Author Topic: TQFP chip de-soldering?  (Read 1160 times)

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Offline David_

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TQFP chip de-soldering?
« on: November 17, 2017, 11:37:46 pm »
Hello.

I own a Atmel Xplained XMEGA A1, on that board are a ATxmega128A1 which as far as I know and can find information about have a lot of faults. Some people have deemed it's ADC as pretty much useless in any other mode than some single particular mode, but many things are not working as it should.

Anyway, I happen to have a few ATxmega128A1U which is the fixed part(Also got a USB transceiver) with the same footprint as the one on the Xplained board and I was about to de-solder it.

But as anyone whom have de-soldered chips know it is possible to complete screw the procedure up making a mess of things, which I don't want.

I will probably have more questions but to begin with, I am using a butane powered solder iron(I bought this because it was cheap and it does work as a hot air solder tool) I will also try to use a FLIR ONE IR camera connected to my Android phone to tell the temperature and know when I should try to pull the IC.

What I want to ask is this, do you think it is beneficial to add extra solder to the pins or should I remove as much as I can of the solder that's already there?

To my mind I would think that more solder would keep the heat for longer thus making the hole procedure easier, but is there any reason for why to not add extra, much extra solder for this purpose?

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Online KL27x

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Re: TQFP chip de-soldering?
« Reply #1 on: November 18, 2017, 12:00:31 am »
The only reason to add more solder is to reduce the melting point, in case they used lead free. You can buy chipquik bismuth alloy solder to help even more.

Otherwise, no, there's no reason to add more solder. And there's no reason to add more flux when desoldering with hot air. Save the flux for cleaning the board after the part is removed.

Just keep the heat source moving and have a stick/implement to intermittently nudge the part so you will not heat the board more than necessary. Take a picture, first. If any of the surrounding passive components get disturbed, don't worry about it. Get the chip off, and clean the mess up, after.

If your torch is too small, you can just dremel the pins off. Clean off the pins and board, and use your iron to solder the new chip. If the board is particularly expensive and I had no experience/practice, and all I had was a small butane torch, I would probably just do it this way.

IME, you are more likely to damage the board than MOST components. The ones to be careful of are ceramic resonators, electrolytics, and plastic connectors. Kapton tape is often used to protect specific components from hot air. IC's/micros, IME, are very much hardier than FR-4 and soldermask. I would focus a lot of the heat right on top the IC, in general, as well as trying to go around the pins. Esp since the IC is worthless, in this scenario, I would really torch the package, itself.
« Last Edit: November 18, 2017, 12:11:35 am by KL27x »
 

Offline alank2

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Re: TQFP chip de-soldering?
« Reply #2 on: November 18, 2017, 12:07:43 am »
I did this the other day with a TQFP44.  I used hot air and one of those aaters meant for the TQFP44.  I actually just sat it on the pins at 350C and let it heat them enough until I could push it off the pads.  Then I pulled the chip and used some flux to clean up the pads and all the smeared solder between them.  Applied flux, put the chip on right and I was going to add solder, but it wasn't necessary.  Touching the pads/pins caused it to wick up into the pin.
 

Offline alpher

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Re: TQFP chip de-soldering?
« Reply #3 on: November 18, 2017, 12:22:59 am »
Another vote for ChipQuick, just use the regular soldering iron not the butane torch.
Even though I have a hot air station I rarerly use it to desolder anything ever since got the chipquick, it does work and most importantly, It really reduces the chances of damaging the board or componets arround the chip.

 

Offline Buriedcode

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Re: TQFP chip de-soldering?
« Reply #4 on: November 18, 2017, 12:35:15 am »
Without hotair, or a preheating plate, if you can, either sacrifice the board, or the chip you're desoldering. It sounds like you won't be needed the chip you're removing.

If you wish to save the board, but don't care for the device you remove - carefully run a craftknife or razor blade along the tops of the rows of pins until you're through them. You're then just left with pins to take off with your iron before cleaning up the pads with desoldering braid/wick and flux. 

Remember, often the glue that holds copper tracks/pads to the board melts at a lower temperature than solder, which means if the solder on the pad is melted - the pad itself is held on the board by surface tension.  Also overworking the same pads (reheating a few times) can cause the adhesive to fail, making it more likely to lift pads each time you heat them.  On most PCB's it's quite hard to actually do this.

An older and much more annoying method is to use fine copper wire under the pins of a row, remove as much solder as you can with braid, and one by one heat the pin and pull the copper wire, lifting the pin, going along like you're unzipping it.  As you're heating the pads *and* putting potential side load on them, there's a fair chance you'll move/lift pads which is why I don't think anyone will recommend it - unless you're really stuck.
 

Offline wasyoungonce

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Re: TQFP chip de-soldering?
« Reply #5 on: November 18, 2017, 04:07:04 am »
Another vote for ChipQuick, just use the regular soldering iron ...

Agree

Made some PCBs up with LQFP64 pin ICs.  Managed to damage one...used chipquick and 2 irons.  Smothered the pins lotsa flux and up it came.  Oh don't blow air onto chipquik...it spreads everywhere!
I'd forget my Head if it wasn't screwed on!
 

Offline BrianHG

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Re: TQFP chip de-soldering?
« Reply #6 on: November 18, 2017, 10:17:31 am »
I've successfully done similar and larger chips carefully with a hot air paint-stripper and tweezers.
__________
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Online Ian.M

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Re: TQFP chip de-soldering?
« Reply #7 on: November 18, 2017, 10:49:57 am »
I agree with all the other ChipQuik recommendations.  Its the simplest way of reducing thermal stress on the board to reduce the risk of pad damage.   If you've got a variable temperature hot air gun, you can improvise an underside board preheater and vastly increase the working time or even keep the mixed alloy fully molten.  Tap the chip gently to be certain all its legs are fully free before attempting to lift it, with a suction tool or bluetack on a stick.

Due to ChipQuik's tendency to get everywhere if you knock the board during cleanup (or blow air at it), it would be a good idea to  mask off as much as possible of the rest of the board with foil and tape, so you don't have to be quite as careful when cleaning it off the pads.

If you are trying to do it *without* ChipQuik,  probably the best option would be to bridge all the pins with Sn60/Pb40 solder to reinforce them so no individual pads get over-stressed, then dremel with a cutting disk round the edges of the chip to sever all the pins, remove the chip body, desolder the loose pins, and clean up with desolder braid, wiping each pad away from the track, and *never* wiping cross-ways (along the row of pads).
 

Offline AndyC_772

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Re: TQFP chip de-soldering?
« Reply #8 on: November 18, 2017, 11:26:46 am »
ChipQuik is all well and good, but you do need to be able to remove it all before you can re-solder anything to the board. I've used it a few times, but generally wished afterwards that I hadn't.

Plan ahead, make sure you know how you'll complete every stage of the process before starting. I'm afraid that if you add any extra solder (including ChipQuik) that you'll just end up with a mess that you'll really struggle to clean up without damaging the board.

The mention of a gas powered soldering tool scares me. They don't come with the small, properly coated and easily tinned tips that you'll need to fit the new IC, and forget any idea of temperature control. I wouldn't touch a TQFP with one.

Getting the old chip off is the easy part. Stick a scalpel blade under one corner of the chip and apply very, very gentle upward lift while you heat it using hot air. Start gently, and if the chip hasn't come off in about a minute, increase the heat and try again.

You're much more likely to do damage by using physical force rather than heat. Don't be afraid to apply a bit more heat if you need it to make the chip lift easily.

Once the old chip is off, remove the excess solder from the PCB pads using solder wick (2 or 3 mm wide is ideal). Always move the wick in a direction which is along the length of each pad, never drag it along the row from one pad to the next.

To fit the new device, have a search for video tutorials on 'drag soldering'. A 0.5mm TQFP is perfectly solderable by hand if you have the right tools and your eyesight is up to the job. But, IMHO, a gas powered iron is about as far from being the right tool as it gets.

Online Ian.M

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Re: TQFP chip de-soldering?
« Reply #9 on: November 18, 2017, 11:43:41 am »
You *can* do good SMD rework with a gas iron, but to do so needs the right sized bit and an extreme level of competence that's best acquired by hundreds of hours doing rework with the proper tools and a good variable temperature controlled iron and station, + more hundreds of hours doing through-hole rework with the same gas iron so you are familiar with all its quirks, like changing orientation altering the gas flow rate.

ChipQuik is a PITA to clean up, but is really the best chance of avoiding pad damage if all you have is an iron with poor temperature control, as long as you can turn it down enough to avoid cooking the pads.

Using a hot air gun rather than a hot air pencil tool doesn't give you much control of what gets heated.   The slightest slip or too much airflow and you'll be scattering passives everywhere, and its difficult to get enough heat into the joints round the chip without cooking nearby parts and the board.  You can mitigate most of the problems by masking the rest of the board with foil and tape, with a sheet of paper between the foil and the board for thermal insulation, but the tape needs to be heat resistant, and few newbies have Silicone adhesive Kapton tape handy.  You may be able to get away with Aluminum foil tape, but the odds are its adhesive wont stand up to the temperatures involved.

If you are only masking to protect against ChipQuik splatter and you are *NOT* using hot air, ordinary masking tape or aluminum foil tape is good enough, because it isn't being directly heated.

Personally, I regard cleaning up the footprint and drag soldering the replacement chip as the easy part - if you've got good technique it will all go as planned and it doesn't take much practice to become competent unless you are either totally cack-handed or have a tremor or other fine motor control disability.    OTOH large SMD chip removal without a rework station including a board pre-heater is always a PITA with a risk of pad damage.   Some boards have very robust copper to substrate bonds, others shed pads if you merely look at them wrong, and unless you have reworked that specific make of board previously, you've got no idea which is which till you've ripped up half a dozen pads and learned the hard way.   Generally, the better quality the board, the more robust its pads will be, but sometimes even leading brand manufacturers get it wrong, and then that model gets an evil reputation in the repair trade.
« Last Edit: November 18, 2017, 11:59:06 am by Ian.M »
 

Offline Buriedcode

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Re: TQFP chip de-soldering?
« Reply #10 on: November 18, 2017, 05:47:55 pm »
Whilst I'm sure some have had success with chipquik.. I'm not sure why one would resort to using it?  The OP clearly doesn't need the chip he is removing, so why try and keep it intact, it just makes life harder.
What can ruin the board isn't just max temperature, but also heating time.  Sometimes it is better to go to hot, but do it very quickly rather than constantly trying to keep huge blobs of solder molten along each row of pins.  ChipQuik always sounded like a bit of a novelty to me.

A large heatgun has far too much power, and with a large nozzle you would have to mask off everything with something heatproof.  Due to its uncontrolled temperature, even if the chip survives (most do) it can ruin the board. 

I still say, play it safe, sacrifice the micro that's already on board.  If you are willing to spend $30,
https://www.amazon.co.uk/Reelva-Desoldering-Unsoldering-Station-Digital/dp/B01FXAYXDG/ref=pd_lpo_sbs_60_t_0?_encoding=UTF8&psc=1&refRID=SPV244DPKEAJ7WFT7WFN

these aren't great, but they do the job.
 

Online Ian.M

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Re: TQFP chip de-soldering?
« Reply #11 on: November 18, 2017, 06:00:07 pm »
My experience with cutting chip pins is: Its far far too easy to wrench or mechanically shock one while cutting it and break the pad to board bond.  That's why I recommend blobbing all the pins together first for mechanical support. 

If you don't want to do that, you can desolder one pin at a time, lifting it with a sharp bent pointed  pick and bending it back over the package to get it out of the way for access to the next pin.  Leave one corner pin on each side till last and hold the chip in place with bluetack while you lift the last four pins to avoid wrenching their pads.
 

Online KL27x

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Re: TQFP chip de-soldering?
« Reply #12 on: November 20, 2017, 01:51:35 am »
Quote
Whilst I'm sure some have had success with chipquik.. I'm not sure why one would resort to using it?  The OP clearly doesn't need the chip he is removing, so why try and keep it intact, it just makes life harder.
What can ruin the board isn't just max temperature, but also heating time.  Sometimes it is better to go to hot, but do it very quickly rather than constantly trying to keep huge blobs of solder molten along each row of pins.  ChipQuik always sounded like a bit of a novelty to me.

IME doing the chipquick thing, there is almost no way you will damage the board, unless you accidentally scrape on a trace while heating the blob. You can generally use regular soldering temperatures, and you can leave the iron on there seemingly indefinitely. The adhesive holding the copper on there is going to be weak, at this temp, but there's no real time pressure to hurry up and get-er done. Just care to not push pads/traces with any force. Don't care how big the blob is, it can't heat the board higher than the temp of your iron. But the bigger it is, the more it will hold that temp while you get the other sides.

On/off fast is the general rule I live by for contact soldering. With hot air, the better guideline is low and slow. You're heating the soldermask and epoxy as much as the thermally conductive copper. When you overdo it, these poor thermal conductors burn right away. Hot air is pretty much the opposite of how you woudl want to do it. By nature, the surface of the epoxy/mask is going to be reach a given temp sooner than the joint you're trying to flow. Also, you can cause the board to flex and stress fracture other joints. So of you want to do the fast vs slow game, hot air ain't exactly the hare, as far as damage to the board goes. At least with the chipquik, you are reducing the melting point, and the heat is going directly to the conductive copper, not the epoxy.

I use hot air, first, because it's more convenient. Chipqwik would be something I dig out where I want to be more careful, in conjunction with either iron or hot air. It works very well, at very low temp. Reflow oven soldering is how the board was prolly made to begin with. This probably did more to the board than the chipqwik. IMO. Bottom line, Chipqwik is not necessary, in most cases, if you have hot air. But it is more gentle to the board, in case that becomes necessary. And it's probably a lot easier to not screw up, if you don't have experience using hot air.
« Last Edit: November 20, 2017, 02:25:49 am by KL27x »
 

Offline jmelson

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Re: TQFP chip de-soldering?
« Reply #13 on: November 20, 2017, 02:14:38 am »
Depends on whether you are trying to reuse the chip, or the board.  If the chip is toasted, or just an obsolete part, I often use an X-acto knife to scribe the pins on 3 sides of the chip (assuming a quad with leads on all sides).  Then, I put the blade in the notch in the lead and snap it off the chip body.  With leads on 3 sides severed, I lightly scribe the 4th side and then lever the body up and down a few times.  the rest of the leads fatigue and the chip comes off.  Then, I use a soldering iron to "sweep" all the leads off the pads.  Then, use braid to remove excess solder, and you are ready to install the new chip.
Do use a brush or whatever to remove tiny conductive dust from the scribing operation before replacing the chip.

Now, of course, if you have a hot air rework station, that is the best way to go for chip removal.

Jon
 


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