Removing large through-hole PSU capacitors with Chipquik bismuth solders

[tggzzz]

I've got an HP spectrum analyser that shows spurs originating in its SMPS, so I ought to replace some of the large aluminum electrolytics. There are also some "delayed action smoke generators" (Rifa capacitors), and they definitely have to be replaced.

All of those are through-hole components on a densely populated 4-layer PCB, so there is an increased chance I'll damage the PCB when I extract them. This question is about reducing that risk.

Chipquik makes DMD, a low-temperature solder based on bismuth which melts at about 58C. https://www.chipquik.com/store/product_info.php?products_id=210002 Chipquik regard the principal maket being removal of SMD ICs, but I have PTH components.

So, has anybody used Chipquik SMD to remove large PTH components? If so, what hints and tips can you pass on?

Thanks

Background: I've decapped and replaced many capacitors in Tek scopes over the years. So I know some come out easily, some, um, "less easily".

[bob91343]

Your casual tossing about of acronyms without definition prevents me from giving any suggestions. rifa?  PTH?

[wraper]

Why would you need this crap to remove capacitors? Also you are forgetting the fact that you need to flush pads on PCB and soldering iron with regular solder twice before soldering new parts. Otherwise solder will be contaminated and solder joints won't be reliable. That certainly does not reduce stress on PCB.

[wraper]

Your casual tossing about of acronyms without definition prevents me from giving any suggestions. rifa?  PTH?

If you don't know what those are, he probably does not need your suggestions. RIFA - paper capacitors which explode. PTH - plated through hole.

[TheMG]

For stubborn capacitors I usually heat one lead, then gently rock the capacitor to one side a bit so the heated lead starts to move out of the hole ever so slightly, heat the other lead and rock the capacitor the other way, repeat until capacitor leads are out of the holes.

[Photoman]

I was just in the similar situation.  A MosFet with chunky legs, bent at an odd angle and a small thru hole.
No matter how much new solder, flux and sucking from my hakko desoldering gun, they wouldn’t come out.
I used FAST CHIP from SRA (same kind of bismuth alloy) https://smile.amazon.com/gp/product/B07NPZTR8J/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1.
Probably an inch of the stick was needed for all 3 legs.  Ran over it with the iron and I was able to wiggle it out.

It wont hurt to try the chip quick or the sra one.  After I got the part out, I reflowed the open holes with normal solder a few times.
Sucked it clean between then cleaned with a flux cleaner.  Just to make sure I don't have the alloy left to mix with new solder.

Just a note of caution, the alloy stays molten for a long time.  Its easy to flick off a drop on to the board or your hand.
So be careful.  Solder burns suck!

[wraper]

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.

[Rasz]

but why?
$32 + shipping for consumable is almost as much as cheapest hotair station

[WattsThat]

but why?
$32 + shipping for consumable is almost as much as cheapest hotair station

Hot air station? The OP is talking about through hole parts.

Cutting the out part out first is always the best way to start when possible.

When you cannot cut leads, I re-solder the part first to preheat the area then desolder. When the vacuum drops out, keep the pin moving with the desoldering tool until it cools. Parts will fall out. Every time.

[tggzzz]

For stubborn capacitors I usually heat one lead, then gently rock the capacitor to one side a bit so the heated lead starts to move out of the hole ever so slightly, heat the other lead and rock the capacitor the other way, repeat until capacitor leads are out of the holes.

That's a technique I've used in the past when recapping.

In this case the geometry makes it look difficult in some, if not all, the cases. As you can see in the pictures, the 6 Rifas (count them!) and 4 of the electrolytics are closely packed and the latter are rather squat

In addition, I've read reports that the board is a "fragile 4 layer board", so I'm prepared to take a belt and braces approach.

EDIT: see reply #28 for better pictures

[tggzzz]

I used FAST CHIP from SRA (same kind of bismuth alloy) https://smile.amazon.com/gp/product/B07NPZTR8J/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1.

Just a note of caution, the alloy stays molten for a long time.  Its easy to flick off a drop on to the board or your hand.
So be careful.  Solder burns suck!

Thanks for the pointer to the alternative supplier, and the tip about the length of time it stays molten. Given the lower melting point that seems obvious - after thinking about it

I still have a small and unimportant solder blob scar from 40 years ago

[tggzzz]

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's one of my normal techniques.

Unfortunately in this case it is impossible to cut the legs of radial capactors, plus the caps are large and attached to a ground plane.

Hence there is a lot of thermal mass to be heated via the vias

[tggzzz]

but why?
$32 + shipping for consumable is almost as much as cheapest hotair station

Hot air guns are problematic for large components attached to groundplanes. Surrounding components/joints would get up to temperature long before the relevant joints have melted.

[tggzzz]

I re-solder the part first to preheat the area then desolder. When the vacuum drops out, keep the pin moving with the desoldering tool until it cools. Parts will fall out. Every time.

I've only got an unheated solder sucker pump without a continuous vacuum.

However I've just blundered across a suprisingly cheap heated solder pump https://www.amazon.co.uk/dp/B01N2H1CLS/ref=pe_3187911_185740111_TE_item and ordered it.

Looks like I'm going to have some experiments to do before attacking the objective

[Haenk]

the 6 Rifas

They made really, really sure, it will explode one day 

[tggzzz]

the 6 Rifas

They made really, really sure, it will explode one day 

You can't see it in the low resolution photos, but cracks are visible I've had them remove the prepreg from a PCB

Having just paid for almost the right schematics, the line input module also has intrnal capacitors. Oh joy.

Bonus: you can still get exactly the same Rifas, so that should remove the need for paying very close attention to the dimensions

[brabus]

Am I the only one thinking that those electrolytics may be perfectly OK and don't need changing at all?

If noise is coming from the SMPS, I would be looking at the secondary side of the converter first.

If you absolutely need to remove those caps, I would use my favourite method: cut the soldering joints flush and drill them through with progressively larger bits, from 0,5mm to 1mm, to 1.25, 1.50 and even 2mm, until the hole is clean. Make sure to be dead center, though.

[tggzzz]

Am I the only one thinking that those electrolytics may be perfectly OK and don't need changing at all?

Harumph. Yes, having been able to look at the circuit diagram since my first post, I am tending to think that for the electrolytics. However, they are 1989 vintage, so 30 years old...

<expletive deleted> Rifas have to come out, though.

If noise is coming from the SMPS, I would be looking at the secondary side of the converter first.

Most definitely; I'm just going through the parts list.

In fact there are two separate switching PSUS, one for the main body and one for the CRT.

Having waved a scope probe around in the air, it looks like the CRT is at the right frequency, and that matches something I've seen on the groups.io mailing list.

I'm also checking a few tant beads, too.

If you absolutely need to remove those caps, I would use my favourite method: cut the soldering joints flush and drill them through with progressively larger bits, from 0,5mm to 1mm, to 1.25, 1.50 and even 2mm, until the hole is clean. Make sure to be dead center, though.

I haven't heard of that interesting technique, thanks. I'm unconvinced I could satisfy that caveat

[Mechatrommer]

but why? $32 + shipping for consumable is almost as much as cheapest hotair station

i bought Zephyrtronics's low melt in 2010 and i only used a bit from it to desolder few many legged ICs (including recent soic8) and keep reusing the used blobs over and over until when i consider them "not low melt" anymore, so far the blobs can still be used for the purpose where the rest of 99.9% of the stock are still untouched. for any repairers, i will say low melt is a must have item on their bench. imho whoever says we have to re tin the pads with normal solder 2 to 10 times are too academics, i will usually clean the IC pins and pads with solder wick once until there is no more significant leftover and resolder the old or new IC into place once and i dont see any problem so far. we cant argue that there will be 1 or 2 low melt molecules got blended, but like i care when there are 99% of normal solder mixture in there. for stubborn board that even low melt and wiggling kungfu cant solve it, i think we need to go back to how to they were made... ie complete reflow (cheap oven).

[wraper]

If you absolutely need to remove those caps, I would use my favourite method: cut the soldering joints flush and drill them through with progressively larger bits, from 0,5mm to 1mm, to 1.25, 1.50 and even 2mm, until the hole is clean. Make sure to be dead center, though.

Nice way how to destroy hole metallization.

[WattsThat]

Rifa: tick tick tick BOOM

[edpalmer42]

I re-solder the part first to preheat the area then desolder. When the vacuum drops out, keep the pin moving with the desoldering tool until it cools. Parts will fall out. Every time.

I've only got an unheated solder sucker pump without a continuous vacuum.

However I've just blundered across a suprisingly cheap heated solder pump https://www.amazon.co.uk/dp/B01N2H1CLS/ref=pe_3187911_185740111_TE_item and ordered it.

Looks like I'm going to have some experiments to do before attacking the objective

I do NOT recommend that tool.  I have one that looks identical.  The temperature is NOT controlled and I found it was very easy to overheat the board.

There is a reasonably priced, heated, variable temperature, vacuum desoldering gun that has had good reviews.  I picked one up and it has been okay in light usage.  I did have a heating element go open so you might want to pick up a spare.  I also recommend that you get the three different size tips.  Search for S-993A.  There are multiple sellers and youtube reviews on it.

Ed

[Rasz]

but why?
$32 + shipping for consumable is almost as much as cheapest hotair station

Hot air guns are problematic for large components attached to groundplanes. Surrounding components/joints would get up to temperature long before the relevant joints have melted.

This thread should have a subtitle 'I have bad equipment therefore I am considering bad solutions'
like this:

I've only got an unheated solder sucker pump without a continuous vacuum.

and doubling down on more crappy tools:

However I've just blundered across a suprisingly cheap heated solder pump https://www.amazon.co.uk/dp/B01N2H1CLS/ref=pe_3187911_185740111_TE_item and ordered it.

you should transfer this one straight to the bin


Hotair in this case is for preheating pcb before desoldering components. Controlled preheating is very important when dealing with large ground planes, helps with bubbling, deforming and delamination.

Am I the only one thinking that those electrolytics may be perfectly OK and don't need changing at all?

You are not. Input caps rarely die, they live an easy life. Their size makes people jump into wrong conclusions.

imho whoever says we have to re tin the pads with normal solder 2 to 10 times are too academics, i will usually clean the IC pins and pads with solder wick once until there is no more significant leftover and resolder the old or new IC into place once and i dont see any problem so far. we cant argue that there will be 1 or 2 low melt molecules got blended, but like i care when there are 99% of normal solder mixture in there.

You are applying something akin to industrial waste. Highly corrosive to dissimilar metals, high resistance, lower thermal conductivity. All because you are probably using 40 year old 60W soldering iron.

Its not entirely the same, but good analogy:





Same deal as people playing with liquid metal in cooling applications.

[rcbuck]

drill them through with progressively larger bits

That is a really, really bad idea for a 4 layer board. The power and ground traces may be connected to the inside layers. Even if they are not, you risk shorting the internal layers together or to the outer layers when you install the new caps.

[NiHaoMike]

In my experience, a soldering gun and desoldering pump has always worked for removing large capacitors with the capacitor getting slightly warm at most.