I want to unsolder a surface mount quad flat pack chip to repair traces under it. The area was affected by a leaking capacitor. The IC legs are dark dull gray, solder isn't going to stick to them to assist removal so I need to try cleaning it first. It's the PCB from an IBM floppy drive and if I'm SOL if the IC legs break during removal since I don't have a replacement. Are there liquids I can use to help remove corrosion before trying mechanical cleaning? I have an ultrasonic cleaner, if appropriate. Similarly I have a Mac Plus mainboard in a bad state which I don't want to repair at this time but would like to attempt some cleaning/neutralizing before putting it in storage for a future project. Thank you.
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How to clean corrosion prior to unsoldering?
- Thread starter Dagwood
- Start date
NeXT
Veteran Member
If the legs are already at risk of coming off, nothing's gonna save you. SMD hot air gun doing circles around the part while applying a lifting pressure with tweezers is all you got.
tingo
Veteran Member
You could try with flux and hot air - I haven't tried it, but it seems like the "lowest stress" option.
Try it on a pin or two and see if it works.
Try it on a pin or two and see if it works.
Hugo Holden
Veteran Member
The grey oxide is not only a thermal insuator, but an electrical one.
The only way is to manually scrape it off the surface with a small tool such as a jeweler's flat blade screwdriver and then attempt to add a high quality multicore flux Tin/Lead solder such as Loctite brand available from Newark Electronics in NJ. Don't use any additional fluxes or acids, or any chemicals in attempts to remove the oxide, it will just further contaminate the pcb.
Also if electrolyte has leaked to the board surface, it is in the conformal coating and will be conductive and not clean off with regular contact cleaner. Some remove the coating, I never do because it ruins the appearance of the board. Instead I use a leaching method I developed. This involves running a small stream of warm to hot water over the affected area and letting that run off the nearest board corner holding the board at a 45 degree angle (not doing anything silly like dunking the whole board in water) It requires holding it there for about 1 hour, the ions diffuse out of the coating and into the water stream and are washed away. After that the board should be cleaned with regular contact cleaner. If the electrolyte is also not washed out of places such as plated through holes, they can corrode out later.
Electrolyte initially leaked from a capacitor is not very electricaly conductive at all and close to pH neutral, or corrosive for that matter. However, this changes over time with reactions with metals and there is an alkaline shift. After a while it becomes highly corrosive and electrically conductive. I wrote an article about this problem in reference to repairing 2465B scopes:
The only way is to manually scrape it off the surface with a small tool such as a jeweler's flat blade screwdriver and then attempt to add a high quality multicore flux Tin/Lead solder such as Loctite brand available from Newark Electronics in NJ. Don't use any additional fluxes or acids, or any chemicals in attempts to remove the oxide, it will just further contaminate the pcb.
Also if electrolyte has leaked to the board surface, it is in the conformal coating and will be conductive and not clean off with regular contact cleaner. Some remove the coating, I never do because it ruins the appearance of the board. Instead I use a leaching method I developed. This involves running a small stream of warm to hot water over the affected area and letting that run off the nearest board corner holding the board at a 45 degree angle (not doing anything silly like dunking the whole board in water) It requires holding it there for about 1 hour, the ions diffuse out of the coating and into the water stream and are washed away. After that the board should be cleaned with regular contact cleaner. If the electrolyte is also not washed out of places such as plated through holes, they can corrode out later.
Electrolyte initially leaked from a capacitor is not very electricaly conductive at all and close to pH neutral, or corrosive for that matter. However, this changes over time with reactions with metals and there is an alkaline shift. After a while it becomes highly corrosive and electrically conductive. I wrote an article about this problem in reference to repairing 2465B scopes:
lots of heat from a hot air gun, heat it up, then let it cool then hit it with a soft tooth brush very lightly, then heat again with flux and clean it again. I seen good things with vinegar being used too. and a pray the chip is not corroded on the inside.
wile I agree fully, I would be worried about doing that with a board with corrosion, I'd hate for something to fall off and get lost down the drain.
DeltaDon
Veteran Member
Why not try some vinegar and a toothbrush and see how much it cleans up before going to the step of trying to get it off in one piece. The cap's internal "juice" needs to be neutralized even if you do remove the chip in working order. Flush with distilled water after the vinegar.
seconded. Mike tech over on youtube does that a lot on his older videos with 486 and older systems and it works good
Al Kossow
Documentation Wizard
wrong...
the electrolyte is an acid
clean with supersaturated sodium bicarbonate in distilled water
Water-based Electrolytic Capacitors: From Plague to Essential
Water-based electrolytic capacitors: From plague to indispensable component. Learn about the benefits of these components.
channelmaniac
Veteran Member
I do arcade game PCBs in the kitchen sink with full hot water (142F at the tap here), a toothbrush, and liquid dish soap, to clean the PCB after cap removal. Gets all the electrolyte and oils from NiCD/NiMH batteries off the PCB. Also works beautifully for cleaning cartridge slots of dirt and sprays that attract more dirt. Shake the water from the slots, pat dry with a loop free towel, and dry in the oven at minimum temps (170F)...
Done hundreds over the years with zero returns for cleaning related issues.
pbirkel
Veteran Member
Point that needs to be emphasized! I suspect that some folks are applying the leaky (alkaline) battery clean-up philosophy indiscriminately ...wrong...
the electrolyte is an acid
clean with supersaturated sodium bicarbonate in distilled water
Water-based Electrolytic Capacitors: From Plague to Essential
Water-based electrolytic capacitors: From plague to indispensable component. Learn about the benefits of these components.www.rutronik.com
Hugo Holden
Veteran Member
Never use a dishwasher and never use vinegar. There are sound reasons for this advice. There are places on pcb in crevices and inside various components where you do not want water to infiltrate ever. This is because with certain anatomical structures the surface area is very low, with respect to the volume and you won't get those areas to dry out for months and corrosion gets induced. I'm sorry I don't have time right now to go into the the mathematics and the details, but, I promise you it is a very foolish idea to put an entire pcb, of any complexity, into a dishwasher.
The last thing you want to do is to add any chemicals with ionic species, like Vinegar. Unfortunately this idea infiltrated the internet and youtube as a solution to many pcb dilemmas and many thought it was a plausible idea to chemically neutralise pcb contaminants. Don't do it. If you want to remove ionic species from a pcb, forget about the concept of chemical neutralisation, unless you propose to calculate the exact number of cations & anions involved, which you can never know, the only way to do it properly is with water leaching, preferably with de-ionised water, but most people do not have that on hand, or a system to create an appropriate flow rate stream of it. The next best thing is tap water followed by a contact cleaner wash, but like I said in a previous post, never dunk the whole pcb in water. You need to pay attention, because as always, the Devil is in the detail.
The last thing you want to do is to add any chemicals with ionic species, like Vinegar. Unfortunately this idea infiltrated the internet and youtube as a solution to many pcb dilemmas and many thought it was a plausible idea to chemically neutralise pcb contaminants. Don't do it. If you want to remove ionic species from a pcb, forget about the concept of chemical neutralisation, unless you propose to calculate the exact number of cations & anions involved, which you can never know, the only way to do it properly is with water leaching, preferably with de-ionised water, but most people do not have that on hand, or a system to create an appropriate flow rate stream of it. The next best thing is tap water followed by a contact cleaner wash, but like I said in a previous post, never dunk the whole pcb in water. You need to pay attention, because as always, the Devil is in the detail.
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Hugo Holden
Veteran Member
Electrolyte is not very acidic. While it might be a tad below pH 7 initially in some cases, mostly close to pH 7 to a tad Akaline. In fact as capacitors age (before they leak) many undergo an alkaline shift in the pH of the electrolyte. The electrolyte formulations are proprietary. To convince yourself open some capacitors and test the pH. Its corrosive effects occur because its pH shifts more Alkaline after interactions with metals (the explanation for this is in the article I linked in post #5) Tin, Lead Copper etc on the pcb. The electrolyte which leaks out of capacitors initially at least, in most cases, is not very electrically conductive or corrosive. However it becomes very conductive after interactions with metals and then its corrosive properties rapidly escalate with an alkaline shift. Initially when the electrolyte leaks out and contaminates the pcb, many pcb's do not malfunction at that time, it takes some weeks/months for the problems to start, and they happen sooner in high impedance circuitry first. In low impedance circuits, there can be devastating corrosion before the circuit malfunctions and the problem of severe corrosion at that point is discovered by the technician.wrong...
the electrolyte is an acid
clean with supersaturated sodium bicarbonate in distilled water
Water-based Electrolytic Capacitors: From Plague to Essential
Water-based electrolytic capacitors: From plague to indispensable component. Learn about the benefits of these components.
It is a very foolish move to add either an anionic or cationic compound to a pcb in an attempt to neutralise a chemical compound to a neutral pH when you do not know the concentration required, the ionic species you need, or the molar amount to add for a perfect balance. It just further contaminates the pcb. The better thing to do is to leach out the ionic species into a pH neutral solution, like deionised water.
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