Soldering flux: the basics?

Thread Starter

rambomhtri

Joined Nov 9, 2015
295
Hi,
I've been soldering for a few years and I've always done it directly with the solder, which has a flux core if I'm not wrong. Nevertheless, I see a lot of people, specially in small surface electronics, use flux all the time. I really don't know what's the purpose of flux. Sure, I've read that it cleans the area and helps to solder, but I haven't had any problem with my solder wire alone.

Anyway, I just ordered a La-Co flux, black canister with red top, very famous. I've used it to solder here and there, and I've not noticed any important improvement. Then today I put it to test and tried to use it to help me solder on a 3.8V battery. You know, batteries normally have a treatment or something that makes it really difficult to solder a cable on them, specially the positive. So, I put a little bit of flux on the positive, heated it up with the soldering tip, I see flux evaporating, then try to solder the wire to the positive, and nothing, not the the slightest improvement over trying it without flux. I cleaned the area, may be you had to clean it completely, although I see that when they solder chips with 20 pins or so, the flux is all around... but nothing, the solder won't stick to the battery.

So, if I really don't notice the difference when soldering is easy, and when the solder won't attach to a surface, the flux don't make any difference, what is it for?

You can explain from experience also why do you use flux, what am I doing wrong, what is it for... may be I can learn the basics.
 

SLK001

Joined Nov 29, 2011
1,448
Adding flux is just extra insurance that you will burn off all the oxides when soldering. Rosin core solder does a great job of removing the oxides - adding extra flux does an even better job.

Day to day soldering of thru hole components and tinning wires, etc, get by just fine with rosin core solder. Surface mounted components, especially with pads on the bottom of the device that can't even be reached with a soldering iron, need the extra flux to ensure a good metallic bond.
 

SamR

Joined Mar 19, 2019
1,060
Flux creates a chemical reaction to neutralize surface oxidation which prevents solder from bonding to the surface. With new clean components and PCBs, the flux core of the solder is usually more than enough. It doesn't really hurt to add it but may not be really needed. I use liquid flux on solder braid when removing components because after a while the surface of the solder does oxidize and it helps the braid to suck up old solder. If the surface of the solder is not "bright shiny" it is either oxidized or wasn't brought up to temp to prevent a "cold joint". I rarely do surface mount so most of my experience is with through-hole soldering. YMMV
 

MrSoftware

Joined Oct 29, 2013
1,511
Aside from cleaning, flux also makes solder flow more like water. And can help spread heat, especially with SMT devices. Ever have solder sticking to your iron and making little pointy globs all over the place? Add flux and it will finish to a smooth rounded shiny result. Flux is your friend, especially with surface mount parts. Use a lot of it, clean it up with denatures alcohol, a brush and a towel when finish.

IMPORTANT: Use the no-clean flux unless you plan on really cleaning your boards well. The type that requires cleaning can eat the board if left on too long.
 

SLK001

Joined Nov 29, 2011
1,448
IMPORTANT: Use the no-clean flux unless you plan on really cleaning your boards well. The type that requires cleaning can eat the board if left on too long.
It shouldn't bother anything if it is left on a board (rosin based fluxes). Now acid fluxes are a different story!
 

SamR

Joined Mar 19, 2019
1,060
I'm a "neat-freak" and hate the messy sticky residue from rosin. Usually, a few drops of cheap 99% isopropyl and a q-tip to mop it around to dissolve and soak up the rosin gum will do it when I finish a solder job. If you are in a hurry to dry up the alcohol a paper towel soaks it up fast. For the really grubby stuff on an old board, I use an industrial "toothbrush" to break up the grime so the alcohol can dissolve or loosen it and get down to what I want to desolder.

Edit: Don't use Isopropyl Rubbing Alcohol. Not only is it very low percentage of alcohol (mostly water) it also contains additives such as witch hazel, mineral oil or even aspirin which is an acid (acetylsalicylic acid).
 
Last edited:

SLK001

Joined Nov 29, 2011
1,448
I'm a "neat-freak" and hate the messy sticky residue from rosin. Usually, a few drops of cheap 99% isopropyl and a q-tip to mop it around to dissolve and soak up the rosin gum will do it when I finish a solder job. If you are in a hurry to dry up the alcohol a paper towel soaks it up fast. For the really grubby stuff on an old board, I use an industrial "toothbrush" to break up the grime so the alcohol can dissolve or loosen it and get down to what I want to desolder.

Edit: Don't use Isopropyl Rubbing Alcohol. Not only is it very low percentage of alcohol (mostly water) it also contains additives such as witch hazel, mineral oil or even aspirin which is an acid (acetylsalicylic acid).

No such thing as cheap 99% IPA. 91% is the cheap stuff that I use and is the highest percentage you can get from simple distillation. I mostly use your technique, but I use a 1/2" acid brush cut to 3/8 to 1/2" long bristles as my scrubber. I get my IPA from Walmart.
 

SamR

Joined Mar 19, 2019
1,060
Cheap as in cheaper than denatured ethanol, Everclear 190 proof ethanol, or Coleman Fuel/lighter fluid or whatever the Brits call it (which I used before IPO and works quite well also but lighter fluid leaves a faint bit of residue/odor as it evaporates slower). I get the Amazon Solimo 99% Isopropyl @~$2.80/pt. which lasts me a long time. It's not Reagent Grade but works fine and dries fast. Probably couldn't tell the difference with 91%. It's just about the same price as the 91%. What I use would be known in the military as a "rifle cleaning" brush. Which I rarely use since most gets done with a Qtip I steal from my wife's vanity.
 

Sensacell

Joined Jun 19, 2012
2,430
Nickel plating hates solder.

The battery terminals you mention are probably plated with a metal that is not compatible with solder. No amount of flux can fix this.
You can grind through the plating to the base metal to solder these, as long as the base metal is solder-compatible.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
295
Thanks everyone!

Now I got a better idea about flux. Now, 2 questions:

1. What kind of trick do you use to solder to the, normally, "impossible to solder on" positive terminal of a battery? If you sand it, you expose a probably solder friendly material to work with? Are there special solder wires that work with there platings?

2. In surface mounted soldering, which I neither happen to do very often, many times I have trouble soldering the tiny pins, and I end up creating solder bridges between them, short them in other words, and I have to remove it and start again. Is flux used to avoid this?
I've seen videos and the guys that solder these type of things, they use "a lot" of solder but never run into the bridge/short problem, it's as if the solder only wanted to stick to the metal pins and not to the PCB between them, is it does when I have not used flux. Is this correct?
 

atferrari

Joined Jan 6, 2004
3,382
Nickel plating hates solder.

The battery terminals you mention are probably plated with a metal that is not compatible with solder. No amount of flux can fix this.
You can grind through the plating to the base metal to solder these, as long as the base metal is solder-compatible.
The many times I tried with common solder, it never worked. Seen them frequently instead, with terminals spot welded using a very fine point.

My very last in a hurry I adhered the terminal with kind of electric tape tightly wrapped around.

Awful kludge...but worked.
 

MrSoftware

Joined Oct 29, 2013
1,511
Flux will definitely help reduce solder bridging. If there's too much solder you need to remove some, a solder wick is one productive way, but flux will help a ton. I like paste flux in a syringe, it's easy to apply where you need it. Don't be shy with it, use a bunch, clean it up when done.

When soldering batteries, one big challenge is the battery terminal has a lot of heat capacity. i.e. it takes a lot of heat to get the terminal hot enough to melt solder. Remember solder will not stick properly to any surface that is not at least hot enough to melt the solder. So if you're not getting the battery terminal itself hot enough to melt solder, it won't stick. This is one reason that many (most?) battery tabs are welded on, not soldered. This guy has a great home made battery terminal welder, maybe something like this would help you:

 

jpanhalt

Joined Jan 18, 2008
7,697
Nickel plating hates solder.

The battery terminals you mention are probably plated with a metal that is not compatible with solder. No amount of flux can fix this.
You can grind through the plating to the base metal to solder these, as long as the base metal is solder-compatible.
That's interesting. Maybe it's something to do with the microscopic structure or surface. I have soldered nickle (pure) strips on NiCd's many times with ordinary flux-cored solder.
 

Tonyr1084

Joined Sep 24, 2015
3,470
Thanks everyone!

Now I got a better idea about flux. Now, 2 questions:

1. What kind of trick do you use to solder to the, normally, "impossible to solder on" positive terminal of a battery? If you sand it, you expose a probably solder friendly material to work with? Are there special solder wires that work with there platings?

2. In surface mounted soldering, which I neither happen to do very often, many times I have trouble soldering the tiny pins, and I end up creating solder bridges between them, short them in other words, and I have to remove it and start again. Is flux used to avoid this?
I've seen videos and the guys that solder these type of things, they use "a lot" of solder but never run into the bridge/short problem, it's as if the solder only wanted to stick to the metal pins and not to the PCB between them, is it does when I have not used flux. Is this correct?
1) Soldering to a battery is difficult because of the large area that needs to come up to temperature. Most often the negative end of the battery is also the battery case. But some rechargeable batteries are opposite with the positive being the case. The surface of the battery terminal is like a heatsink that pulls the heat away. That's PROBABLY the biggest reason why you have difficulty soldering to a battery terminal. PLUS - - - heating a battery like that is ill-advised.

2) SMD soldering is difficult. When I solder QFP's (Quad Flat Packs) I'll solder one, two or three pads on one corner holding the chip as close to where I want it to rest. Then do the opposite corner while pushing the QFP closer to alignment. Then when I have that last corner in place I'll go back to the first corner and make final adjustments. Then, using flux and solder, I'll start on one of the sides where I haven't started soldering yet and go ahead and add enough solder to scare the hell out of novices and drag that blob of solder all the way down that side. With sufficient flux and heat the solder will adhere to the pin and pad without bridging. At the end of that row I may end up with a solder bridge or two. No problem. Just wick some of it off. You may be left with too little solder. Again, no problem. Add more flux and drag the iron across all those pins. You'll end up with a series of joints that look like they were machine made. Then do the opposite side the same way. Then do either the side where I started with the first corner or last and repeat until I have all joints soldered. People will think you're some kind of soldering master. As for BGA's (Ball Grid Array's) you need flux and a hot air pencil.

On the subject of sufficient heat, some boards I've worked on were heavy with copper. Soldering on those boards could NOT be done without first bringing the entire board up to 125˚C. Then you can get solder to flow with either an iron or a hot air pencil.
 

jpanhalt

Joined Jan 18, 2008
7,697
Re: Post #13
Here's a photo to show what I meant. Substrate: nickel (pure) strip about 6 mm wide. Solder: Kester rosin core ("44") with 63/37 alloy.

upload_2019-10-10_9-22-26.png

The solder flows and wets the surface normally. After placing the solder, I bent the strip to show it does not pop off. No special preparations or care taken. Just used my regular electronic soldering iron.
 

Yaakov

Joined Jan 27, 2019
1,481
Flux cleans the surface but it also has a very important function: it prevents reoxidation. For that to happen, the flux must be there at the time the solder is applied. The tacky fluxes stay around even with a lot of heat, but even fluxes that will burn off will do the job if they are present at the time the solder is applied.

The flux you have will almost certainly facilitate soldering to a battery terminal if you DON'T heat it up first. Apply the flux, then make the joint as you would, right on top of the flux.

Here's a wire soldered to the terminal of a AA cell. I used Kester 959T no clean flux, but could easily have used a rosin flux. The solder is a Kester 63/37, though for the tinning, a 60/40 would have worked even better with the better wetting action it offers (it wasn't needed). The iron was a 350°C, and I applied a drop of the flux, then tinned the terminal, then tacked the tinned wire on and flowed a bit more solder.

The joint is very solid, mechanically, and of course electrically sound. The flux allowed the solder to flow on an oxide free surface, and it makes getting the terminal tinned easier. It can be done with the rosin core solder alone, but not as easily, and it would likely mean heating the work more.

IMG_6829.jpg
 

Yaakov

Joined Jan 27, 2019
1,481
If this is acid flux, it will "work" but then it will destroy the work so, yes, stay away from such things.

Rosin flux (colophane) or a more modern, no clean flux is best. Look for "RA" type flux that says the residue is non-corrosive.
 
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