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Feeling stupid about solder / flux
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Feeling stupid about solder / flux

by stephanie on Fri Jan 07, 2011 9:00 pm

I've been playing with electronics for over 25 years - assembling kits, etc. etc. I learned to solder before I learned to drive. My problem is that I take a few years (or a decade or so) off then come back to it, and some subtle changes seem to have come along and caught me in the rear.

When I learned to solder, the choices were 'use rosin based flux, not acid based flux'. Now I've just found out that there's water-soluble flux, which is a very different beast from rosin based. Found this out because some projects I've built have inexplicably failed after a short time in-use. Trying to diagnose the problem and noticed that the flux residue seems to be still wet, sticky, messy, and in some cases, it looks like it's been bubbling. Huh?!

Sure enough, the last 1lb roll of 63/37 I picked up is 'water soluble'.

My understanding is that with water soluble, one must clean the board after assembly. Do you need special equipment for this? I have a banned sink...

Is there a time-limit to cleaning? Can you clean the board off after say, a week? Two weeks? A month? Is the flux residue conductive? It seems to be, or at least, it conducts enough to confuse the heck out of microcontrollers.

Sigh and grumbles. I hate when the world changes and nobody tells me.

Cheers & thanks in advance.
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Re: Feeling stupid about solder / flux

by neutron spin on Fri Jan 07, 2011 9:45 pm

I would not use the water soluble flux for hobby use. The water soluble flux needs to be removed entirely or it can lead to corrosion if some is left behind. But it is easier to remove from underneath surface mount components. I personally only use the rosin based fluxes but again I am old school...regards... :)
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Re: Feeling stupid about solder / flux

by stephanie on Fri Jan 07, 2011 10:11 pm

Thanks for the quick response. I'm feeling 'old school' as well. Had I known this stuff existed, I would have gone out of my way to avoid it. I'll pick up some 'real' solder next time I'm at the local electronics place.

In the meantime, I guess the damage is done. I've got to go back now and inspect everything I've put together in the past 5 or 6 weeks, and hopefully it isn't too late to wash the stuff off.
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Re: Feeling stupid about solder / flux

by franklin97355 on Fri Jan 07, 2011 10:15 pm

I almost fell for the term "water soluable", What could be more natural than water? WS is a very aggressivs flux and is good for old boards/parts with oxidation but most of my work these days is done with "no clean" flux. Harder to get off but not as destructive if not removed.

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Re: Feeling stupid about solder / flux

by stephanie on Sat Jan 08, 2011 12:27 am

I'm picking up some real rosin-core flux tomorrow, and am getting rid of the 'water soluble' stuff.

What I'm wondering now, though, is what I can do about the boards I've assembled already? Do you just dunk the board in water for a few minutes? Or hold it under running water?

And can I still do that with a board that was assembled as long as 5 weeks ago?
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Re: Feeling stupid about solder / flux

by franklin97355 on Sat Jan 08, 2011 1:27 pm

I would use HOT water and a brush sometimes adding detergent is advised. After doing that you might still go over the board with isopropyl just to be sure.
Here is a good forum post http://www.smtnet.com/Forums/index.cfm?fuseaction=view_thread&Thread_ID=15237

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Re: Feeling stupid about solder / flux

by zener on Sat Jan 08, 2011 4:37 pm

I always had good results with water washable flux. It is pretty simple stuff. You solder, then you rinse it off with water and a brush. Some people use a dishwasher if they have a lot of boards. It is a lot easier than using flux cleaner. Rosin is fine also, and you generally don't need to wash it except for appearance. No clean is OK but does leave some residue and some works better than others. If you had washed the water soluble flux as you are supposed to then you would not be having any problems. It was developed so that industry didn't need to use lots of solvents and worked well for that, until no clean came a long.

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Re: Feeling stupid about solder / flux

by stephanie on Sun Jan 09, 2011 12:29 am

Well I've had fairly good success with cleaning boards after-the-fact. I found the datasheet for the solder, it said to clean within 3 hours using just warm water. 3 hours, five weeks, same difference, more or less.

I took 5 or 6 small project boards I've built, and used hot hot hot water, a toothbrush, did some scrubbing where the guck was most noticable or most built-up. Then I left the boards soaking in hot water for 15-20 minutes, and then finally set them out to dry. For once, the super-dry winter air is helpful.

One of the boards was a kit I had assembled at the start of December, and it had failed immediately. I thought the kit was bad, as I found other people describing similar failures. It's now fully functioning.

Most of the other boards were working but close inspection showed the flux residue was looking a bit nasty; they are all still functioning just fine and are now shiney clean to boot.

One board however has not fared well. My monochron, ran for 36 hours and then the display went wonky. It was while trying to diagnose that, which led me to this huge lesson in water-soluble solder. Alas, the monochron has not recovered and I've been unable to solve it. (I started a new thread in the clocks section, seeking help about that.

Zener, I am sure that when it is used correctly, the water soluble stuff is just fine. Although it seems that there can be problems with it 'hiding' in some places that you can't easily wash, such as beneath ICs, inside DIP pin connectors, or inside female header connectors, and under male header connectors. My problem was basically that I didn't know this stuff existed, and erroneously assumed I was using standard rosin-core solder -- the stuff I've been using for 25+ years without any problem.

I've decided not to throw out my roll of water-soluble; I'll give it another shot in the future, now that I know how to use it correctly.
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Re: Feeling stupid about solder / flux

by Akhilesh on Wed Nov 06, 2013 5:01 am

Hi, need to understand something about rosin fluxes..
Are they safe? What is the rosin content like 5% or so?
And which is the best brand to get it from, please help
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Re: Feeling stupid about solder / flux

by adafruit_support_bill on Wed Nov 06, 2013 6:39 am

Rosin core solder is not particularly dangrerous as long as you follow basic precautions and avoid breathing the fumes during soldering. For more information see the article below:
http://www.lbl.gov/ehs/ih/pdf/safeSolderingFinal.pdf

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Re: Feeling stupid about solder / flux

by Renate on Wed Nov 06, 2013 9:32 am

Flux is usually not conductive.
The problem is that it will absorb water over time.
If you have circuits with lower impedances they couldn't care less.
It's when you get into higher impedances and precision circuits that they wreak havoc.
You'll find this often with older equipment and especially hand soldered connectors.
They usually forget to deflux those.

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