Mosfet AOD472  Desolder [SOLVED] - Page 1
Forum section: electronics / beginners

author: SteveO
author_rank: Newbie
timestamp: February 26, 2013, 01:57:28 pm
content: Hi I am having a real pig of a time trying to remove a Mosfet AOD472 from a board, It has two legs and a base plate at the top for the drain. I wanting to take two off to compare them as one seems to test damaged. Now I've broken the damn leg off of one trying to get it off. Because the one contact is a long surface it' just won't seem to come off. Any Tips appreciated. Should I use Hot Air or a Solder tip?Thanks for any helpThought I would add this image to show the Balls up I having just nowSteve

author: Kremmen
author_rank: Super Contributor
timestamp: February 26, 2013, 03:01:56 pm
content: First amputate the remaining leg just to get rid of it. Then apply hot air to the component, and assist with an iron to the tab. Large planes can be a bitch to heat and what you most need is heat capacity. The component is a DPAK so the tab will extend almost all the way to the legs.

author: ConKbot
author_rank: Super Contributor
timestamp: February 26, 2013, 05:20:47 pm
content: +1 to clip the remaining lead and hot air. If you dont have any hot air tools, for starting on a budget, I'd suggesthttp://www.circuitspecialists.com/csihotgun-2.htmlI use the same one and I have no problems soldering or desoldering a TO-220 package to an unetched PCB.If you want to use an iron and not mangle your board, youre going to need a high end iron, even with a fairly chunky tip the 60w/iron (IIRC) weller station I used at work would have difficulty with pagkages with that. I now have a metcal (80W) which can solder D-pack chips in ~15 seconds which is still longer than you want but not too horrible. An underpowered, or excessively hot tip will get it up eventually, but you'll most likely have overheated the trace/pad weakened the glue, and it would be pretty easy to lift the pad then.

author: dr_p
author_rank: Frequent Contributor
timestamp: February 26, 2013, 05:48:47 pm
content: You can manage without a hot air station:What I used to do is set the iron at high temp and adda lotof solder to the pins until they are shorted with solder. Then add some more to the heatsink (the part touching the PCB). I then alternately heat up the legs and the heatsink until the component comes off. This way you ensure proper heat transfer and also thermal inertia - the solder and thus the terminal remains hot for longer while you move the soldering iron from one spot to the other.It might not be safe for the component, but usually it takes less than 10s.

author: Smokey
author_rank: Super Contributor
timestamp: February 26, 2013, 07:46:08 pm
content: Ya you almost never need to clip the legs of a package. Especially if you are going to use hot air.There are a couple methods for removing stuff and not damaging the chip or the board:1) Use hot air. Done. Just be careful not to melt the board/damage other parts and don't blow off adjacent chips.2) Flood the part with solder and your basic iron. This works really well for SMD passives and SO-8 size packages but gets harder the more pins there are. Essentially put a TON of solder all over the pins holding the part on till you have one solid blob on each side. Then try to keep it all melted at the same time by heating one side then the other quickly. Then just lift the part off with tweezers, and clean up the extra solder with wick. This also works really well even though it makes a flux mess sometimes.3) Use solder wick on all the pins as best you can, then pop the pins off the pads with a scalpel. You really need to get as much solder off as you can, so use the trick where you put some liquid/paste flux on the wick first, then hit all the pins. At this point, if you did it right, there should only be a very little bit of solder under each pin holding it on. You can take the tip of a scalpel under each pin and go one by one and pop them off just enough to break the solder. Don't bend the pin way up. You run a little risk of damaging the pads if you didn't remove enough solder with the wick, but most of the time you will be fine.4) Get as much solder off as you can with wick like #3, then use a really pointy tip on the iron and a really pointy pair of tweezers or pick and use the iron to heat each pin while you lift it with the pick. This method is essentially the same as #3 but you run the risk of bending the pins more this way. The more solder you left on the pad, but further you have to bend the pin to get it free. Sometimes they break off. This is also the preferred method to lift individual pins if you don't have to remove the whole chip.5) For parts with a big thermal pad like those DPAKs, either use hot air for the whole thing or pop up the small pins using method 3 or 4. Then flood the exposed tab with a lot of solder and wait for the PCB to heat up enough to release the part. This can take a while, be patient. Then lift it off with some tweezers. Sometimes you can get away with just heating the thermal pad tab and the whole chip will get hot enough to melt the other pins. Just don't do that on thermally sensitive parts.Of course if you don't want or need to save the part, go ahead and clip all the leads. Otherwise just keep the tip on the part with some fresh solder and wait wait wait.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 26, 2013, 08:41:04 pm
content: That's definitely a job for a hot air gun. Unless you have a very powerful soldering iron and a tip to match, you won't get enough heat into the copper plane under that device otherwise.I have one of these:http://uk.farnell.com/steinel/hg-2310lcd-uk/hot-air-gun-230v-uk-lcd/dp/1702591It's a slightly unconventional choice for SMT rework I know, but with skill and experience it can work perfectly well for anything from an SO8 upwards. Getting that DPAK off the board would be a breeze.

author: SteveO
author_rank: Newbie
timestamp: February 26, 2013, 11:00:13 pm
content: Wow, Thanks for the comprehensive replies, everyone. It seems that hot air might be my best route. Any Ideas at what temperature and how long (Approx) it might take. Damn annoying. The thing was I just wanted to take this off and test it, now I need to find a replacementAnother Question, please. You might ask why I was taking it off? Well there are several of these on the PCB and I thought they could be tested in circuit? but 5 out of 7 of them show a short between the Drain and Source. Now I figure this meant they were spent but I wanted to take them off to see if it was the circuit causing the error. Can these chips be reliably tested in circuit?ThanksSteve

author: Smokey
author_rank: Super Contributor
timestamp: February 27, 2013, 01:06:10 am
content: As for temperature, if you check the data sheet it should have a reflow temperature profile used for professional assembly (pre-heat, ramp up, soak, cool down). If you wanted to be really safe and professional and all, you would follow that same temperature profile when you were taking it off to not damage the chips. Just heat up slowly and try not to exceed the maximum temp of the profile and you will probably be fine.The thing to keep in mind when using hot air to do rework is you do need to heat up the chips and board gradually. If you don't, its like taking a cold glass and sticking it in boiling water. Things tend to crack. This isn't as sensitive for big power devices as it is for ICs, but the other chips around the FET and the PCB itself could get damaged if you heat it up too fast. One of the big problems with PCBs is you can crack vias internally in the barrel since both sides of the board don't heat up evenly and you'll never know it visually.

author: SteveO
author_rank: Newbie
timestamp: February 27, 2013, 04:23:36 am
content: Thanks Smokey. I kinda wish I had a board heater. But normally I use Hot Air at about 100 for a few minutes and gradually crank it up. I can report that this Chip did Give Way in the end but I took the Air Up to 300 degrees over about 15 minutes. I have another 4 or so to remove, so I am not to pleasedBut thanks for your feedback.Can you reliably test these chips in circuit? (I think not) but I would be happy to know if you can?Steve

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 27, 2013, 09:06:59 am
content: Are you sure they are really shorted? There is a diode from the source to the drain that can make it look like it is shorted depending on how you take the measurement.

author: dr_p
author_rank: Frequent Contributor
timestamp: February 27, 2013, 09:30:25 am
content: Quote from: SteveO on February 27, 2013, 04:23:36 amWay, way too much time at that temprature!! You can damage components as well as the PCB itself. Be a good sport and flood it with solder, use a good powerfull iron and it's out in 15 seconds.

author: SteveO
author_rank: Newbie
timestamp: February 27, 2013, 10:28:15 am
content: Quote from: dr_p on February 27, 2013, 09:30:25 amI flood it and Flood it etc. etc but the little bugger wouldn't shift The Chip is Soldered a good 4mm under the board it covers almost the entire foot print of the chip and the heat just was not getting in. My Iron is Variable up to 480 Degrees. Even cranking the hot air up to 300 only just did it after some time. I don't think and Iron will do it but I am happy to be corrected.I just took a second one off and it took the best part of 25 minutes. This is a real pain of a chip to de-solder I was at the point of belting it off with a cold chissel (if you have ever been there you know what I mean)Steve

author: Nermash
author_rank: Frequent Contributor
timestamp: February 27, 2013, 10:43:44 am
content: Recently I have desoldered and soldered some DPAK2 transistors on a PC motherboard (8+ layers, huge thermal mass), using cheapish chinese hot air gun. I usually preheat the mosfet at 150°C for few minutes, and then I crank it up to 320-350 and move in close, maybe 1 cm above the transistors. It lets go in under a minute. It is important to have a proper size nozzle, so that hot air coming out can encompass the entire transistor.

author: SteveO
author_rank: Newbie
timestamp: February 27, 2013, 10:55:09 am
content: Quote from: Nermash on February 27, 2013, 10:43:44 amI'm working on another Board for testing. Just tried again dr_p's suggestion and flooded the one of them with copious amounts of solder. Despite looking like the Terminator Reborn the damn thing still wouldn't budge and in the end it Broke.I took another one off at 300 but that didn't go as well as the first. I had it up there for 15 minutes and it didn't budge. Maybe I should have gone the extra 30 Degrees plus?I really find these hard to get off and I am practicing on the easy ones. Some of the really tricky ones are surrounded by chips.Any other tips much appreciatedANd I was still wondering what anyone thought of testing these in circuit?SteveO

author: ConKbot
author_rank: Super Contributor
timestamp: February 27, 2013, 12:46:06 pm
content: Quote from: SteveO on February 27, 2013, 10:55:09 amYes, definitely hotter air. Tin lead solder melts at 183C, and the air coming out is 300C, by time the air mixes with cool air, and the board sinks away heat, you're struggling to get. If nothing is melting, give it more. Even with a slow reflow oven like profile, with preheating included you should only need 5-8 minutes or so. For the devices that are near more sensitive components, you can use aluminum foil as a heat shield, and kapton/polyimide tape to hold the foil down.What kind of flux are you using? If youre using rosin/RMA, dont worry about it smoking, rosin solder starts to smoke shortly before tin/lead solder melts, so the smoke just means youre close. If youre getting excessive amounts of smoke it could mean your heat gun is too hot and youre burning off the flux before the solder can take the heat in, but it sounds like youre going slow and careful, so probably not a problem.

author: SteveO
author_rank: Newbie
timestamp: February 27, 2013, 01:36:56 pm
content: I'm Using ROHS F130 Liquid SolderWell I can report back a "Success Story" Here. I'm Using a Aoyue 968A+ Hot Gun. Starting Off with 3 Minutes at 100 Degrees to warm the Chip and Board. Crank up to 250ish for a further 2 Minutes and then up to 350 For about 4 minutes. And that Chip Plucks off the board Healthy and Clean with a Bent Nose Tweezer.I hope this Helps others. I might even do a Video at some time, because this really bugged me. You Guys have been FANTASTIC support. Thanks to you an Dave for the Site.All the best.Can I mark this Thread as Solved? Certainly Helps with Google searches. I "" Solved in a lot of searches and it narrows down the fieldI will look after PostingSteveO

author: SteveO
author_rank: Newbie
timestamp: February 27, 2013, 01:44:46 pm
content: Quote from: TerminalJack505 on February 27, 2013, 09:06:59 amNo I wasn't sure TJ. When I take a reading across the Source and Drain (As in your image). It reads as short both ways, another chip on the Board of the same Spec a little further up the board does not. A few more on circuit OAD472's also read shorted whilst others don't. I wondered if reading them on the PCB could have caused this I wanted to take a faulty one of the board to test.What might Cause several to short? That is if they are short?SteveWoops didn't hit the post on this Message so it's kinda older than my Last Post

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 27, 2013, 03:48:49 pm
content: I'm certainly no expert in this area. I have fried my fair share of MOSFETs from ESD, however. I don't know if that would be the cause or not. I can't remember if that causes a short between the drain and source.You might check outthis documentfrom NXP.Here's a quote from the introduction:Quote