Author Topic: How can I desolder a QFN without accidentally desoldering nearby components?  (Read 3538 times)

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Offline dicky96

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Hi Guys
I have to change this TPS54620 as it is short circuit.  See the attached photo, the component is a small QFN and closely surrounded by a lot of very small passive SMDs and there are also a few more passive SMD and SOT packages on the underside of the board in the same area.

How should I replace the faulty component without accidentally unsoldering any of the surrounding parts or melting the nearby plastic coaxial power jack socket and optical audio socket?

I have the usual kit, soldering iron, hot air station, suitable magnification, various flux, quick-chip etc.... but no preheater (yet).

best regards
Rich
 

Offline anishkgt

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Apply heat under the pcb. The longer it takes less likely other components would be affected. In short lower the fan speed and apply heat under the pcb. I would add some solder flux as well m, helps desoldering faster.


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Offline sokoloff

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Kapton tape around the part you're trying to desolder will help shield them from some of the hot air (as well as help keep them in place).

Pre-heating the board from underneath (as anishkgt suggests) will also help, but I've gotten by with just hot air and Kapton from the topside.
 
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Offline sstepane

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Cover passives with capton tape. Or with aluminium sheet (even that is for cooking, cut an appropriate hole and try to stick it to a pcb).
 

Offline dicky96

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Sorry I forgot the pic lol

I do have some experience of reworking QFP etc I am just a bit worried about all the stuff close to this one and I bet it has a ground/heatsink connector in the middle of the QFN package as well  >:D

Ther are also some SMDs on the otherside of the board in this area.  Wouldn't they fall off first if I heat it from underneath?

Pic Here!



 

Offline sokoloff

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By the time you heat the whole board enough to blow parts off the backside, you're well past the temp needed to remove the QFN from the front.

I'd put a piece of Kapton across the parts on the back just because, but for small parts with low thermal mass like this one, it's usually not too hard to remove them from the front without drastically overheating things.
 

Online NivagSwerdna

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Kapton tape around the part you're trying to desolder will help shield them from some of the hot air (as well as help keep them in place).
^
 

Offline T3sl4co1l

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Tape or foil (insulation / heat shielding) around the plastic components to prevent them from melting.  Hot air at the QFN, with preheat of the board as needed.  Don't touch the other components and they won't move.  Pick up the QFN with tweezers.  Use paste flux.

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Offline JFJ

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ChipQuick has worked for me, in the past. Please see Dave's video review:

https://youtu.be/UmD7F0--7Lc
 
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Online wraper

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ChipQuick has worked for me, in the past. Please see Dave's video review:
It's QFN, there is a pad on the bottom.
« Last Edit: November 05, 2018, 04:25:49 pm by wraper »
 

Online wraper

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I would place a piece of thin sheet metal between IC and plastic connectors to prevent melting plastic. It should not directly touch plastic, otherwise it may malt a little bit. Then just heat with hot air and remove with small tweezers or vacuum pickup if you have one. There are not that many tiny parts around so even if you screw up, it's easy to put them back.
 

Offline alpher

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Since there is something  "bad" :-DD :-DD  lurking beneath, :) why not use a soldering iron with a flat tip like 5C for example,
and just heat the whole chip from the top.
It's a dead chip anyway.
 
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Online wraper

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Another option is to desolder two nearby connectors before replacing IC. There would be easier access and less risk to move tiny parts or melt plastic.
EDIT: likely I personally would use this method because such connectors often melt at quite low temperature.
« Last Edit: November 05, 2018, 04:33:31 pm by wraper »
 

Online wraper

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Since there is something  "bad" :-DD :-DD  lurking beneath, :) why not use a soldering iron with a flat tip like 5C for example,
and just heat the whole chip from the top.
It's a dead chip anyway.
Very likely will end up as screwed up PCB as well.
 

Offline KL27x

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I would try to mark those little capacitors. Clean with alcohol and mark with a different color sharpie. And take another pic before starting the surgery.
 
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Offline dnwheeler

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When Louis Rossmann removes chips with hot air, he relies on the surface tension of the solder to keep all the components in place. He then uses tweezers to remove the part. When he removes the heat, the solder quickly hardens, reattaching all neighboring components.
 

Online wraper

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When Louis Rossmann removes chips with hot air, he relies on the surface tension of the solder to keep all the components in place. He then uses tweezers to remove the part. When he removes the heat, the solder quickly hardens, reattaching all neighboring components.
FWIW one should learn soldering from someone else, not him. He himself often says that he sucks at soldering and frankly it's true. Just please don't make off topic discussion out of this comment because I already had bad experience about mentioning his soldering skill.
 
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Offline cdev

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ChipQuik is easy to control (a little bit goes a long way) and it seems to me that unless something mixes the chip quik with solder its not supposed to be in, there is little risk of accidentally melting other components and having them move around or fall off.
« Last Edit: November 05, 2018, 10:43:25 pm by cdev »
"What the large print giveth, the small print taketh away."
 

Offline KL27x

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dnwheeler, yeah, if all goes well, it's really that easy.

On a multilayer board with internal ground pours, you can burn the soldermask, char and bubble FR-4, and/or delaminate the traces before the chip comes off. And your hot air station can also hit max output with the chip still stuck. Then you might have to turn up the air flow or add a preheater and whatnot.

And while picking up the chip, you can inadvertently bump other parts. You never know. Louis has schematics and spare parts/boards in case he accidentally moves a few ants.
 

Offline T3sl4co1l

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On a multilayer board with internal ground pours, you can burn the soldermask, char and bubble FR-4, and/or delaminate the traces before the chip comes off.

This is impossible to do with correct settings.

Only increase the hot air temperature once you've tried preheating and cranked airflow to maximum.  Be patient.  Do not use a paint-stripping heat gun!

Delamination can occur due to rapid heating and absorbed moisture.  Bake out first, to be sure.

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Offline dicky96

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Thanks for the advice guys

Can I just mention though, some folks suggested Quik Chip and I did buy some Quik Chip but I find it next to useless when the QFP has a heatsink pad underneath it, which unfortunately seems to be most of the time, from my experience!

OK so following various parts of the above replies... here is how I did it and some pics.  Hopefully this quick description may help others who read this

I first de-soldered the power jack socket and the optical audio socket .  I did this with my soldering iron, first applying some 60-40 solder to all the pins, then using desolder braid dipped in the flux that you can see in the pic, then once I removed as as much solder as I could I heated the pin side of the pcb with my hot air station and pulled the sockets from the component side with long nose pliers.  A quick clean using a bit more desolder braid and job done (see pic 1.)  I am not sure what this flux is.  It is hard and comes in a tin and I have to melt it first with my soldering iron tip before i can dip the braid in it.  I says 'Pasta Lutovania' on the tin and at first I thought that must mean it comes from Lithuania  :-X

Next I put the kapton tape around the QFN to be removed (pic 2)

Then I de-soldered the QFN with the hot air station, having put some Topnik liquid flux on it first..  I think I may have got the heat set too high as the kapton melted or curled up a bit.  It was set to 420C on my cheapo  858D.  I angled the hot air from vertical a bit so it was blowing a little towards the empty area of the board where i desoldered the sockets. Once I removed the QFN device with tweezers I let it cool down and removed the kapton then cleaned the area with Isopropyl Alcohol and a cotton wool bud.  Using a jewelers loop for a close up look I found the QFN had de-soldered quite nicely (I used a bit more flux and solder braid to clean the pads) but noticed I had accidentally un-soldered and moved a capacitor (circled in the attached pic) even though it was under the kapton.

I then inserted a knife blade between the two capacitors and used my soldering iron to first apply a bit of 60-40 solder to each end of the mis-aligned capacitor, then put the soldering iron bit sideways so I could heat both ends of the cap at the same time and used the knife blade to push the two caps apart. 

This did not work perfectly to plan as the cap moved suddenly and moved a bit too far.  ::)

So I then used the same technique of heating both ends of the cap at the same time and used tweezers to reposition the cap, then de-solder braid and flux to clean excess solder from the ends of the cap..    (pic 4)

That all looked good so I cleaned everything with isopropyl again and examined again with the jewellers loop.  The last pics show the PCB and the QFN after removal. 
Both the device and the PCB were undamaged

I think I did a pretty decent job??? 

Kit used
ZD-915 solder station
Atten 858D hot air station
8 diopter (3.25x magnification) illuminated bench magnifier
x10 jewellers loop
Flux Paste, Topnik Flus, Flux pen, 60-40 lead solder, desolder braid, isopropyl alcohol

You may notice I use the top off an old satellite receiver when working with hot air so it does not char my workbench

I just need to order a replacement part (get a pack of 10 from aliexpress for about €4) and solder that back in place.  I will put 60-40 on all the pads and try to get the hot air temperature a bit lower so it does not quite melt the lead free solder on other components.  I find from experience that method seems to work quite well.
 
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Online Rasz

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I says 'Pasta Lutovania' on the tin and at first I thought that must mean it comes from Lithuania  :-X

"pasta do lutowania"? from AG Termopasty aka AG Chemia. solid Polish, family owned company. I have been using their products since ~1998.

I think I may have got the heat set too high as the kapton melted or curled up a bit.

you didnt have Kepton tape, you had shitty Chinese "KaptAn" made in "Califomia" garbage, thats why it melted :)
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Offline cdev

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While you are waiting for the replacement is a good time to look at the area and the other side really closely.
"What the large print giveth, the small print taketh away."
 

Online wraper

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you didnt have Kepton tape, you had shitty Chinese "KaptAn" made in "Califomia" garbage, thats why it melted :)
Tape itself is not shitty and it does not melt. But adhesive is crap compared to expensive tapes. More often I see kOptan. But using it like this should be avoided, if adhesive was good it might be even worse. If it attaches itself to components well, when solder melts it may lift the parts.
Quote
"pasta do lutowania"? from AG Termopasty aka AG Chemia. solid Polish, family owned company. I have been using their products since ~1998.
IME half of their products are junk. Once I purchased their solder paste (with solder) to try it out. It was like 2-3 months old and already completely useless, barely any of it melted and most of it remained as grey dirt. Even considering it might be stored without refrigeration at TME, it's just the worst shelf life I've seen ever.
 

Online Rasz

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IME half of their products are junk. Once I purchased their solder paste (with solder) to try it out. It was like 2-3 months old and already completely useless, barely any of it melted and most of it remained as grey dirt. Even considering it might be stored without refrigeration at TME, it's just the worst shelf life I've seen ever.

as a counter example I still use their silicone thermal grease from 20 year old 100g container, looks and works great, bought it when I started selling Celeron 300 computers, still about 1/3 left.
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Online wraper

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as a counter example I still use their silicone thermal grease from 20 year old 100g container, looks and works great, bought it when I started selling Celeron 300 computers, still about 1/3 left.
I have such old container as well. Well it's just a thermal paste that works but has a piss poor thermal conductivity.
 

Offline dicky96

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Don'y know about their other products but the Pasta de Lutowania works really good to 'activate' desolder braid.  I honestly thought that desolder braid was pretty useless stuff until I bought a tin of that flux!! Now I think it's wonderful.  I just melt a 'trough' in the hard flux with my soldering iron and dip my desolder braid into it, then use the braid and it sucks solder like a good' un.   I always add some 60-40 to the existing solder joints before using the braid.

That technique has worked really well for me, and I still have loads of flux in the tin after several years of use.  I can't recall why I bought that particular brand from ebay, maybe it was just cheap.  But I will definitely buy another tin when it runs out.

Other than that I use a Maplin flux pen and recently got some Topnik TK83 flux to try, which comes in a little bottle with a brush to apply.  Both these seem really good when desoldering QFP and similar.  Basically I use these where I want to use a more liquid flux.  Oh and then is the gooey stuff that came with the quikchip.  That seems to work fine too.

Rich
 

Offline helius

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JBC makes protectors and extractors for isolating heat to a single component. You can also use copper or aluminum tape to protect adjacent board areas.
 

Offline KL27x

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Good job, OP, but just a small note, here.

Adding flux is a reflex that is out of place, here. With an iron, it helps to form a solid solder bridge between the iron and the pads/leads. You certainly don't have to add flux when desoldering with hot air. (You can just intermittently poke/prod the chip to know what it's done.) IMO, save the flux for when you're putting the new chip back on.
 

Offline helius

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Flux does serve a purpose when desoldering. Sometimes it's more trouble than it's worth.
An activated flux will remove oxides from the the solder joint and pads, which means the device can be reflowed (oxides won't melt). It also protects the solder and pads from oxidation caused by the heat and oxygen in the air (remember, hot air contains oxygen and you are pumping in a lot of it). To a limited degree it also conducts heat into the solder joints from the board surface, similar to how it helps make heat bridges when using a soldering iron.

In certain situations flux is more of a hassle. It clogs up through-hole desoldering guns.
 

Offline perieanuo

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Stick some aluminium foil, the one with one sticky side, not cheap and cut just the part which covers your chip, in fact a little larger.low-cost but works.otherwise don't use massive airflow...


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Online wraper

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Stick some aluminium foil, the one with one sticky side, not cheap and cut just the part which covers your chip, in fact a little larger.low-cost but works.otherwise don't use massive airflow...


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You would need to have special foil with high temperature resistant adhesive. If you will use regular foil, there will be ton of smoke and board covered with burned sticky junk.
But IME it's not worth covering anything other than easily melting connectors and similar.
 

Offline perieanuo

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No you don't need high temp adhesive foil.I did this a lot of times in laptops, with alu foil non-stick type, when I was home I picked from my wife's kitchen a bigger surface and rolled over the pcb.
have you tried and it didn't work or you like arguing?
Pierre


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Online wraper

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have you tried and it didn't work or you like arguing?
I wrote this because I tried using regular tape from hardware store in the past. Most adhesives obviously will burn when heated above 250oC.
 

Offline dicky96

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OK.... so it turns out that once desoldered it was not that QFN that was short circuit, it was a small smd cap very close to it. Not one of the  2x10uF ones in parallel on the same rail (I also desoldered those two first).  Probably a 100nF decoupling cap or similar.

Which now begs the question, how best solder the QFN back on again.   There are little exposed parts of the QFN solder pads down the sides of the QFN package itself but it is too narrow to get my soldering iron tip down there because of close mounted SMD passives.  I have seen some videos that reflow that way.    At least the pads on the pcb are nice and clean now and have a bit of 60-40 on them.

best regards
Rich
 

Offline helius

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Soldering the center pad of QFN is difficult unless there are vias to the other side, in which case you can apply the soldering iron to the other side and it will conduct heat through the vias. The side pads can be drag soldered using a knife tip and a large solder blob.
Otherwise hot air is used to reflow everything at once. When the chip floats and self-aligns over the pads it is fully reflowed and is allowed to cool.
 

Offline coromonadalix

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To solder it back you use solder in paste form and a hot air station,  first you clean all the pads with copper wick, but be careful not to lift the pads,  once everything cleaned  you put an small dab of it on the pads with an tooth pick or needle ... and the center dissipating pad.

Put the ic back and center it ,  with hot air comming from the top, not in an angle, rotate slowly around the chip to heat the board and go in circle to the center of the chip always in circular motion , it should center itself on the pads ... and let go of the heat.

I do it with QFN 32 pins chip at my job doing repairs.

I use kapton tape to hold all the surrounding parts for the QFN removal.
« Last Edit: November 09, 2018, 12:09:37 am by coromonadalix »
 

Offline KL27x

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Wraper will disagree, but I would

1. put a nice bead on all the pads, esp a good fatty on the center pad. Well, what I means is it's better to have too much than too little. Too little and the chip will stick to the board like glue without self-centering. Then you basically have to push the chip to where you think it goes or, better, pull the chip off and start all over.
2. Plenty of flux.
3. pin the chip down with tweezers while shooting the hot air. When it reflows, you will automatically end up pushing the chip against the board, temporarily. Don't squish it too hard, though. Then you can let go.

The excess solder, if any, squeezes out. And it will bead out around the chip, clinging around the sides. If the chip snaps out of alignment to the wrong pads, just nudge it back to place and remove the heat. As long as you have enough flux on the board and you squeezed it a bit against the PCB, there will be no bridges underneath the chip, IME.

Then go around the chip and reflow the sidepads, removing excess solder and bridges, if any, with the iron. If you have to remove some surrounding components first, then that's what ya gotta do. Or get a new tip for your iron that can squeeze in there.

Or you can try solderpaste and hot air. In many cases, I think solder blob and flux is easier and more surefire.
« Last Edit: November 09, 2018, 12:53:48 am by KL27x »
 

Online wraper

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Wraper will disagree, but I would

1. put a nice bead on all the pads, esp a good fatty on the center pad. Well, what I means is it's better to have too much than too little. Too little and the chip will stick to the board like glue without self-centering
Yep I certainly will, it's the worst idea to do. You need only a little bit of solder on the center pad and as much solder as you can get on all other pads (equal amount on all pads). Also good idea to apply solder on the pads of IC itself too (except center pad). Clean flux residues, apply a little bit of tacky flux and heat with hot air, done. And don't bother reworking the mess you would get with suggestion by KL27x.
 

Offline perieanuo

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OK.... so it turns out that once desoldered it was not that QFN that was short circuit, it was a small smd cap very close to it. Not one of the  2x10uF ones in parallel on the same rail (I also desoldered those two first).  Probably a 100nF decoupling cap or similar.

Which now begs the question, how best solder the QFN back on again.   There are little exposed parts of the QFN solder pads down the sides of the QFN package itself but it is too narrow to get my soldering iron tip down there because of close mounted SMD passives.  I have seen some videos that reflow that way.    At least the pads on the pcb are nice and clean now and have a bit of 60-40 on them.

best regards
Rich
Next time try some other technique I used in laptops:identify voltage (e.g. 19V or 3.3V or other ), set your 20A-capable power supply with that value and apply it.The shorted piece will show itself, if you increase current above 5 or 8 Amps it will smoke very little or just became hot.Don't apply 20 amps at first, increase current protection to find it.
I repaired more than 80 mobos like this, searching on 3V3 rail a shorted cap in a laptop is time consuming.
Best regards,pierre


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Offline dicky96

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@Perieanuo. 
Thanks.  Yes that 's how I found the short circuit part - put the bench supply on current limit and see what got hot.  Unfortunately the faulty decoupling capacitor was very close to the chip and I got it wrong first time. I only use about 2.5 A and let it get warm not hot, I would worry 8A may damage PCB tracks, also my bench PSU only goes to 5A.  My other dual output linear 0-30V goes to 6A in parallel mode but that ones still in the UK. I didn't bring it over to Gran Canaria yet, it's one the pile of 'must take soon' stuff along with my variac and some other heavy kit lol maybe next time I'm back in the UK

So basically I do know the technique but I'm not perfect lol

OK so I resoldered the QFN using the 'wraper' technique.  Not to stoke the fires of any argument here but because I don't have any solder paste and never used that before TBH.

Anyway, mister Wraper your technique worked just fine  :-+.  I used some of the flux that came with the QuikChip as that is kinda gooey.  I'm not sure if that is what was meant by 'tacky flux'.  I didn't use Kapton this time as I reduced the air flow and heat on my hot air station a bit and was quite easily able to resolder the QFN using 60-40 solder without desoldering anything else that was lead free solder.  It took me two attempts.  first time the QFN moved due to the hot air flow, I added a bit more of that gooey flux then it soldered nicely on the second attempt.

Once the QFN was back on, I soldered a couple of leads from my bench PSU, set to 12V which is the correct voltage input and powered up the DUT.  It worked fine  ;D

So then I resoldered the 2 x 10uF caps. fitted a new 100nF - just a guess as the original one was short circuit but all three 10uF + 10uf + ??? were in parallel on the12V input power rail after a serial diode presumably to prevent damage from an incorrect polarity PSU being plugged in - so 100nF seemed like as good a quess as any.  These are all obviously smoothing/decoupling capacitors.  After that I resoldered the Coaxial Power Socket and Optical Audio Socket, fired it up once more from the original PSU and it's running good.

Here's a couple pics of the reassembled device (MAG 256 IPTV box).  OK so I unsoldered the wrong part but if nothing else I gained some experience of reworking QFN for the first time.  Also the Mag Box owed me nothing (I already sold the customer a new one as they needed it working quick) so I will have a good profit when this one gets sold.  I always have customers  wanting to save a few euros against buying a new one.  If this thread helps anyone else in future then that's a win win for all.   Thanks guys.  Good job done.

Rich
« Last Edit: November 09, 2018, 05:39:18 pm by dicky96 »
 
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Offline perieanuo

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Not bad.
Sometimes in cases like this I don't even replace the capacitor..usually they put severals with safety margin, but it's pro to replace it.usually I put some low-z one.
My technique in find the sucker is to grow the current limit and you risk nothing (like I did in research company with power regulator boards at the beginning of the test routine).
Best regards,pierre


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Offline dicky96

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Thank you Pierre  ;D

Note: I did this job with my Maplin ZD-916 soldering station and conical tip which I have used for years, but I read on the internet that conical tips are rubbish.  I also used my trusty 858D hot air station which internet forums will also tell you is junk.  It seems interesting I can do a half decent job with apparently rubbish kit.  :P  However, they do say a picture paints 1000 words.

I do have one of the cheap T-12 soldering irons on my 'to buy' list with a selection of tips so I can play around with them.  And also a Quick 861DW and a 853B hot air pre heater on the list to buy.  Probably when I sell this box I fixed I will buy something with the proceeds....





 

Offline tehmagicsmoke

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I have seen people use kapton tape,  then aluminum tape over that. If you try to just use aluminum tape then the adhesive will make an uncleanable mess!
 

Offline perieanuo

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The mess cleansup witk acetone in 5 mins :)


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Offline perieanuo

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Thank you Pierre  ;D

Note: I did this job with my Maplin ZD-916 soldering station and conical tip which I have used for years, but I read on the internet that conical tips are rubbish.  I also used my trusty 858D hot air station which internet forums will also tell you is junk.  It seems interesting I can do a half decent job with apparently rubbish kit.  :P  However, they do say a picture paints 1000 words.

I do have one of the cheap T-12 soldering irons on my 'to buy' list with a selection of tips so I can play around with them.  And also a Quick 861DW and a 853B hot air pre heater on the list to buy.  Probably when I sell this box I fixed I will buy something with the proceeds....
Yep, a preheater helps very good, on new boards when I solder Bga or high pin count chip I use a heating plate home-made, aluminium plate that I usually put on 100 deg C.the hot air preheater is better, I remember the metcal 5000 in 2010...
Concerning the tip, for me conical is best, just change size.blades imho is for big stuff If you don't have something like a metcal soldering station with enough power behind and bigger tip inserted.
And you're right, cheap tools in hands of smart people does miracle, high end ones and 'left handed' are a disaster.me, I'm passionate with all type of soldering and welding, so the subject really touches me.fiability of a board is 90% soldering quality if the research part was been done correctly.
So, I say preheater if you have big ground planes and bga or something similar,if not just buy the specific heads (forgot the word) for your hot air station for your most used chipcases, changes your life.no more bigger temp for changing a processor, smaller time etcétéra..but if you can afford the preheater,go ahead, I did it for home hobby stuff, arduino raspi and the rest.
Nice talking with you!


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Offline dicky96

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And nice talking with you too

Is it possible to buy additional nozzles for the 858D in various shapes?  When I look on ebay it is hard to search as normally I find all the hot air stations on sale with 3 nozzles.  I did manage to find these sets of 6 nozzles https://www.ebay.es/itm/162099743211 and this set of secen on aliexpress https://www.aliexpress.com/store/product/6pcs-BGA-Circular-Nozzles-858-Hot-Air-for-SAIKE-ATTEN-858D-858-Hot-air-soldering-station/1758634_32324738291.html but not square shaped ones

Rich




« Last Edit: November 11, 2018, 02:33:57 pm by dicky96 »
 

Online wraper

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Square nozzles are not worth it. Basically you can do any job except large BGA with just 2-3 round nozzles.
 

Offline cdev

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I have several squares of liquid crystal plastic sheeting that acts like a poor man's thermal imager, placed over a PCB it lets you identify what is getting warm instantly.
"What the large print giveth, the small print taketh away."
 


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