Author Topic: How can I desolder a QFN without accidentally desoldering nearby components?  (Read 3534 times)

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Offline 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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Offline 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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Offline 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 »
 

Offline 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 »
 

Offline 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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