Soldering flux advice - Page 1
Forum section: electronics / beginners

author: gentrym76
author_rank: Newbie
timestamp: June 15, 2025, 01:39:45 am
content: want to step up my soldering a notch. I need some flux again. I was using some brush on yellow paste stuff that I don't remember what it was. Doesn't matter.My requirements:I'm doing through hole, repairing SMD, reworking, using braid. I also will be cleaning after completion with rubbing alcohol (unless someone has something better to suggest). I want to complete the work then remove residue.I use kester 245 solder Sn63p37 no clean. My irons are a weller wes51 and a zeny 852d+. I use a plunger type desoldering pump with the rework heat gun.So I was considering some form of either pen or syringe type.And I need some braid. Also want to buy from cont. USA.What brand and type flux? And what applicator does everyone recommend

author: xavier60
author_rank: Super Contributor
timestamp: June 15, 2025, 03:16:02 am
content: I have settled on Caig RSF-R80 for SMD and general purpose. It works as well as any other RMA flux I have used. The manufacture is known for its Deoxit products. Also, it's available to me locally from Jaycar. Noticed recently that Digikey also stock it.I apply it with a 5mL rubber-less syringe and applicator needle.https://vi.aliexpress.com/item/1005006768606634.html?

author: gentrym76
author_rank: Newbie
timestamp: June 15, 2025, 07:12:57 am
content: Lol. That's what I've been using already.Hopefully I can find my can after the move. I used that stuff on my guitar amp build but I couldn't remember the name of it.Its my understanding that the no clean solder should be left alone, even if using the above flux?I think I'm gonna change to a flux free solder. Sucks cause I have a lot of the no clean left.

author: xavier60
author_rank: Super Contributor
timestamp: June 15, 2025, 07:29:57 am
content: Quote from: gentrym76 on June 15, 2025, 07:12:57 amI did use a "no clean" flux some years ago and found that it caused some electrical leakage which required the PCB to be thoroughly cleaned anyway. I understand from what others have said that there will be no need to clean if the flux has all been heated during the soldering process to fully deplete its chemical activity.I have been using the Caig RSF-R80 flux long enough to trust that if some residue remains under SMD parts, it's not likely to cause problems.

author: xavier60
author_rank: Super Contributor
timestamp: June 15, 2025, 07:47:44 am
content: Using a solder that contains No Clean flux might not be a problem. I check for suspect leakage by putting solder blobs on adjacent Veroboard tracks, then check with an insulation tester.

author: Warhawk
author_rank: Frequent Contributor
timestamp: June 15, 2025, 08:01:21 am
content: Quote from: gentrym76 on June 15, 2025, 01:39:45 amHi, I use Rosin based flux for hand soldering. FL 22 in my case. For hot-air type of soldering I sometimes use another one (don't remember the brand) that is white. and not rosin based.I always clean all kind of flux with flux remover / IPA.PS: Louis R. has been using this:https://store.rossmanngroup.com/amtech-nc-559-v2-30-cc-16160.html

author: Ian.M
author_rank: Super Contributor
timestamp: June 15, 2025, 11:12:11 am
content: The risk is incompatible fluxes can form more hard to remove polymerised residue than usual. Kester 245 flux core is classified as ROL0, so any rosin fluxwillshouldbe compatible. There shouldn't be any extra problems from cleaning this 'no clean' flux core as its a normal fairly low activity rosin flux and as core, its guaranteed that all the activators in it have been heated to soldering temperature. You certainly don't need to swap to a flux-free solder, and doing so will likely cause you far more problems!I wouldn't necessarily go with Louis Rossman's flux preference and methodology - he is doing fine pitch SMD rework on often badly compromised boards commercially (i.e. as fast as possible), and puts every board through an ultrasonic cleaner afterwards, so flooding the area with gel flux is his habit as it is quicker than a more topical application, and he isn't manually cleaning it.Edit: Tooki made a valid point about compatibility below.

author: tooki
author_rank: Super Contributor
timestamp: June 15, 2025, 11:23:22 am
content: Quote from: xavier60 on June 15, 2025, 03:16:02 amPlease truncate to the bare minimum any AliExpress links; aside from the rest being unnecessary, the forum software chokes on things like the quotation marks in the URL, resulting in the weirdness seen here.This is all you need:https://www.aliexpress.com/item/1005006768606634.html

author: CrazyTiger
author_rank: Regular Contributor
timestamp: June 15, 2025, 11:41:06 am
content: I use SMD291 from Chip Quik. Digikey and mouser sells them. I found it creates very little fume and smoke.In the past I found some flux caused allergic reactions when I inhaled the fume.The SMD291 and the SMD291-NL is advertised as "Has a pleasant odor" on the datasheet.

author: tooki
author_rank: Super Contributor
timestamp: June 15, 2025, 11:48:47 am
content: I also like the SMD291NL.

author: tooki
author_rank: Super Contributor
timestamp: June 15, 2025, 12:16:07 pm
content: Quote from: gentrym76 on June 15, 2025, 07:12:57 amAll bets are off once you mix fluxes; it’s probably fine, but there are no guarantees. Solder manufacturers sometimes expressly state which external fluxes to combine with a particular flux core, as they have known compatibility.For example, in the Kester 245datasheet, they expressly say to use Kester 951 or 952-D6 liquid fluxes with it.(The 245product pagealso lists those as “process compatible” products.)Even within a particular flux classification (like ROL0), compatibility is not guaranteed, since different fluxes can use different activators, solvents, and additives. So it’s probably better to clean.Fundamentally speaking, no-clean flux is designed for its residues to be OK to leave on for most applications, when used during normal industrial manufacturing. (In particular, tacky flux is fundamentally designed for careful, minimum-amount dispensing (most often as the vehicle in solder paste), followed by reflow soldering. The safe-to-leave-on properties do NOT apply when the flux is used for hand soldering, where incompletely heated flux goes all over the place, in particular when large amounts of flux are applied.)Quote from: gentrym76 on June 15, 2025, 07:12:57 amThat would be a terrible idea, since you can’t tip your soldering iron tip with no-flux solder.

author: gentrym76
author_rank: Newbie
timestamp: June 15, 2025, 01:48:41 pm
content: Hey, thanks for all the advice!I'll can the flux free solder idea.I originally considered kester 291. I'll be getting a flux pen.Again thank you for the help.

author: eugene
author_rank: Frequent Contributor
timestamp: June 15, 2025, 04:33:38 pm
content: I have not tried a variety of different fluxes because what I've been using for decades seems to work really well for me: any solder wire from the Kester 331 line along with Kester 2331-ZX flux, which is available in pens or in gallon jugs. Use plenty of flux and the solder joints are perfect and the flux is effortless to wash off with hot water.I've heard that there are now no-clean fluxes that can be washed with water. I suppose that would be ideal if it works as well as the 331, but honestly I just don't bother with no-clean unless it's something that can't be washed. What I've been using works so well for me that I have no interest in trying anything different.

author: radiolistener
author_rank: Super Contributor
timestamp: June 15, 2025, 08:21:37 pm
content: Quote from: tooki on June 15, 2025, 11:23:22 amIt's unusual to see this coming from you, but I completely agree - those long URLs with a bunch of tracking id's are inconvenient, as they have to be manually trimmed before opening.

author: digger
author_rank: Regular Contributor
timestamp: June 15, 2025, 11:56:43 pm
content: the "by-the-book" answer depends on the flux in your solder. kester 245 is ROL0, so you will want to use a type RO flux. (even more by-the-book is to use a flux that kester says is compat with 245)i looked into it a lot and settled on STIRRI V3-TF which is type RO and available on amazon. i like it and can recommend it. it comes with its own syringe applicator and includes a couple of blunt syringe tips. i'd get the 10g one ($18). it is the successor to the excellent amtech v2-tf.interestingly, i am seeing now that they have recently released V4-TF and V5-TF. if i were you, i would probably just get V5-TF (never tried it), but V3-TF is for sure great. more info in this post:https://www.reddit.com/r/soldering/comments/1jneowe/comment/mow5s9t/i am not a fan of flux pens. imo they are needlessly messy.the main things that distinguish braid (aside from the overall manufacturing quality) are the fineness of the strands and the type of flux present (if any). eventually, the flux on the braid does go bad because it's all exposed to atmosphere and has a lot of surface area, so for occasional hobbyist use, getting plain braid without flux might be a decent way to go. you can apply your own flux on-demand, and perhaps it will have a longer shelf life that way which could matter if it takes you forever to get through a thing of braid. in any case, i suggest one with super fine strands.i am personally working through a thing of MG Chemicals 427 Desoldering Braid #5 Fine Braid Super Wick with RMA Flux, 5' Length x 0.125" Width, Brown. i got it from amazon and like and recommend it.general infohttps://mgchemicals.com/knowledgebase/what-is-solder-wick/good advice about selection and techniquefwiw, if you ever want to switch to lead-free, i strongly recommend kester K100LD. it's seriously almost as good as Pb to the point where i don't miss it at all for hand soldering.K100LD's available in 275 (rol0) and 48 (rom1). 275's a true no-clean, and 48's a successor to 44 which is a very popular solder core.48 sounds like it's basically no-clean, idk. probably not strictly no-clean but usually fine to not clean it.i have both and find that the gentler flux in 275 is good enough, and it's all i use now.more info: there are silver-free Sn alloys featuring nickel doped with trace amounts of shit like Ge or Bi. different companies have their own special alloys. AIM's got SN100C, kester's got K100LD, felder's got Sn100Ni+. chipquick actually has CQ100Ge. not sure yet which is best, but these alloys flow better and look shinier than SAC305. it's the closest you can get to Pb solder so far.

author: digger
author_rank: Regular Contributor
timestamp: June 16, 2025, 12:23:45 am
content: also, regarding 'no-clean' flux, technically it does not become no-clean until it has been heated to the activation temperature. a no-clean core in solder wire automatically satisfies this just by being melted with your iron. it's always safe to not clean that off.however, if you apply additional flux (like the V3-TF i recommended above), there is some chance that not all of it will get that hot depending on how you're using it. for important shit, you should probably clean it off to be safe. i usually do anyways to satisfy my OCD. i like neat, clean boards.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 16, 2025, 03:44:11 am
content: I can't be the only one with bad experiences of "no clean". Last time I used it, there was enough leakage current to prevent the timer function of a lithium charger IC from timing out, and when I tried to clean it - it got significantly worse until I used an ultrasonic cleaning bath. Amtech NC-559 works just fine for me, and cleans off easily.

author: tooki
author_rank: Super Contributor
timestamp: June 16, 2025, 07:20:42 am
content: Quote from: Buriedcode on June 16, 2025, 03:44:11 amI don’t think you are alone, I’m sure I’ve seen several people on the forum report similar things.Was this leakage current with added no-clean external flux (liquid, tacky, etc), or only with a no-clean flux core solder wire or solder paste?

author: tooki
author_rank: Super Contributor
timestamp: June 16, 2025, 07:33:49 am
content: Quote from: eugene on June 15, 2025, 04:33:38 pmWhat do you do for things where water-soluble flux must not be used, like stranded wire, or parts that cannot be washed?Quote from: eugene on June 15, 2025, 04:33:38 pmI’ve been experimenting with some of these.Chip Quik has water-washable no-clean in both flux core solder wire and in paste flux and solder paste (4300 series). They come off fairly easily in hot water, and nearly instantly in a hot ultrasonic bath. My hunch is that it’s basically a “normal” no-clean flux with saponifier built in, because when hand-washing with a brush, it actually lathers a bit.SRA also has a line of water-washable no-clean solder wire. I am not impressed by it; the residues don’t seem to come off easily, even in hot ultrasonic. I ultimately had to resort to using solvent. (I almost wonder if they didn’t accidentally mislabel what is actually regular no-clean solder wire.)On the other hand, SRA has an interesting water-washablecleaningflux, #441, which not only works well as a flux, but when heated, actually takes existing non-water-washable flux residues and makes them water-washable. It’s fantastic for getting rid of stubborn no-clean residues.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 17, 2025, 01:25:13 am
content: Quote from: tooki on June 16, 2025, 07:20:42 amThis was external - liquid flux, in a pen format - the firstand lasttime I've used the pens. Now, this "sample of one" doesn't means that they are all like that, or that the pens are useless, it's just this particular case it was unpleasant to work with, and put me off both "no clean" and pens.

author: tooki
author_rank: Super Contributor
timestamp: June 17, 2025, 07:18:02 am
content: Quote from: Buriedcode on June 17, 2025, 01:25:13 amGotcha!I don’t hate flux pens, but they aren’t my absolute favorite, either. For liquid fluxes, I prefer the Bonkote brush pens. I usually use liquid flux when soldering virgin joints and I just want to give the fluxing action a little boost. But I have found that as of lately, I’ve been tending to use tacky flux (SMD291NL) transferred to a thin syringe, with an extremely fine dispensing needle to apply just a tiny amount of flux. (That doesn’t work with fluxes that are too thick, or which tend to clump up, like MG Chems’ no-clean tacky flux.)About no-clean liquid fluxes: because “no-clean” doesn’t really mean anything other than “low-residue”, the formulations vary wildly. While most no-clean flux cores and tacky fluxes tend to be rosin- or resin-based, no-clean liquid fluxes can be rosin- or resin-based, just using a much lower solids concentration, or they can be entirely organic acid based, which seems a bit more common. I haven’t tried too many different liquid fluxes, but so far, I have preferred the ones based on rosin or resin. But I prefer traditional high-solids liquid RMA fluxes over no-clean.