Isopropyl alcohol conductivity

[netdudeuk]

Hi

I soldered some pins on a 0.1" header and did some TTL logic probe tests.  I found a low on an unsoldered pin adjacent to a proper low.

I'd cleaned the area with 99.9% IPA before the testing.  I'd used a Chemtronics flux pen beforehand.

There were no shorts so I removed the header frame today and found that there were no shorts under it either. It was still wet though.

I cleaned up and there were no issues on further testing.

So, does IPA cause shorts ?

Thanks

[Kjelt]

IPA should evaporate. That it is still wet after a longer period really puzzles me, you must have used a lot, in the future heat the pcb at that location a little bit with an airgun (50^oC or about)

But to answer your question, according to this pdf:
www.panachem.com/msds/iso_propyl_alcohol_IPA.pdf

the electrical conductivity of IPA with the D4308 method (no idea what that is) is about 6*10⁶pS/m = 0.000006 S/m = 166666 ohm/m

so yeah it is conductive but not very much, between two legs of an ic is aprox 8mm so a drop of IPA would be around 20Mohms.

But I might miss something here like there is no 100% IPA so there will be some kind of water? in it etc. etc., so let the real chemist stand and speak up 

[coppice]

Hi

I soldered some pins on a 0.1" header and did some TTL logic probe tests.  I found a low on an unsoldered pin adjacent to a proper low.

I'd cleaned the area with 99.9% IPA before the testing.  I'd used a Chemtronics flux pen beforehand.

There were no shorts so I removed the header frame today and found that there were no shorts under it either. It was still wet though.

I cleaned up and there were no issues on further testing.

So, does IPA cause shorts ?

Thanks

IPA is a good insulator, but a lot of fluxes conduct fairly well. They won't give you a short, but they can give you a low enough impedance to cause trouble in a high impedance circuit. Messing up LCD displays is a common effect of flux residues around MCUs, for example.

If you found the board appeared wet after a few minutes, it wasn't wet with IPA. That evaporates quite quickly. The IPA had probably dissolved some flux, spread it around, evaporated, and left a shimmering wet look film of flux.

[Ian.M]

It absorbs moisture from the air and with trace ionic contamination from the flux, that will cause shorts.   I hope you keep your stock of IPA in a *TIGHTLY* sealed container. Next time, after all excess IPA has been shaken or mopped off and all visible remains have evaporated dry with hot air at 105 deg C for several minutes to get the last traces out from under parts.

However the high conductivity indicates you need better washing to remove flux residue - with an aqueous phase to remove ionic contamination.   Try an initial scrub with your preferred flux remover to remove all visible flux, followed by scrubbing with  IPA with 20% distilled (or deionised) water added (as recommended by H.P. back in their 1982 'Bench Briefs'), followed by rinsing with pure dry IPA to reduce the remaining moisture level and promote total evaporation.  Between each solvent, blot with a clean dry lint-free cloth to remove as much as possible of the previous solvent.

[Howardlong]

I had a hell of a time debugging a scope front end after doing a small amount of rework on it. Nothing made any sense, it went into oscillation sometimes but not others, and I was seeing some weird stuff on the meter that I simply thought were caps charging/discharging as the resistance changed.

It turned out to be the cocktail of IPA/flux/fluxcleaner being used on the high impedance board was messing things up, and once it'd got itself inside the relays all bets were off.

Almost all of my experience is in low impedance and digital applications, this high impedance stiff isn't something I generally do, but I learned the hard way.

[netdudeuk]

It absorbs moisture from the air and with trace ionic contamination from the flux, that will cause shorts.   I hope you keep your stock of IPA in a *TIGHTLY* sealed container. Next time, after all excess IPA has been shaken or mopped off and all visible remains have evaporated dry with hot air at 105 deg C for several minutes to get the last traces out from under parts.

However the high conductivity indicates you need better washing to remove flux residue - with an aqueous phase to remove ionic contamination.   Try an initial scrub with your preferred flux remover to remove all visible flux, followed by scrubbing with  IPA with 20% distilled (or deionised) water added (as recommended by H.P. back in their 1982 'Bench Briefs'), followed by rinsing with pure dry IPA to reduce the remaining moisture level and promote total evaporation.  Between each solvent, blot with a clean dry lint-free cloth to remove as much as possible of the previous solvent.

It was between the plastic frame of the header and the board so I was stuck.

I've seen videos where they apply a sheet of something and then brush with IPA or the like.  I saw this one today.

https://youtu.be/Jgpo406pJzE?t=16m55s

I was thinking about buying this today.

http://uk.rs-online.com/web/p/products/2987921/

I like the brush idea but maybe that's no better than my IPA and cotton buds ?

[coppice]

I had a hell of a time debugging a scope front end after doing a small amount of rework on it. Nothing made any sense, it went into oscillation sometimes but not others, and I was seeing some weird stuff on the meter that I simply thought were caps charging/discharging as the resistance changed.

It turned out to be the cocktail of IPA/flux/fluxcleaner being used on the high impedance board was messing things up, and once it'd got itself inside the relays all bets were off.

Almost all of my experience is in low impedance and digital applications, this high impedance stiff isn't something I generally do, but I learned the hard way.

Even if you get a board super clean, you can still have issues with high impedances. Some PCBs have moisture trapped under the solder mask, due to poorly controlled manufacture. I don't know a way to fix that. You just have to throw out the boards, and get a batch made with tighter control. Good boards get dirty, and as soon as the surface has any texture moisture can accumulate on the surface at fairly low humidity levels. A hair dryer and a spray of silicone grease can fix this in the short term, and are good for diagnosis. However, most silicones are hygroscopic, so they are eventually self defeating. Slotting high impedance PCBs can be a excellent tool for achieving a solid reliable high impedance.