How can I desolder a QFN without accidentally desoldering nearby components?

[dicky96]

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

[anishkgt]

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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[sokoloff]

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.

[sstepane]

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).

[dicky96]

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 

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!

[sokoloff]

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.

[NivagSwerdna]

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).

^

[T3sl4co1l]

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.

Tim

[JFJ]

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

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

[wraper]

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

It's QFN, there is a pad on the bottom.

[wraper]

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.

[alpher]

Since there is something  "bad"   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.

[wraper]

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.

[wraper]

Since there is something  "bad"   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.

[KL27x]

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.

[dnwheeler]

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.

[wraper]

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.

[cdev]

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.

[KL27x]

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.

[T3sl4co1l]

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.

Tim

[dicky96]

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 

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.

[Rasz]

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

"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

[cdev]

While you are waiting for the replacement is a good time to look at the area and the other side really closely.

[wraper]

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.

"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.

[Rasz]

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.