Removing large through-hole PSU capacitors with Chipquik bismuth solders

[Photoman]

I was just in the similar situation.  A MosFet with chunky legs, bent at an odd angle and a small thru hole.

Cut the legs, desolder each leg separately, then use desoldering gun or solder wick to clean the holes. At least 10 times faster and less stress. Or use large tip and a lot of solder to cover all 3 solder joints simultaneously and easily pull transistor out.

That would work on a smaller package but not on this particular fet.  I also didn’t want to cut the legs in case they were good and I could reuse them.
They were a obsolete part only available with a 2 week lead time from china.  Though I did order them 2 weeks ago just to be safe...one was dead the other not.
I will only cut the legs if I really need too.  Legs up the Hakko 808 isn't a good idea.  They sometimes get a little stuck.

Also adding that much solder and heat could delaminate the board, remove pads or damage other components nearby.
Again in this case, the legs were close to other components that would have been damaged.

The SRA solder worked really fast and without any issue.

[wraper]

The SRA solder worked really fast and without any issue.

And then you need to clean everything from this crap at least twice. If package is something Like TO-3P. I would wiggle it out while heating each solder joint a few times in succession.

[Photoman]

The SRA solder worked really fast and without any issue.

And then you need to clean everything from this crap at least twice. If package is something Like TO-3P. I would wiggle it out while heating each solder joint a few times in succession.

I wouldn’t suggest using it for every joint on the board, just for the impossible ones.  Especially if you need to save a component or not damage the board.
Yes cleaning takes a little time, an extra minute or 2.  To avoid destroying the component and the board, it was worth it for me.
Also sometimes leveraging the component can destroy the thru hole.  I had a board a while ago that if you looked at it wrong, the pads would come off.
I used this stuff for one cap that needed to be replaced.  No lifted pads and it only took a few minutes.

Sure its expensive but that tube will last years.

[tggzzz]

In general I take the attitude that you should strive to do "more with less" and become a master of the tools available to you. There's truth in the old saying that "a bad workman blames their tools". However a good engineer/technician always knows the limits of their skills and tools, and chooses the right tool for the job.

I've now removed the board and been able to compare it with the schematic. This is more tricky than other boards I have recapped, so I was right to believe I need to improve my techniques.

The red ellipses indicate the capacitors I want to remove, and some of their pads on the underside. Key points:

• capacitors are close together, so wiggling them will not be easy
• it is a four layer board, with buried groundplane and with some very small pads; getting the right amount of heat to the solder is non-trivial
• given the appearance of the capacitors on the top, I do not understand the groups of three pads under the axial capacitors

So thank you all for your experiences and suggestions; please keep them coming!

[Messtechniker]

Yes these 3-legged electrolytics exist. See link here:
https://www.nmr.mgh.harvard.edu/~reese/electrolytics/cap_gallery.jpeg

[tggzzz]

Yes these 3-legged electrolytics exist. See link here:
https://www.nmr.mgh.harvard.edu/~reese/electrolytics/cap_gallery.jpeg

I've seen those before, and removed them from Tek scopes amongst other places.

But the ones on this board are tall, thin, and axial - there is the wire coming out the top. So what exactly is at the other end?

Oh well, I'll find out in a few days time

[Mechatrommer]

those big caps look easier to wiggle. those axial type also look more easier to wiggle, just in different way. ie visible solder joint on the top side should make the job much more easier. i will hold soldering iron steady there (with boost button keep pressed), from bottom if no access from the top. and pry up the top leg connection with tweezer. i think every hobbiests will try to do more with less at first, its blended in their DNA. only when they know its unfeasible then they start to search for other the right tools. btw i dont own a AIO desoldering gun (yet) so far i can deal with what i already have (hakko sucker hand pump) but calson did a review in case you are interested...

[ogden]

The red ellipses indicate the capacitors I want to remove, and some of their pads on the underside. Key points:

I would want to remove lefmost in black sleeve as well. - To gain room for "wiggle desoldering". When marked area is clean (from caps), only then proceed putting new back.

[tggzzz]

The red ellipses indicate the capacitors I want to remove, and some of their pads on the underside. Key points:

I would want to remove lefmost in black sleeve as well. - To gain room for "wiggle desoldering". When marked area is clean (from caps), only then proceed putting new back.

Removing "extra" capacitors is a dilemma, since it is another chance to damage the board. However, in this case that black capacitor is scheduled for clearance since it is part of the HT supply, and I suspect the HT inverter is the source of the spurs.

I'll also carefully note the type of each capacitor removed from each socket, in case there is something equivalent to the Tek24x5 "swapped capacitors" problem. Now I'm sure HP wouldn't make such a mistake, but there are multiple PSU variants for the 8562.

[tggzzz]

those big caps look easier to wiggle. those axial type also look more easier to wiggle, just in different way. ie visible solder joint on the top side should make the job much more easier. i will hold soldering iron steady there (with boost button keep pressed), from bottom if no access from the top. and pry up the top leg connection with tweezer. i think every hobbiests will try to do more with less at first, its blended in their DNA. only when they know its unfeasible then they start to search for other the right tools.

I'm not convinced those techniques would be sufficient for these capacitors, hence my questions and looking for new (to me) tools.

btw i dont own a AIO desoldering gun (yet) so far i can deal with what i already have (hakko sucker hand pump) but calson did a review in case you are interested...

That video shows some of the usual yootoobe video problems. The first 20 minutes shouldn't be a video, since moving pictures aren't needed - and if in the form of text+pictures, I could absorb the information in 30s flat. However, the second half does make good use of moving pictures, even if a little editing could have reduced it from 20 minutes to 5 minutes. Thanks.

[ogden]

I'll also carefully note the type of each capacitor removed from each socket

Right. I would just label them using sharpie, take a picture.

I second to advice using plain sn+pb solder and powerful enough soldering iron, without overdoing temperature. Initially I just make rounds between cap pins adding solder so much that they can't take anymore. On lead-free board it is good idea to wick/suck first mix of solder away to add sn+pb again and get lower temps in result. After that wiggle-solder step by step it out of the board. Preheating PCB area with (85..105oC) air usually increase chances of success. Even hair dryer can make difference, thou make sure you hold to board ground while doing that or just use ESD strap.

[tggzzz]

I'll also carefully note the type of each capacitor removed from each socket

Right. I would just label them using sharpie, take a picture.

Yes indeed. I'm a belt-and-braces person, and I assume that no one way will be sufficient!

I second to advice using plain sn+pb solder and powerful enough soldering iron, without overdoing temperature. Initially I just make rounds between cap pins adding solder so much that they can't take anymore. On lead-free board it is good idea to wick/suck first mix of solder away to add sn+pb again and get lower temps in result. After that wiggle-solder step by step it out of the board. Preheating PCB area with (85..105oC) air usually increase chances of success. Even hair dryer can make difference, thou make sure you hold to board ground while doing that or just use ESD strap.

This (1989) should be pre-ROHS, so it should be Pb solder - but your point is well taken.

I have various hot air guns, ranging from a temperature/volume controlled solder station with multiple nozzles, through hair dryers, to paint strippers.

I've only used the solder station to remove/replace single SMDs on a Tex24x5 controller board, using polyimide tape to shield other components. I think this may be a case where I will use it to preheat the area.

[edpalmer42]

If you're worried about damaging the board, the 'wiggle' method of capacitor removal should not be high on your list.  This method requires you to heat the pads over and over.  This will put a lot of heat stress on the pads.  Using the 'wiggle' method with ChipQuik might be an interesting combination.  It should reduce the heat stress due to multiple meltings.

The three holes under the capacitor could be for a three-legged capacitor where the third terminal is only for mechanical support.  That's not unusual with large capacitors.  It could also be to accomodate multiple physical capacitor sizes - maybe from different manufacturers.  That way, you don't have to respin the board if you change capacitors.  This could also be to accomodate second sourcing.

I've found that, particularly with pins that are connected to ground, it takes a lot more heat to unsolder that pin.  It's always important to use an iron that has lots of power behind it.  A small iron has to be held in place for a long time to heat up the traces and pin.  This alone can damage the board.  A larger iron will do the job fast so you can get in and out quickly and minimize damage to the board.  The same comment applies to the size of the tip.  Don't try to unsolder a large pin on a ground plane with a tiny tip.

Ed

[ogden]

This method requires you to heat the pads over and over.

Not actually. Thermal inertia helps. Also if you need more than 3..4 "wiggles" - you are doing it wrong. Try PCB preheating?

This will put a lot of heat stress on the pads.

What's that?

[Rasz]

There is using what you have, and there is using wrong tool for the job 

I dont see anything particularly fragile, or especially difficult about this board. Its not like its 8 layer pcb LGA socket replacement job.
Damage usually happens when cheap tools dont have sufficient power/thermal mass and user tries to compensate by bumping temperature.

[tggzzz]

Thank you all for your suggestions. In the end I used several techniques and the board has now been recapped.

I played around with a couple of PCBs:

• a 1972 PCB containing core memory,, fine tracks, double sided PTHs and old(!) solder
• an ATX PSU, ground planes, single sided

As predicted, the cheap heated 40W solder pump was a little fierce for my comfort. However, running it from a variac and reducing the voltage by 15% also reduced the temperature by ~15% (not 22.5%?!).

The Chip-quik bismuth solder might have formed alloys that reduced the required temperature, but definitely had the benefit of providing a good thermal contact at a relatively low temperature.

The heated desoldering pump was good at quickly removing the solder used for large snap-in capacitors. That was mainly because there was a lot of solder in a large hole, and once it started sucking into the pump, it all ended up there. The vacuum on its own is only partly helpful at removing the solder; equally important is that a strong draught into the pump picks up solder and removes it. Hence melting the solder and then holding the pump tip at an angle (to allow a draught) often worked best.

None of the techniques were excellent with small holes and closely fitting leads. I tended to use the pump to remove as much as possible, then desoldering wick with flux.

Sometimes that left a difficult-to-shift solder bridge deep in the hole. Tthe best technique was to melt the bismuth solder and allow it to flow into the hole. Then it could transmit the heat to the lead solder, and both could be removed with a wick.

The three legged capacitors are unlike anything I have seen. The third "axial" leg is just the can, and that is insulated from both the other terminals. I wondered if that is to allow some extra shielding connected to the ground, but that doesn't seem to be the case.

[tggzzz]

Some follow-up questions...

The local hackspace is considering whether it is worth buying a "proper" desolderer, either one of the "S-993A" mentioned above, or perhaps a https://cpc.farnell.com/duratool/d00672/desoldering-station-uk-eu-plug/dp/SD01384 and we feel we would like to hear experiences of using any such tool.

Starting questions are:

• how easy/difficult is it to remove the solder from inside the tool? Previous experience from decades ago leads me to believe it can be a pain, plus the tool will be in a "hostile environment"
• how large is it in the hand, and does that cause any issues?
• where the vacuum is supplied via a hose, does that mean it takes a long time for the suction at the nozzle to increase/decrease?
• where the vacuum is supplied via a hose, is the hose a "tail that wags the dog"?

Your opinions and experiences would be welcomed...

[BreakingOhmsLaw]

To add something to the original question:
After having bought some "bismuth" solder that turned out to be fake, i now learned this:

As of today, you cannot get a bismuth solder wire with flux in it, as the alloy is to brittle for the process that is needed to put in the flux cores. Flux core wire is basically made by first making an aloy pipe, filling it with flux and then continuously rolling it down to the desired diameter. You apparently cannot do that with Bi58Sn42 as it will just break. That's why the BiSn wires as of now are all solid without flux - or a paste.

However!
The German comnpany Felder has announced they will be launching a flux-cored bismuth solder later this year. They apparently found a way to pull it off and have patented it.
As soon as i can get my hands on it, ill be sending some to Daves mailbag and hope he will give it a run for its money.
To answer the obvious question: No pricing available yet. I am guessing it won't be cheap and they will be cashing in on their research.
You can find their press release here: (in German and English)
https://www.felder.de/aktuelles/produktneuheiten/produktneuheiten-details/snbi-l%C3%B6tdraht.html?file=files/felder/content/Aktuelles/Produktneuheiten/Neu2020.pdf
I for one would be very happy with that stuff. I restore a lot of old test gear (GAS, guilty as charged  ) , and this stuff could really help to not lift the pads off from elderly crumbling PCBs