors are also critical components. They must be low esr and rated for the ripple current at operating temperature.
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Old 29th January 2003, 08:08 PM   #7
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Hi all,
For what it is worth from my 5 day a week experience have a look at my method:
http://db.audioasylum.com/cgi/m.pl?f...so&r=&session=
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Old 29th January 2003, 10:02 PM   #8
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Just brought two replacements: a pair of IRF530. Those were the closest in specifications to the old (discontinued) mosfets, and they double the voltage ratings. I just hope it works

Thanks for all the feedback guys!
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