VOGONS


First post, by AppleSauce

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I'm planning on removing some caps on a motherboard and a power supply in preparation for new caps , are there any things to watch out for like how to safely remove the cap , or a good temperature?

Reply 1 of 13, by Deunan

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With what? Soldering iron? I'm going to assume, since you ask, that you do not have hot air station with preheater.

Preferably cut them out and then desolder the leftover pins and clean the holes with a solder sucker. Be gentle with cutting, do not pull or twist, always think ahead of how much strain you are putting on the mobo so that you don't rip the copper away. This method is especially meant for multilayer PCBs with tons of GND/power fills that take the heat away from the iron tip. You can heat the pin remains from either side, pull it to either side. The only downside is you can't test the cap afterwads to see how good/bad it really was.

On simple one-sided PCBs like in PSU you can just heat one spot and twist the cap a bit, repeat with the other side and go back and forth until it's out. That approach is also possible with multilayer PCBs but some old boards can peel from just looking funny at them, so beware, you can damage the copper that way.

Now, some people desolder stuff with hot air guns, but this is not really a tool for removing just selected parts and not others, plus you'd better train a lot on dead PCBs first to get a feel for temperature and time.

Reply 2 of 13, by wiretap

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Through Hole -- Add fresh solder / flux to the capacitor legs from the back side. Use a soldering iron and solder sucker to remove the solder from the mounting holes. A manual hand pump works fine, but a mechanical vacuum pump is easier. Pull capacitor out. (some additional heat may be needed if there is a little leftover solder or the legs are bent) Clean out the hole with the solder sucker after it is removed.

Surface Mount -- Add fresh solder / flux to each pad. Use hot tweezers to lift the component off the board. Don't twist or pull if it isn't fully heated and released because that's how you tear pads. Clean the pads with solder wick braid after the capacitor is removed.

Temperature depends on the type of solder. Adjust the soldering iron temperature to where the solder begins to melt, then add 20-30 more degrees to account for heat wicking.

Surface mount are the easiest capacitors to remove IMO.. many times the through-hole capacitors are connected into a copper plane and it takes a lot of heat to remove them cleanly. If it is just a 2-layer PCB though, it shouldn't be an issue.

It helps to have the right tools for the job to avoid damaging a board. They don't have to be expensive tools, just the proper ones.

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Reply 3 of 13, by BitWrangler

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I don't know of any reason why flux should be needed when desoldering, it inhibits the job while providing no advantages to offset this.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 4 of 13, by wiretap

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BitWrangler wrote on 2021-10-09, 13:35:

I don't know of any reason why flux should be needed when desoldering, it inhibits the job while providing no advantages to offset this.

It aids in the removal so solder doesn't get stuck in tight tolerance areas. (helps it get sucked out easier) It isn't inhibiting the job at all, it makes it far easier. Give it a try.

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Reply 5 of 13, by BitWrangler

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In my experience it only draws heat out of the work and gums up your solder sucker. All it does is clean metal surfaces to make solder stick better, it's does not modify the flowing properties of solder. It makes it wet the metal better, but does not make it runnier.

In particular if you have an IC where the tips of the pins have re-oxidised, and you need to walk it out of the board, if you heat until you can get one side withdrawn into the holes and have not used flux, it probably didn't pull solder into the holes with it, it beaded off the oxidised part, then you do the same with the other side, and it might be loose enough to pull all the way out now, or with a touch on a couple of pins, rather than needing to work each pin row again.

To a less noticeable extent, this will happen all over the board, solder will cling to freshly fluxed stuff instead of beading off the oxidation on it and getting sucked up easier.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 6 of 13, by wiretap

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Well, I don't know what to tell you, because adding flux to aid in removal is per IPC-7711B/7721B and corresponding J-STD-001, used by the global rework industry as the gold standard.

It does aid in removal to a significant degree, and it is obvious once you do it. It clears the vacuumed holes much better.

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Reply 7 of 13, by BitWrangler

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I'll regard that as voodoo until I see a scientific reason for it.

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Reply 8 of 13, by wiretap

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They document everything really well with the scientific analysis. I'm sure if you dig in their archives, you'll find the answer.

But yea, heresy.

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Reply 9 of 13, by Joakim

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I'm more of a theoretical guy, I actually watched through this 1960s educational video about soldering and it was very informative and probably still quite accurate.

They probably do not go through desoldering though.

https://youtu.be/vIT4ra6Mo0s

Reply 10 of 13, by Thermalwrong

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Maybe if the solder has a small amount of flux in it, then adding extra flux is less necessary. IDK if you've ever used solder without flux, but I accidentally bought some and it's quite useless without at least some flux on application.
Solder wick also has flux in it and that's essential for cleaning pads after removing parts - it does leave a lot of flux residue though.

The quality of flux makes a big difference, I got some Chipquik flux from Mouser rather than cheapy / fake fluxes from amazon & ebay. It's a night and day improvement, cleans up better and gets solder flowing more easily. Great for fixing pins that got soldered together on smaller SMD components.

For desoldering parts, I tend to find just flooding leaded solder is enough, but flux can help to desolder parts where the solder has been corroded.
Wiretap's method for desoldering SMD parts is exactly what I do with parts that have legs on each side, very convenient method and little risk of damaging pads or surrounding parts 😀

For temperatures to use, I usually stick around 350C, leaded solder doesn't need such a high temperature but it gets solder flowing quickly. If you're working with parts made after around 2003, where lead-free solder started coming into use, you'll need slightly higher temperatures. When you're desoldering parts, a bigger soldering tip can help because of the greater thermal capacity.

Since you're recapping the parts, you'll come across some cap legs where the hole will just not clear of solder. I use an Abeco Flexivac manual desoldering pump and that helps, but with a motherboard's power stages, the holes can be very hard to fully clear. I cheat, I got some really small drill bits I think 0.8mm - I just turn those in the hole and since I'm just turning them between finger & thumb, the force is only enough to clear the solder, the copper vias haven't been broken yet.

For cleanup, I've found that the easily available WD-40 fast drying Contact cleaner is great at cleaning up flux. Just spray that on, brush a bit, swab with a cotton bud. Then to fully remove all the flux and get it looking like a clean PCB, spray the area to clean, put a paper kitchen towel on it and brush the paper towel onto the area.

Reply 11 of 13, by Deunan

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Flux does mainly two things, the acidic part deals with metal oxides and the rest reduces the surface tension of the solder, and also to some extent shields fresh solder from oxygen in air. Depending on solder type, age and the amount of corrosion it has, adding flux prior to desoldering can be as effective as adding new solder (which usually also has flux in it, by the way). Sometimes both are needed, and if mild flux is not cutting it then there's always more acidic stuff. Obviously the more aggresive it is, the more imporant it is to clean it from the PCB after soldering.

I go with flux, if for no other reason that beeing able to see the solder start flowing properly when removing 100+ pin chips with hot air. For low pin count parts, if someone has a preferred method that doesn't use extra flux, well, whatever works. So long the results are satisfactory I guess any method is valid. Flux does help to deoxidize pads and pins though, and I always follow with solder sucker and/or toothpick to clean the holes, so I like using flux.

Reply 12 of 13, by maxtherabbit

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Deunan wrote on 2021-10-09, 15:33:

Flux does mainly two things, the acidic part deals with metal oxides and the rest reduces the surface tension of the solder, and also to some extent shields fresh solder from oxygen in air. Depending on solder type, age and the amount of corrosion it has, adding flux prior to desoldering can be as effective as adding new solder (which usually also has flux in it, by the way). Sometimes both are needed, and if mild flux is not cutting it then there's always more acidic stuff. Obviously the more aggresive it is, the more imporant it is to clean it from the PCB after soldering.

I go with flux, if for no other reason that beeing able to see the solder start flowing properly when removing 100+ pin chips with hot air. For low pin count parts, if someone has a preferred method that doesn't use extra flux, well, whatever works. So long the results are satisfactory I guess any method is valid. Flux does help to deoxidize pads and pins though, and I always follow with solder sucker and/or toothpick to clean the holes, so I like using flux.

this

Reply 13 of 13, by hyoenmadan

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For flux requirements when soldering, and quantity, I just stick to Louis Rossmann recommendations on the subject.
"Bigger the glob, better the job" (tm), and then clean properly with a brush, IPA and gently scrub.

Flux is cheap anyways, at least compared to many of my vintage patients waiting on the repair line.