Need help desoldering MOSFETs.

[Op4500]

I'm working on a 4000w amp right now, it has 24 large MOSFETs. Took me several hours just to get 4 of them off. I'm using a 25/40w radioshack soldering iron and copper braid to do it. These things are basically giant heatsinks so the solder hardens the second I remove my iron. They all have 3 pins going through the board with a lot of solder on the back. My current method has been to heat up each joint and pull it up with needle nose pliers, then clean up with the desoldering braid. Already broke one of them this way and it very time consuming, is there a better method? I'm doing this at 40w, my 25w weller could barely flow the solder, I don't think I could go much higher without damaging the chips.

[3d0]

Re: Need help desoldering MOSFETs.

I put a little drop of solder on the tip, then go to a joint you want to desolder, it will re-flow much faster. Then I use a desoldering pump to suck up the solder.

I have the cheapest one, it has worked great so far.

[bammbammfran]

Re: Need help desoldering MOSFETs.

You probably need a hotter iron. I mostly use a 65w and sometimes that is rough when the pins connect to a large trace or heatsink.
As mentioned, add flux first to the joint, this helps the heat spread better and quicker. Sometimes adding a glob solder to the tip of the iron helps spread the heat even quicker
With a hotter iron, you'll be able to use solder wick too.
Hotter iron will not be worse on the chip because the heat will be there for a shorter period of time

[mariushm]

Re: Need help desoldering MOSFETs.

If you don't care about the mosfets, cut the leads. This should reduce the mass of metal and therefore you will heat up everything faster.

Heat up remaining lead + solder, when ready grab the rest of lead with some tweezers and pull out.

If you care about the mosfets but be cheap still.... Chipquick: http://www.digikey.com/product-detai...LFP-ND/2682721

It's a special solder formulation that stays semi-liquid for long time. Apply to holes, warm it up, unbend the leads, pull out mosfet. As it mixes with regular solder it loses its properties, but you can still reuse the mixed solder several times before it won't do any good.

Or buy a better soldering station, a 65w Hakko FX888D is under 90$ in some places. Maybe it's time to upgrade from a 40w iron. Add solder to holes, flood the leads with solder, then it's easy.

ps. If you're really cheap, there's also these Hakko 936 clones Hobbyking has for about 17$ (not sure about the shipping costs though) : http://hobbyking.com/hobbyking/store...h_US_Plug.html

The 60w rating is a lie, the transformer inside is at best 40w but still the tips will hold heat better than those 25/40w sticks that don't have temperature adjustment. You can also find tips for those stations easily on ebay (the old hakko 936 tips and fx888 tips are compatible with these clones) so you could use a tip that has a larger surface, to heat up stuff faster.

This clone was reviewed here: https://www.youtube.com/watch?v=0GdV7XBae74

[Op4500]

Re: Need help desoldering MOSFETs.

Never heard of the chipquick before, I might give that a try. Going to try a better solder sucker first since I can pick that up locally, I think a high quality soldering station is in order, but I'm pretty broke right now. I'm doing this job for a friend for around $100, it's pretty much just a hobby for me. I do soldering work maybe once a month. Maybe I will invest that cash in some better equipment, right now all the tools I have are cheap junk except for my fluke 179.

[particleman]

Re: Need help desoldering MOSFETs.

Sometimes I add some flux to the braid to help it along. what kind of tip are you using?

[Op4500]

Re: Need help desoldering MOSFETs.

Sometimes I add some flux to the braid to help it along. what kind of tip are you using?

It's a fine pointed tip, I'd rather have a chisel tip but radioshack doesn't have them and it's basically my only option unless I order online.

[bammbammfran]

Re: Need help desoldering MOSFETs.

this might get you by
and it is not too far ship wise for you
plus it is low cost
http://www.ebay.com/itm/Hakko-503-So...-/251538912062

[capkid]

Re: Need help desoldering MOSFETs.

I use a heat gun on low setting to heat up mosfet pins on heat sinks prior to removing solder. If you're doing a lot of these, I'd buy a de-soldering gun. The Hakko 808 in conjunction with a heat gun works great.

[momaka]

Re: Need help desoldering MOSFETs.

I don't know if I am just too Spartan or what, but most fancy tools rarely work for me when I have to solder or desolder heavy joints (i.e. lots of copper planes and big solder joints).

With that said, I'll share with you my rather simple method of desoldering large parts as I've had great success with it.

Step 1: get a cheap unregulated 60W (or higher) rated soldering iron. eBay is a good place for that (just make sure to get a 120V version since you are in the US, and not 220/230V). Keep in mind though, that many of these cheap irons can have quite shoddy built quality, so don't leave it unattended.

Step 2: get some no-clean flux (liquid or paste are both fine) and preferably 60/40 leaded solder.

Step 3: Let your iron heat up for a few minutes. Meanwhile apply flux to all 3 MOSFET leads on the PCB. Also, if the MOSFET is held with a screw onto a heatsink, make sure to remove the screw and slightly pry the MOSFET away from the heatsink (i.e. just make sure the MOSFET is not sticking to the heatsink in any way).

Step 4: Turn the PCB so that it is flat and the solder side is oriented towards the ceiling.

Step 5: Melt a big blob of solder on the soldering iron tip and apply to all 3 MOSFET pins at once. The melted solder should cover all 3 of them.

Step 6: wiggle the MOSFET a bit. In most cases, it will fall down and out of the board all by itself. In some cases, though, you may still need to pull it. Beware, however, that once the MOSFET is removed, molten solder could feed through the holes where it was and fall on your hands (or feet), so be extra careful there! I suggest wearing thin wool or shop gloves.

Step 7: recover some of the solder you melted with your iron, turn the PCB so that it is facing solder side towards the floor, and just run it over the PCB holes where the MOSFET was. Gravity will pull some of the solder out. Then use a pin or sewing needle (I use a cork board tack) to clear the holes a little better one by one. Clearing the holes fully is not required at all. If you need to reinstall a MOSFET back on the board again, do the same procedure that you used for removing the MOSFET - i.e. use a big blob of solder to melt all 3 holes at once and then insert the MOSFET.

With this method, you won't need any fancy tools. Just a 60W+ iron, solder, and flux.
I don't know why, but desoldering braid never works for me for large joints like this. I only use it for cleaning BGA parts and pads. It's nearly useless for everything else. I've also tried the Hakko 808 before, but I don't find it as amazing as people told me it was. It also struggles quite a bit with multi-layer boards. No good for motherboard recapping, in my experience. Given its price tag, I don't find it justifiable shelling all of that money even if you own a repair shop. Most desoldering can be done quickly and efficiently with much simpler tools.

Also, I've recently developed a gag reflex when I see those cheap temperature-regulated stations (like the 936 clones) get recommended. IMO they are quite overrated. While their temperature regulation is accurate when they sit idle, their tips have horrible recovery time. The iron on my 852D+ station is like that as well. Barely any more power than my 35W unregulated Radio Shack iron. In general, I find that anything that uses the 900W series tips had mediocre performance. Stations like the Aoyue 2600 that use the T12 tips are a much better choice if you want a good temperature regulated iron with good thermal recovery. Of course, the stations are a bit more pricey too. And the tips are usually at least $10 each. But once you've used one, you'll easily find the price tag justifiable.

[budm]

Re: Need help desoldering MOSFETs.

I cut the legs of the device first then I use the same method similar to momaka.