SMD soldering flux? - Page 1
Forum section: electronics / general

author: unknownparticle
author_rank: Frequent Contributor
timestamp: December 16, 2019, 04:09:49 pm
content: My electronics career began and finished (due to a career change) way before SMD technology. I got back into electronics, as a hobby, about 10 years ago but have only really worked on vintage through hole, and even tag board stuff in that time. However, SMD are now impossible to avoid so I am equipping myself to work on such equipment and devices.My question is, what is the best flux to use for new assembly and rework? Louis Rossmann seems to really like Amtech, what is the concensus on that and how about other brands?

author: georges80
author_rank: Frequent Contributor
timestamp: December 16, 2019, 04:33:47 pm
content: If you can keep your designs to components on a single side, then the flux already in (decent) solder paste is more than adequate for soldering (on a temp controlled hotplate) typical r's and c's and ic's in soic/quad flatpack and qfn type packages.I'd recommend some kind of air extraction/filter to keep flux fumes out of your face and preferably heading out a window.I use a hotplate and also a hot air tool.cheers,george.

author: FuzzyOtter
author_rank: Contributor
timestamp: December 16, 2019, 04:34:51 pm
content: I am a fan of MG Chemicals no-clean paste flux (part number 8341). It behaves nicely during and after soldering and is tacky enough to hold components to the board while you tack down a pin or two. I have also used their liquid rosin flux (part number 835) and it performs very well, but is a nightmare to clean, and if you leave it for too long, it turns rock hard. It also gives off a strong smell that sticks to your clothing.I haven't used the AmTech flux, but I'm sure it performs just as well. You're likely to be fine with whatever is easiest for you to get your hands on.

author: m98
author_rank: Frequent Contributor
timestamp: December 16, 2019, 04:48:32 pm
content: I usually use Stannol X32-10I, which leaves only minimal residue. One downside, it comes as a liquid, so you can't use it to hold parts down.

author: unknownparticle
author_rank: Frequent Contributor
timestamp: December 16, 2019, 04:57:59 pm
content: Quote from: georges80 on December 16, 2019, 04:33:47 pmThanks. I'll be discreet soldering, at least initially. So repair work, replacing components and assembling small kits. Can't forsee me needing a hotplate anytime soon, although I will need a hot air tool. Your suggestion to use extraction for flux fumes amused me! When I started in electronics being engulfed in clouds of flux smoke wasn't seen as an issue, actually, I quite like the smell of flux! Although that might be different with newer flux types, my experience has been with good ol' 60/40 lead tin 18 SWG multicore Savbit solder! I got a 2.5 KG drum of the stuff back in the 70's and I'm only half way through it now! I have actually acquired a small bench fan extractor though, although the noise irritates me!

author: tooki
author_rank: Super Contributor
timestamp: December 16, 2019, 10:32:57 pm
content: I also like the MG Chemicals no-clean flux mentioned above.Don’t be afraid of SMD. I also came back to electronics a few years ago after leaving the hobby for over a decade, and I quickly figured out that SMD is not only not as hard as it looks, but in fact is often much faster. (Do not underestimate the amount of time it takes to trim component legs on THT stuff!! And drag soldering an IC is way, way faster than soldering DIP.)

author: Random Model Maker
author_rank: Regular Contributor
timestamp: December 18, 2019, 03:59:00 pm
content: If you plan on hand soldering first, pens like the Stannol one are very convenient and easy to use, although they are more expensive in the long run.I use the Stannol X32 as well, but only on the go, at home, I have Interflux IF6000 and IF8001 respectively, which I apply with a paintbrush.As always with flux, more more is more better.What I can really recommend, is something called "soldering honey" (search for Löthonig). This is actually pure rosin disolved in isopropyl alcohol. It comes as a sticky goo in a little tube. With that you can solder nearly anything, although it's really aggressive, so you need to clean it very well.

author: unknownparticle
author_rank: Frequent Contributor
timestamp: December 18, 2019, 04:08:41 pm
content: Quote from: tooki on December 16, 2019, 10:32:57 pmYeah, I can see what you mean. I think with SMD, for the novice, it's the minute scale of the components, having to work with a microscope and developing a very steady hand that cause apprehension. I love the look of SMD boards, especially when they are done to a high standard,

author: unknownparticle
author_rank: Frequent Contributor
timestamp: December 18, 2019, 04:12:42 pm
content: Quote from: Random Model Maker on December 18, 2019, 03:59:00 pmThankyou for this, just the sort of info I'm looking for. I will be hand soldering initially, so very useful. On the subject of flux removal, is IPA universal or is other stuff needed? I have plenty of IPA as I already use it for cleaning vintage electronics and mechs.

author: tooki
author_rank: Super Contributor
timestamp: December 18, 2019, 04:21:26 pm
content: Quote from: Random Model Maker on December 18, 2019, 03:59:00 pmI actually make that myself! I have a little 10ml dispenser bottle, which I fill about 2/3 with crushed rosin, then fill up with IPA. It can take quite a while to dissolve (less if the bottle is turned constantly), on the order of days. The result is a syrup that works great.I don’t, however, believe it is that aggressive. As a rosin flux without activators, it should actually be quite mild.Quote from: unknownparticle on December 18, 2019, 04:12:42 pmIPA works well, but commercial flux removers do work better in my experience.Note that some no-clean fluxes (none I’ve used myself, just going by what many people have reported) leave residues that are devilishly difficult to dissolve. So if you are the kind of person who likes to remove all flux residues, use rosin fluxes and clean them. If you don’t want to clean, use either rosin or no-clean.

author: m98
author_rank: Frequent Contributor
timestamp: December 18, 2019, 06:49:11 pm
content: If you really need to clean off all residue from a no-clean flux, save the headache of scrubbing in an IPA bath and just use a flux cleaner.

author: ajb
author_rank: Super Contributor
timestamp: December 18, 2019, 11:15:13 pm
content: I like to keep both a no-clean paste flux and a low-solids/no-clean liquid flux on the bench. The paste is great for whole-part rework, especially replacing or touching up fine pitch or leadless parts. I prefer the low-solids/no-clean for spot rework or bodging, especially when having to do multiple rounds, as it doesn't leave a whole bunch of crud that has to be cleaned up. I also tend to use the liquid for tinning wires, as it gets between the strands and helps the solder wick nicely.I tend to use the same MG Chemicals 8341 paste as everyone else in this thread, and for the liquid I've been using Kester 979 which works well but has really unpleasant fumes and so really does want to be used with ventilation.I second the recommendation of a flux cleaner. I have MG Chemicals 4140 on the bench right now which works well, but you'll also want to keep some 99% isopropanol for rinsing. Without a rinse or a good wiping, the flux cleaner will just spread the flux out on the board and leave an ugly white haze.