Beginner flux that won't cause conductivity problems when hand soldering  - Page 1
Forum section: electronics / beginners

author: Heyitsroth
author_rank: Newbie
timestamp: May 18, 2020, 10:55:01 am
content: Hi, I'm a beginner at soldering and I'm trying to research the right kit to start withI’ve got most stuff and now I'm looking at flux. I was about to purchase the amtech stuff that everyone raves about, until I foundthispost regarding conductivity of this flux before it’s heated. This is a concern since I will be not only hand soldering, but beginner hand soldering, so may very well put too much flux or not heat all of it for long enough to leave non conductive residue.I want to minimise headaches to give me the best chance of persevering and improving, so what flux can I use that I simply don’t need to worry about? Or, if that’s not really an option, should just go for basic rosin flux, since, if I will need to clean the board anyway (due to possibly conductive residues), then basic flux is cheaper and easier to remove?Not actually entirely sure why I should be looking for no-clean flux anyway, since the only arguments I can find for cleaning normal rosin flux is to allow for conformal coatingThanks!

author: Ian.M
author_rank: Super Contributor
timestamp: May 18, 2020, 12:26:42 pm
content: If you hand solder with a gel or paste flux you *MUST* clean the board as there is a high probability of leaving residue that hasn't been deactivated by sufficient heating. How thoroughly you have to clean depends on how active and thus corrosive the flux is.Any R or RMA (i.e. rosin) liquid flux that dries hard, if used sparingly can be treated as no-clean for non-critical hand assembly, however if you slop it on everywhere it will probably have to be cleaned if only because it will leave a horrible tacky mess that can take several days to fully dry hard. I'm lazy so use a liquid rosin flux sparingly to avoid unnecessary cleaning, however rosin paste fluxes are much more readily available in small quantities and are usually easer for most novices to use, if you accept that you will have to do some cleaning.N.B. Chose solder wire with a compatible flux core. All rosin fluxes can be mixed. Avoid anything with a water-washable flux core, (and water-washable fluxes in general) as a hobbyist without a high pressure heated wash system or a swept frequency ultrasonic cleaner cant get all the trapped flux residue out from under parts where it will cause pockets of corrosion if any is left. *NEVER* use high activity, halide, or water-washable flux on stranded wire as the residue will get up under the insulation, cannot be effectively cleaned and will cause the wire to corrode till it rots out.

author: Heyitsroth
author_rank: Newbie
timestamp: May 18, 2020, 01:13:16 pm
content: Thanks for your replyQuote from: Ian.M on May 18, 2020, 12:26:42 pmThis is what I'm worried about. Obviously, avoiding water soluble fluxes avoids corrosion as you said, but my concern is if the flux is conductive, then it getting into places I can't clean it out from would also cause problems. So I was looking for a reliably non-conductive flux, which apparently doesn't necessarily include 'no-clean' fluxes.Quote from: Ian.M on May 18, 2020, 12:26:42 pmBut might they not a) be conductive if not heated thoroughly, b) run/flow into places that can't be cleaned without specialist equipment? Therefore potentially ruining a board?

author: Ian.M
author_rank: Super Contributor
timestamp: May 18, 2020, 01:37:55 pm
content: Dry rosin is a reasonably good insulator. As long as the board isn't for high voltage, doesn't have any nodes that require extremely low leakage current, and isn't going to be exposed to extremely high humidity, and you haven't left a lot of charred flux bridging pins and tracks, there is no need to clean small traces of dry R (Rosin) or RMA (Rosin, mildly activated) flux. Most RA (Rosin, Activated) fluxes should be cleaned unless the datasheet specifically states the residue is inert and non-conductive at ambient temperature.N.B if a board was heavily loaded with rosin flux and has been cleaned with IPA or other alcohol it may take up to 24 hours with good air circulation for any remaining trace residue to dry out enough for the leakage to become negligible. Using lots of warm air (not exceeding the max. temperature rating of the most delicate part on the board) can speed that up considerably, to under an hour or maybe even a few minutes.

author: David Hess
author_rank: Super Contributor
timestamp: May 18, 2020, 02:44:19 pm
content: Pretty much any common paste or liquid rosin flux will be suitable.

author: Heyitsroth
author_rank: Newbie
timestamp: May 18, 2020, 03:15:59 pm
content: Quote from: David Hess on May 18, 2020, 02:44:19 pmThat's what I thought, but the post I linked (https://www.eevblog.com/forum/beginners/flux-conductivity-(nc-559-as)/) suggests that certain fluxes are conductive until fully heated, and can't guarantee I would completely heat all the flux as a beginner hand soldererQuote from: Ian.M on May 18, 2020, 01:37:55 pmAre there any gels/pastes that are R/RA? The ones I've looked at seem to just call themselves No-Clean with no additional info. Also, when you say dry rosin is a good insulator, is that straight out of the bottle, or requiring heating up first? If so, then I come back to my original issue of not being able to guarantee proper heating from hand solderingQuote from: Ian.M on May 18, 2020, 01:37:55 pmWhat would you consider high voltage in this context? I know I'll be working on some e-bike stuff, so that would be ~60v DC

author: wraper
author_rank: Unknown
timestamp: May 18, 2020, 03:18:28 pm
content: Alpha OM-338 tacky flux is completely safe as it's not conductive even in unactivated state. But for hand soldering you might want liquid flux instead.https://www.tme.eu/en/details/topnik-gel-bga/fluxes/alpha/om-338/

author: David Hess
author_rank: Super Contributor
timestamp: May 18, 2020, 04:29:43 pm
content: Quote from: Heyitsroth on May 18, 2020, 03:15:59 pmConductivity of the flux will only matter for high impedance and precision circuits which will require their substrates to be thoroughly cleaned and baked out anyway. It might also matter for high voltage circuits, but this does include line voltages.

author: wraper
author_rank: Unknown
timestamp: May 18, 2020, 05:02:17 pm
content: Quote from: David Hess on May 18, 2020, 04:29:43 pmIf you did actually check how conductive they are, you would not write this. But chipquick garbage is very conductive even after full activation when heated to >50oC temperatures. Tests in second part of the video.

author: Heyitsroth
author_rank: Newbie
timestamp: May 18, 2020, 07:20:17 pm
content: Quote from: wraper on May 18, 2020, 03:18:28 pmThank you! Was hoping someone would make a recommendation since I'm getting tired of all the research and just want to start making things!Could I ask why you suggest liquid flux might be better for hand soldering?

author: Gyro
author_rank: Super Contributor
timestamp: May 18, 2020, 07:35:04 pm
content: A rosin based flux pen is a convenient way of applying just enough extra flux when reworking or need a bit more than the rosin cores of the solder itself. It avoids accidentally flooding the board. eg.https://cpc.farnell.com/chemtronics/cw8200/rosin-flux-dispensing-pen/dp/SA00860?st=flux

author: wraper
author_rank: Unknown
timestamp: May 18, 2020, 07:43:56 pm
content: Quote from: Heyitsroth on May 18, 2020, 07:20:17 pmLess residue unless you apply tacky flux in very thin layer. Tacky flux has long working time so is more suitable for hot air or IR soldering.

author: Ian.M
author_rank: Super Contributor
timestamp: May 18, 2020, 08:03:04 pm
content: It all comes down to less and easier cleanup.A liquid rosin flux will dry, eventually to solid rosin. Drying is sped up by application of heat, but most R and RMA liquid fluxes are safe one dry even if they haven't been heated to activation temperature as time alone will render them non-conductive.If you are assembling bare boards, a flux pen of liquid R or RMA flux is ideal for getting a thin film of flux just where you need it and once dry and soldered the extra residue will be negligible, so cleaning can often be skipped. A flux pen is less suitable for rework as fluxing SMD and thru-hole IC pins tends to chew up the nib unless you stick purely to a dabbing action. A small brush holds up better for applying flux for rework, but if you are a klutz, for %DEITY%'s sake work from a small bottle or jar in a low-profile holder with a wide stable base, *NOT* from the cap of the bottle the flux was shipped in as cleaning up even a smallish flux spill on your bench is a PITA, let alone when you've just knocked over half a litre of it and its dripping off the edge of the bench onto your chair and floor and soaking through your trousers!If you are assembling low cost or home-etched bare copper PCBs, you can also clean the tracks to bright copper with a mild abrasive then wipe the whole board with liquid R or RMA flux and let it dry, which will reduce oxidization and preserve easy solderability for quite a long time.OTOH paste or gel flux is better for getting a lot of flux to stay around the joint as you are soldering, and if you are applying flux by the bucket full its easier to keep paste/gel flux on the board not spread over your bench!

author: KL27x
author_rank: Super Contributor
timestamp: May 18, 2020, 08:16:51 pm
content: QuoteYeah, and it's kinda like painting. If you're gonna let the (rosin or other suitable) flux just dry on its own, without soldering/heating it, then it's better to apply a very thin layer that will dry quickly. And then add more when you actually solder the parts on there. Especially if you're using SMD parts, it helps to apply fresh flux if for no other reason than so the parts will stay put while you move the board around. But additionally, you really want a bit of a pool or palpable thickness of flux when doing SMD wave/drag type of soldering.If you apply a thick layer of RA onto copper, and you leave it there, you can watch the flux turn green within minutes or hours. On most manufactured PCB's there is very little exposed copper. You usually have some kind of finish on there. Or solder/tin. So even if you leave wet residue, it won't be nearly as big a problem. It will eventually just dry. The tin/solder oxidizes way more slowly than does copper, so it won't make the same problem.

author: Heyitsroth
author_rank: Newbie
timestamp: May 19, 2020, 10:27:24 am
content: Quote from: Ian.M on May 18, 2020, 08:03:04 pmDo you know of any fluxes that meet this criteria, i.e. are definitely non-conductive after time? I'm really just looking for specific recommendations so I can get on and buy something. And how much time are we talking (assuming a light coating with brush or pen)?Quote from: Ian.M on May 18, 2020, 08:03:04 pmThe amount of times I've swept a load of carefully placed screws/part from something I'm disassembling onto the floor as I turn round to get something.. yes I will definitely do this, thank you!

author: KL27x
author_rank: Super Contributor
timestamp: May 19, 2020, 09:45:47 pm
content: If it's rosin flux, it's more than likely a standard formula. It's an actual specification that goes back to the 70's or something. If you want to be sure, try to make sure it is designated byR plain rosin, no organic acids addedRMA mildly activated by addition of some organic acidsRA maximum strength, by addition of organic acids.There are some paste fluxes that are called rosin flux. I don't think that meets a standard of any sort, but maybe I'm wrong. As long as you use a "real" rosin flux, sometimes it might be called "mil-spec," it will be safe to leave on most low voltage circuit boards. Old TV's are slathered with rosin flux residue, and if not for being obsolete, many would more of these would still be in service, today.OTOH, stuff that is called "no clean?" Well, that can mean a lot of things. Many of them are basically identical to rosin fluxes, but some are for special purposes and will not be safe to leave on there unless used exactly as proscribed.I don't have a dog in the fight, either way, but I use rosin flux because it's a "known quantity" without having to remember a part number. And because it's standard, it is dirt cheap. MG chemicals sells it in smaller bottles. But any pen called rosin flux and is not shipped from China (if a pen is what you prefer) will be fine. China sellers stock all kinds of flux with nitric acid and other weird stuff in it. They're usually upfront about the contents, but if you want to be sure, just buy it from somewhere with more stringent controls. Same goes for no cleans. If it comes in a pen from a repuatable source, it's most likely appropriate for hand-soldering/rework. If you want more info, check the datasheet; the ones you want to avoid for hand-soldering are sometimes called "low solids."A lot of people are sold into specific "no cleans," because of salespeople. They are told a bunch of features of various no cleans in such a way that they come away with the impression that it's better. The most appropriate no cleans for hand soldering are no different than rosin fluxes, other than they might be less brown/yellow. There are other no cleans that are used for manufacturing, because they leave no visible residue and won't affect ICSP pads or the like. But they have to be used in the right quantities and have to be heated correctly. The residue of these is actually conductive, but there's so little of this residue (when used correctly!) that it doesn't matter for most things. Some people think no clean is automatically better because they combine the best features of different no-cleans in their own mind. It doesn't work like that. Well, AFAIK. Maybe there's been a real advance in the technology in the past 10 or 20 years that I missed. And/or maybe the industry has developed new ways to overcomplicate a simple thing.

author: enjoy.cowboy
author_rank: Contributor
timestamp: May 20, 2020, 11:11:07 am
content: What about cleaning the flux? What is the proper way to do it? Isopropanol and rub it off with a q-tip?

author: Zero999
author_rank: Super Contributor
timestamp: May 20, 2020, 03:53:27 pm
content: Flux: what's that? Nasty sticky stuff!I use rosin cored solder, without any additional flux most of the time. Some people like to pile flux on, especially when soldering fine pitch surface mount components, but I find it makes a mess and is difficult to clean. Solder paste and preferably a reflow oven is the correct way to solder fine pitch SMT devices. Relatively large SMT parts such as SOIC, 805 and SOT-23 are easy to solder with ordinary rosin cored solder.The only time I use extra flux is when I have to solder something very big or very badly oxidised. I can be cleared with deionised water and alcohol.

author: KingSolomon
author_rank: Contributor
timestamp: May 20, 2020, 05:32:25 pm
content: Quote from: enjoy.cowboy on May 20, 2020, 11:11:07 amAlcohol and a tooth brush or q-tips will work well. If you really want to be professional, you could purchase an ultrasonic cleaner like this:https://www.amazon.com/VEVOR-Ultrasonic-Cleaner-Stainless-Professional/dp/B083KBN9M5/ref=sr_1_7?dchild=1&keywords=ultrasonic+cleaner&qid=1589995558&sr=8-7.This website has some more information:https://www.digikey.com/en/maker/blogs/cleaning-pcbs

author: Ian.M
author_rank: Super Contributor
timestamp: May 20, 2020, 07:53:39 pm
content: Caution: RMA and RA fluxes can leave a residue of salts that aren't alcohol soluble. To avoid objectionable streaking and the risk of increasing leakage current as the residue adsorbs humidity, you need to keep the board wet with alcohol while scrubbing off the flux and rinse with enough alcohol for it to flow off the edge of the board, or be blotted up with a lint-free wipe, carrying off the suspended salts. If you still have difficulty with streaking and leakage current, after the initial alcohol cleaning to remove the flux resins, you may need to wash with hot water and a little detergent, then rinse, then rinse again with distilled/deioniosed water, followed by shaking off as much water as possible, and rapidly drying the board (which may be aided by a final alcohol rinse to remove more of the water and form an azeotrope with the little that is left).

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2020, 07:58:10 pm
content: Quote from: enjoy.cowboy on May 20, 2020, 11:11:07 amFlux thinner is often 50/50 isopropanol and toluene but for rework where I do not have to worry about plastic parts, I use acetone or lacquer thinner with swabs or tissues.

author: enjoy.cowboy
author_rank: Contributor
timestamp: May 20, 2020, 09:48:19 pm
content: Quote from: Ian.M on May 20, 2020, 07:53:39 pmthe specific flux seems to not form that yellow crust after using, especially because its grey. I've seen some people run pcbs under a tap, and thought it was a bad idea

author: xavier60
author_rank: Super Contributor
timestamp: May 21, 2020, 05:09:56 am
content: For anyone considering trying an RMA-223 labeled flux on ebay, most are fakes, usually colored petroleum jelly with no activity at all.If nothing has changed since my last purchase, this one actually behaves like an RMA flux. I'm not claiming that it's genuine.It becomes conductive when heated, then non-conductive again when it cools. It also thickens into a soluble gum after being heated.https://www.ebay.com.au/itm/AMTECH-Soldering-Flux-Paste-RMA-223-TPF-UV-10CC-USA-Free-Needle/262244648798?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649