Seeking solder/flux recommendation - Page 1
Forum section: electronics / general

author: Involute
author_rank: Contributor
timestamp: December 27, 2016, 01:15:54 am
content: My standard soldering "recipe" is Kester 245 flux cored solder with Kester 951 liquid flux. Most of my work is SMD by hand (little through-hole, no reflow). My goal is as pristine a PCB as possible after soldering, with as little post-processing as possible. Nevertheless, while Kester describes the results of 245 as "visually acceptable," there's still plenty of residue (either from the 245 or 951), most of which comes off with some mild isopropyl scrubbing with an anti-static brush. Unless I put in a lot of effort, there's still residue visible. Just wondering if someone here has a better recipe/process. Thanks.

author: elecman14
author_rank: Unknown
timestamp: December 28, 2016, 11:13:08 pm
content: You could look for a flux that requires cleaning. They generally are more soluble than a no clean variety. You also could look into flux cleaner.Quote from: Involute on December 27, 2016, 01:15:54 amThis probably refers to the IPC/J-STD standards. Since you are buying name brand solder and flux there probably is little risk from ionic contamination if you leave the board "dirty".

author: tautech
author_rank: Super Contributor
timestamp: December 28, 2016, 11:18:09 pm
content: Quote from: Involute on December 27, 2016, 01:15:54 amWhere are you: no flag or location in your profile.You missed some free flux yesterday:https://www.eevblog.com/forum/chat/auckland-beer-and-flux-xmas-get-together/

author: KL27x
author_rank: Super Contributor
timestamp: December 30, 2016, 08:49:23 pm
content: Try using 91% iso for the extra water content. Some of the ionic residue (the white powdery stuff) won't dissolve in pure alcohol. Hot water and dish soap might work even better, followed by 99% iso to dry the boards.If low solids no clean is not esthetically clean enough for you, then there's not much point in using it. The residue is no easier to clean than rosin flux; in some cases it is much more difficult to remove. The whole point of no clean is to not clean it. Try using less additional liquid flux, maybe, and see if it's not clean enough without any additional post-processing?Some PCB manufacturers use relatively expensive water-based post-cleaning for no clean residue, simply because they can use no clean flux with no post processing to satisfy most of their customers... and the extra time/expense of cleaning the few who require it ends up less expensive than changing a bunch of parameters to use different flux/solder. But if you need board to be post-cleaned all the tie, anyway, maybe just use rosin flux?I could be wrong, but the way I understand it, rosin flux will create just as much or more of the ionic contaminants in the residue. But instead of drying/hardening directly in the crevices of the board, it is bound up in rosin. The rosin is easily dissolved in solvent, so most of this ionic (white stuff) is easily washed away with solvent, and what little is left directly on the board can be largely removed in the same process with just a little water in the solvent. So 91-95% alcohol cleans rosin flux relatively thoroughly/easily with less mechanical agitation/ultrasonic/detergents needed compared to low solids no clean flux.

author: trampas
author_rank: Regular Contributor
timestamp: March 01, 2017, 12:36:08 pm
content: My experience with no clean flux is that it is no clean as long as it is ran through reflow oven and heated up to burn off.I have used 951 flux for years and had boards returned with pins acid etched off board with white residue where 951 was used. The issue was I was hand assembling boards and not cleaning afterwards, or baking to clean.I have tried various means of cleaning boards, ultrasonic, flux off, etc. The best I found to date is a tooth brush and dish soap, followed by a rinse with water and air dry. I worry about flux under pins and such so I am still looking for a better method if anyone has one.Also can others confirm that no clean is only "no clean" if baked.Thanks

author: HHaase
author_rank: Regular Contributor
timestamp: March 01, 2017, 02:03:49 pm
content: Involute:You are unfortunately stuck with incompatible criteria there, or at least you'll need to compromise on which one is more important. You're using no-clean flux while looking for pristine boards. The thing is that a "no-clean" flux essentially means it's safe to leave on the board without cleaning (assuming it was activated fully). However, as you notice, it still leaves residue which can be difficult to clean off easily. The main reason people use no-cleans is so they can just push things through without ANY cleaning process as the residue is non-corrosive. Doesn't help that you're hand soldering too. This tends to leave more flux residue than a properly controlled spray fluxer in a wave solder machine, which is what the 951 flux is designed for. No-cleans are just difficult to get to the standard you're looking for.If having pristine PCB's is your primary focus you may be better served by moving to a water soluble process, both in flux and solder. Residues from water-soluble rinse away with just hot water. They also tend to have better solder performance than no-cleans too. But it's not ALL good news, there's some serious drawbacks too. The caveat is that you MUST get those residues off completely, and within a short time frame, as the flux is otherwise highly corrosive. Getting those residues out from under IC's, particularly BGA's and low profile leadless parts, can be difficult. Could also consider going to an RMA flux instead of a no-clean, which would be easier to clean than what you have now but still needs to be cleaned a bit aggressively with IPA if you want pristine boards.Another option would be a pneumatic dispenser and paste solder, instead of wire and flux, for your surface mount work. This tends to leave much less residue even if you're using no-clean paste. It's actually pretty easy to use this kind of setup for hand-assembly of surface mount parts. I find it easier than wire solder to be honest as you don't have to worry about holding as many things at once. Since you can control paste volume with a good dispenser you'll also end up with nicer looking solder joints since you won't be over applying solder like it's so easy to do with wire solder.Lots of ways to handle it. Just a matter of picking what your priority is and finding the right process.-Hans

author: KL27x
author_rank: Super Contributor
timestamp: March 04, 2017, 06:44:43 pm
content: There's the saying that you cannot use too much flux. If you are using low solids "no clean," and you plan to not do any post-cleaning, this is not true. This type of flux is so designated because it does not have much, or in some cases ANY, resin with which to bind up ionic compounds thus rendering them inert. Instead, they aim to be so inactive that they don't make enough ionic compounds to cause a problem. Of course, this requires a fresh, clean board to begin with, to have any chance of actually working as intended, and it is fairly difficult to get any consistency if hand soldering. If you are soldering by hand, it is probably much easier to use rosin flux which fairly easily washes off with 91-95% alcohol and a brush.Rosin flux will create plenty of ionic residue. The longer you leave it on before soldering, the more time for the copper to RE-oxidize and dissolve again and again. And the more green ionic residues you will make. But eventually you either solder the joint or the flux dries out and all solvent evaporates. The rosin that is left turns into a waterproof solid that encapsulates the ionic waste products. Unless you expose the board to solvents, this residue is inert. And it is (IMO) easy to remove if desired.There are also some "no cleans" which act much like rosin flux. They have a synthetic resin binder. The main improvement is that the resin is less yellow/brown, more clear. But it will be similar volume of solids as rosin flux. Perhaps this is the kind you are already using. In this case changing to rosin flux may make no difference. In my experience with the synthetic resin no cleans and the purified rosin fluxes, they are essentially same as rosin flux in terms of ease of cleaning off residue with alcohol. E.g. AIM "glow core" and Indium purified rosin flux core solders. So if this is what you're using, you might want to try the "true" "no cleans." The residue is certainly less, but it IS conductive, without question. And it IS trickier to remove, if you decide it is not clean enough.

author: richardlawson1489
author_rank: Regular Contributor
timestamp: March 09, 2017, 04:52:50 am
content: So, Cleaning Flux would be a better solution?

author: KL27x
author_rank: Super Contributor
timestamp: March 09, 2017, 05:03:21 am
content: Yep. Cleaning Flux for the win.If you want a clean-looking board without any cleaning, no clean might be your ticket.If you can't abide trace conductivity of no clean, and you don't care about esthetics, rosin all the way.If you want a clean-looking board without any trace conductivity, then .... you need to clean the board. And it is easier to clean Cleaning Flux (rosin) than no clean. I have heard that the only reason manufacturers ever clean no-clean flux is because the majority of their orders don't need to be cleaned. And it would cost more to change the paste and change reflow settings for the minority. So they spend more time/money on cleaning no clean rather than using rosin flux for these orders.