RMA/RA flux compatability - Page 1
Forum section: electronics / beginners

author: int10h
author_rank: Newbie
timestamp: October 11, 2017, 01:19:19 am
content: Hi there, first post here.I have some Kester 44 rosin-cored solder (they seem to advertise it as somewhere between RMA and RA flux). I noticed in the datasheet that Kester specifically lists their 186 and NF372-TB fluxes as compatible with 44, but they say that external flux should only be used as a "last resort."I want to be able to use external flux, and I have some RMA and RA fluxes lying around from various brands. Are there any specific considerations or caveats associated with mixing fluxes in this way (i.e, using off-brand RMA or RA flux instead of 186 or NF372-TB)?By the way, I should point out that I don't have any specific requirements; just general tinkering. I'm pretty sure I could get away with mixing fluxes and be just fine. However, I think it's useful to understand different processes and the tradeoffs between them. For example, I don't use NASA's soldering processes (cleaning everything with alcohol, etc.) all the time, but I still find it helpful to understand the "right way" of doing things.

author: nanofrog
author_rank: Super Contributor
timestamp: October 11, 2017, 01:40:51 am
content: FWIW, Kester 44 uses an RA paste contained within the wire core. The liquid equivalent is 1544 (RA). These can be mixed with R, RMA, or RA from other vendors, such as MG Chemicals' 835 (RA).Kester 186 or 186-18 are both RMA that borders on RA, with the latter version containing solids down to 18% (vs. 36% for standard 186).The biggest issue is getting good quality liquid flux at an affordable price. Kester for example, sells their stuff in 1 gal. containers or larger generally speaking. In some cases however, they also offer a disposable pen. The downside being is disposables are too expensive IME.MG Chemicals 835is sold in 125ml & 1L sizes for example, which are far more hobbyist friendly.CML Supplywill break down some Kester products into hobbyist/small repair shop sizes, but it's only certain products ATM (i.e. Easy Paste 256 <63/37>, RF741, 951, and 186).There's some eBay sellers that also break down larger containers for sale. Even CML Supply sells via this route (though they do not offer international shipping if through eBay; international shipping is just through their own website).

author: int10h
author_rank: Newbie
timestamp: October 11, 2017, 02:00:19 am
content: Thanks for the info and for pointing out that vendor.The stuff I have currently is an RMA pen sold by SRA Soldering Products as #99-20 (SRA appears to be an importer; they don't have datasheets for their stuff), and the rosin paste tub from Radio Shack that used to be really common before Radio Shack went belly-up.It sounds like from what you're saying, R, RMA and RA fluxes are generally compatible. I guess then the main consideration is if you want to clean, you've got to use the strongest cleaning chemical among the fluxes you're using. For example, SRA advertises #99-20 as being cleanable w/ isopropyl, but Kester specifically says that a strong cleaner designed for no-clean flux is required for 44. So, if I used these together and wanted to clean, I guess I'd have to use something like MG Chemicals 4140, which should take care of the 44 and have no problem with the SRA stuff.

author: nanofrog
author_rank: Super Contributor
timestamp: October 11, 2017, 02:25:50 am
content: Quote from: int10h on October 11, 2017, 02:00:19 amAny Rosin based flux will be compatible with another, so there's nothing to worry about there.Some no-clean variants will also mix with Rosin fluxes quite well; just be sure to read the datasheet on the no-clean first. What ever you do however, Do Not Mix rosin based or no-clean formulations with water soluble flux; they're just not compatible.As per cleaning, IPA, ethanol, methanol, acetone, xylene, toluene, and methyl-ethyl ketone (MEK) are common solvents used in cleaning. Some components may not tolerate all of these well though, such as plastics.In my case, I primarily use denatured alcohol (brand I use is a 50/50 mix of ethanol & methanol). If it's too stubborn, I'll mix in ~ 20% of xylene or acetone by volume. MEK I use straight and spot clean with it (for really nasty looking joints). I keep IPA and distilled water around as well.Lots of threads on this that will offer up some additional details, so worth a search IMHO.

author: KL27x
author_rank: Super Contributor
timestamp: October 11, 2017, 09:32:18 pm
content: QuoteBetween rosin fluxes, they will all clean the same way.There are many considerations for using additional flux, but depending on how you are soldering, many of them take care of themselves.There are some things that can happen if you use liquid flux inappropriately. Liquid flux is active. Until all the solvent has dried/boiled away, the flux is active/acidic AND the solvent allows oxygen exchange to the traces/pads/pins as long as it keeps the flux in solution. So this can cause a problem in a number of ways.Ideally, you want the flux to remove the existing metal oxides, just once.* But if you fluxed parts/pads with a pool of liquid flux and leave it for a while before soldering, you may come back and find the flux has turned green. The pads/traces have eroded a bit more than necessary. The already clean pads/pins will slowly reoxidize and redissolve until the solvent has evaporated.If you do not boil off enough of the solvent while making the joint, ditto. You could leave the flux "wet" enough to be active. And if leaving a thick enough layer/pool, it can take awhile to evaporate. You can end up with more tin salts in your flux residue, making it is more conductive than it could have been. Which if you're going to wash it off, anyway, doesn't much matter.But if you solder more or less immediately and you provide enough heat to "dry out" the flux, the issues of using too much rosin flux are very minimal, other than bulk/esthetics of the increased amount of residue.*But you also want enough flux to dissolve the oxides that will form on the surface of the solder bead while flowing the joint. These oxides kill the surface tension. Keeping the solder free from oxides is how you get nice filets and smooth joints without dragging spikes/blobs and making bridges when you remove the iron.All that said, it's very difficult to use "too much" rosin flux. Esp if you are going to clean the board, anyway. For low temp app, I rarely find it necessary to clean rosin flux residue.I mix synthetic resin flux and rosin fluxes with no problem. They work essentially the same way. The ones I watch out for (and generally avoid when possible) are "low solids no cleans" "water soluble no cleans."

author: Spooky558
author_rank: Newbie
timestamp: November 14, 2017, 10:11:55 am
content: Quote from: int10h on October 11, 2017, 01:19:19 amHello. I'm new here, first post, though I've been browsing the forum for a few days. I've got a similar question, didn't know whether to start a new post or post here, but it's along the same lines, so I'm posting here.I want to get into soldering, and I'm confused about flux. What type of flux do I need to use? I'm looking for a liquid flux (I think lol) but I have no idea exactly what type. I'm going to be using 60/40 solder. I don't want to really use a pen, I think a bottle with a brush might be my best option, but I'm lost. I've been looking for days online, and can't make a decision what's going to be best.I'm in the UK, I don't want to spend lots on flux, so can anyone recommend something that will be suitable for me, but cost less than £5 ideally?Thanks.Peter

author: Ian.M
author_rank: Super Contributor
timestamp: November 14, 2017, 11:35:38 am
content: Personally I prefer liquid Rosin (R) flux. The brand doesn't matter. As its unactivated its minimally corrosive even before it dries and cant interact adversely with the activators in any other Rosin flux that may be present. Used sparingly, you can treat it as no-clean. I don't mind cleaning pads with a fibreglass pencil and through-hole leads with a green pan-scourer, before soldering if working with N.O.S parts, so unactivated Rosin suits me well enough.However a mildly activated Rosin (RMA) flux may well suit you better if your parts and pads are even slightly tarnished, or if you are using old stock SMD parts. You should be able to get some within your budget. Search EBAY for:RMA liquid flux 50mland pick one that's in a brush bottle.The other DIY option is to get a 20g tin of solid Rosin flux or get 20g of lump Rosin, whichever works out cheapest, then powder it and add it to 100ml of IPA (Isopropyl alcohol, 95% or better) in a glass jar with a good lid seal, shake daily for a couple of days, let it stand so any impurities separate out then decant enough to work with into a clean brush bottle, (again with a cap that seals well), leaving any milky layer at the bottom behind. That gives you an approximately 20% dissolved solids Rosin (R) flux comparable to many commercial products. Its not cost-effective unless you already have IPA (which any electronics workshop should have), but if the alternative is buying 1L of brand name flux and possibly paying for courier delivery or waiting six weeks for the slow boat from China, it may be worth the hassle.You can also get alcohol resistant new empty paint pens for a reasonable price annd load them with the liquid flux of your choice. Don't use metal bodied ones with any activated flux.

author: Spooky558
author_rank: Newbie
timestamp: November 14, 2017, 11:01:18 pm
content: Thanks for the help and advice Ian.M, much appreciated.