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Zinc cloride paste problems

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TheMerovingian

Thu Mar 27 2008, 10:04AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I'm finalizing my microcontroller-driven 5-stage coilgun submachinegun, setting the pulse widths to obtain best speed. For first i tried the first stage maximizing the speed. After some right shots it started to became unstable. THe bargraph started to empty slowly during standby, while the microcontroller tried to switch on the charger to reload the bank. Before realizing what was happening and turning off the charger i heard a click from the coil (a pulse) and the microcontroller went in error state, telling that the charger output was shorted. After inspection I realized that the IGBT of the first stage died, probably because the gate was driven to a low voltage, and the IGBT exceeded the istantaneous power dissipation. Since both photogates where low and the microcontroller didn't send the pulse to the driver (discrete driver), the driver itself must have some sort of failture. Since it isn't the first time i have problems with zinc cloride paste, i know that some residues of zinc cloride past got electrolized and became conductive turning the igbt slightly on. THe removal of this paste is boring and time consuming (acetone, cotton and a brush), but it is the only thing to do. Anyone had similar problems with zinc cloride paste?

Marko

Thu Mar 27 2008, 01:17PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I could not realize from your post, what are you using zinc chloride for?
Some sort of thermal grease?

(It's nasty, hygroscopic, cellulose-dissolving stuff which reacts quite exothermically when mixed with water. I was attempting to use ZnCl2 to dissolve paper in toner-transfer PCB making but didn't work too well and was too messy to store, even in zip-lock bags it turned into brown goo within weeks).

In air it does this in hours and creates very good electrolyte, not something I'd want near high voltage electronics, so are you sure about everything in your post?

Marko

TheMerovingian

Thu Mar 27 2008, 11:06PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

THis is used for solder-aiding since it deoxidizes surfaces and helps tin turning shiny. I found 1IGBT dead plus some bjt's of the level shifter. I'm trying to remove any residues with acetone and a cotton stick. I'm monitoring now the power dissipation in transistors with the circuit turned on and in standby.

Marko

Thu Mar 27 2008, 11:32PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Oh, I see.

I'd strongly advise against such kind of flux. Can you buy ''lotlack'' spray over there (it's insulating varnish containing solder flux, great for PCB's), I had really great use of it.

Zinc chloride absorbs moisture like crazy, and as electrolyte solution it has excellent electrical conductivity (last thing you want on your PCB's!)

cbfull

Fri Mar 28 2008, 11:35AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

Do you have access to plain rosin? Rosin is a mild organic acid that is hydrophobic, so it will not absorb moisture. I have a jar of rosin chunks and so far I haven't had a need for anything stronger. It dissolves easily in 99% isopropyl alcohol, so cleanup is a snap.

IIRC, ZnCl flux should really only be used for soldering copper pipes, and never for electronics. It's far too corrosive to electronics.

Steve Conner

Fri Mar 28 2008, 12:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, I've seen several electronics books that warn against using this kind of plumbing flux. The flux in flux-cored solder, which is mostly made of rosin AFAIK, is all you need.

If you have a board so dirty that it won't solder, try an abrasive cleaner, something like wire wool with alcohol, or the small fibreglass brushes that electronics retailers sell, or even just scrape the grot off the spot you want to solder with a pocket knife, which is what I do.

Rosin smells great though

TheMerovingian

Sat Mar 29 2008, 12:18AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I removed all residues with acetone and now it works great

. I never learn the lesson to not use it except for wire soldering...

where i can find rosin?

ps: I have tested three stages ( on 5) so far and it works very good, more information about the project will be displayed in "electromagnetic projectile accelerator" as soon as all stages are tested and final speed obtained

cbfull

Sat Mar 29 2008, 02:34AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

TheMerovingian wrote ...

where i can find rosin?

I was afraid you might ask that. I've never actually shopped for it, but where I work I found a 5 gallon pail full of it. It just looks like light brown pieces of transparent rocks, and it is very brittle. I think I have seen it dissolved into a solution using electronic grade (99%) isopropyl alcohol. Some people like to dip their leads into it before soldering. It's actually something I never tried, the chunks are a bit of a pain in the neck.

I would recommend checking (oh jeez, I can't believe I'm going to say it!) Radio Shack or a computer supply store. Look for the 99% alcohol in these places too for cleaning off the residue. A clean board is a happy board (well, hopefully).

Happy soldering!

Craig

...

Sat Mar 29 2008, 03:05AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I find it almost hard to believe that you have managed to survive without flux in your life, it is the magic juice that makes solder stick. I prefer to use flux pens (you can get them for about $2 at any half decent electronics house), but you can get it in a sponge, brush-in-can, spray bottle, etc, The most common is rosin flux, or a multitude of specially formulated mixes (water soluble, no clean, etc, etc) if you feel like being adventurous.

Bjørn

Sat Mar 29 2008, 03:08AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If all fails you can make rosin of sap from pine trees or possibly some other trees.

As Steve pointed out the smell can be couriously pleasant. I put forward a hypothesis that people that did not like the smell all died houseless, toolless and foodless.

I used to have a big box of rosin and I nevere found any use for it. I just clean the parts instead. An old toothbrush and toothpaste seems to do the job brilliantly and not a single solder joint has ever failed.

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