What Solder Flux Are You Using?


At the moment, I am hand-soldering prototypes. I have an aging bottle of flux that seems to have turned into the consistency of pancake syrup.

I’m going to purchase Kester 186 RMA for general purpose prototype work and re-work.

However, I am wondering if Kester 959T might be better for manufacturing production on the few parts that will need to be hand soldered.


I really like Kester 951 (similar to 959 as far as I can tell from the datasheets). They are both clear no clean fluxes.

I also use MG 8341, which is a paste. Not sure if I like it more than the kester liquid yet. They both help with wetting and make nice solder joints.

They are both no clean, but I prefer to clean up any residue with an acetone / 99% isopropyl mix.


Amtech NC-559-V2-TF Tacky Flux

Any other too liquid will burn way too fast for hand soldering
in particular if using LEAD-FREE… and mostly LEAD-FREE is
really crappy stuff.

Which BTW … if not really properly “profiled” will most likely crack
tomorrow next week or next month…

Amtech NC-559-V2-TF Tacky Flux gives you time enough
for hand soldering



I use MG 8341 also… though I might have to try that Amtech tacky flux.


A few drops of isopropanol (or, if you can’t get it easily, 91% Isopropyl alcohol from the pharmacy) may revive your flux. Don’t use the 70% “Rubbing Alcohol” from the grocery store “Beauty Supply” section.

I don’t recall the brand or formula for liquid flux they tell me to use at work. I only use it for tinning wires in the solder pot. @Sprig has his feces amalgamated but others stumbling across this thread need a reminder to use ONLY rosin-based fluxes or others intentionally formulated for electronics - NEVER the zinc-chloride stuff from the hardware store or hobby shop.

I don’t recall when I last used a standalone flux on a PCB assembly. For thru-hole parts it’s just a desktop, regulated, soldering iron (Hakko FX-888) and thin-gauge, flux-core, wire solder. If a large solder joint doesn’t want to flow smoothly I grab the soldering iron from the next worktable (and maybe a co-worker) and use two irons on the joint. For surface-mount parts I usually use the same solder paste that would be used in a reflow process. " CML Supply Paste Syringe " (and other vendors) make it available in small syringes. With solder paste under the part you only need to touch the smallest tip of your iron to the pad, then wait a second for the whole drop of paste to melt and flow. I know the paste advertises a shelf life of 6 months but the two-year old syringe I keep in the refrigerator is still quite usable for manual rework or small assemblies. You can get syringes of paste that fit whatever process requirements you have - leaded, lead free, hi-temp, no-clean, solvent clean, etc.

If I’m doing a whole PCB assembly of SMT I order a stencil from OSH Stencils, apply paste and place components, and do the board in my “Table-top Reflow Chamber” (also known as “Electric Skillet from the Second-Hand Store”).



One of my friends swears by having the extra flux and will argue your ears off until you conceed; 'cause he won’t. I agree that extra flux is not always needed. I figure that if Kester engineers have 3.3% rosin flux in their solder, then 100% more flux is probably not needed.

However, with small stuff, I have seen the extra flux help with the thermal capillary action and it seems to reduce the chances of solder bridging.

I just, tonight, purchased a Hakko FX-888D to replace my very old Radio Shack iron. I had hacked the RS iron to accept an aftermarket tip and it was quite satisfactory for most tasks. Apparently something in the RS iron went Tango Uniform as of yesterday.

Despite the pancake syrup texture of the flux I have, the new iron made all of the difference. Only thing that irks me about the iron, and I knew it going in, the control interface is not at all intuitive.

@dchisholm Thanks for pointing out the link to the solder paste. I have not tried that yet. My prior employers had strict procedures for rework and all the repair operators used wire solder even for SMD re-work. It takes a little bit to get used to moving SMD parts due to the “sticktion” of the molten solder, but it can be made to work with a little practice.


Pick up a handful of tips for the Hakko while you’re at it. (Unless your wife informs you that her NEXT husband won’t blow the budget on electronics-related stuff.) I have a couple sizes of chisel-pointed tips - I think one was original with the iron; a slim, very pointed, conical tip; and a long, slim, beveled tip that gets used on a lot of SMD stuff.


I think I can make the original equipment tip work for everything that I am currently doing. However, since I’ve been soldering for as many years as I have, I might wager that I could solder anything with one of those old design hand held 45W pistol grip models… (but my eyes aren’t 20 years old anymore, they are a tad bit older).

Do you have a part number for the tip mentioned in the quote?


I don’t have it here at home, and Hakko Soldering Tips shows no fewer than 14 (!!) variations of that shape, but I think it’s their model number T18-CF15 or T18-CF2.