Flux questions. What to buy from ali, and what is wettability?

[epigramx]

I figured out I'm in need of taking flux seriously; I even have literal rosin but it doesn't seem great if I just ground it and mix it with alcohol; I have another "soldering grease" flux which appears to work (to achieve wetting) but I fear it might be unhealthy with its fumes and it might not be the best possible so I wonder if you have suggestions on what to buy from ali (or elsewhere) that has the highest quality possible for the purpose.

And how exactly does wetting work I wonder; I know it seems to split the solder when there is the plastic of the PCB between the different metal contacts; but why does the solder not just keep connecting to itself between those metal contacts since it has already connected to itself before flux and it seems to separate instead into different parts between the PCB contacts after adding flux?

[Solder_Junkie]

You cannot bridge gaps between pads on a circuit board with solder, you need a cut off component lead, or a piece of wire to link the pads.

In general, you do not need additional flux, use cored solder which contains flux.

Additional flux can be added when using hot air to solder an integrated circuit, or some transistor types, to a board, where the component has an underside that needs soldering to the board. You can tin the board, add a thin smear of flux paste, place the component and heat it using hot air.

The gel type of flux in a syringe will last a long time, as you rarely need it, and when you use it the amount is minuscule.

SJ

[Doctorandus_P]

I figured out I'm in need of taking flux seriously;

Yes, indeed, flux is essential for soldering, but good solder has the flux already included. Lots of SMT soldering techniques do require adding extra flux though. Solder wick also contains (powdered) flux, but adding more flux often helps there too. There are lots of good (and also bad and ugly) video's on youtube about the use of (extra) flux during soldering. Go watch them to see what a good soldering process looks like. (Hint: "GreatScott" is terrible at soldering. Avoid him at all costs when it's the soldering you are interested in.)

I even have literal rosin but it doesn't seem great if I just ground it and mix it with alcohol; I have another "soldering grease" flux which appears to work (to achieve wetting) but I fear it might be unhealthy with its fumes and it might not be the best possible

Rosin isn't healthy either. The only solution I know is to use filtering and/or ventilation.
I've heard you can use (dissolved) citric acid as a flux, with some other liquid to make it "thicker" and stick more. As it's acid based you have to clean it afterwards, but as it readily dissolves in water, it's also easy to clean.
And yes, you can wash and scrub a PCB in a bowl of water. Just let it dry thoroughly before you apply power. (Some parts such as buzzers do not tolerate this treatment).

so I wonder if you have suggestions on what to buy from ali (or elsewhere) that has the highest quality possible for the purpose.

Quality from Ali? I wish yo lots of luck.

Wetability is the same principle as with water.

If you pour a glass of water over a T-shirt it gets wet (and sometimes other nice side effects).
If you pour a glass of water over a raincoat it rolls of.

With soldering, there is always a bit of dirt and oxidized stuff on everything you want to solder. It's (partly) a result of our atmosphere consisting of 19% oxygen, which is quite aggressive stuff. This all started with https://en.wikipedia.org/wiki/Great_Oxidation_Event and now we have to live with it, but it also has it's advantages.

In practice, the solder sticks to the clean other metals, but not to the dirt and rust, and a part of the job of the flux is to remove the dirt.
I think the flux also lowers the surface tension of the solder (but I'm not entirely sure). But if it does, it is (a bit) comparable to mixing in a bit of soap with the water before you pour it over the T-shirt. (Although the flux does not dissolve into the solder, so it's a mediocre analogy).

[tooki]

I figured out I'm in need of taking flux seriously; I even have literal rosin but it doesn't seem great if I just ground it and mix it with alcohol; I have another "soldering grease" flux which appears to work (to achieve wetting) but I fear it might be unhealthy with its fumes and it might not be the best possible so I wonder if you have suggestions on what to buy from ali (or elsewhere) that has the highest quality possible for the purpose.

If you want to take flux seriously (which I recommend), then don't bother with Aliexpress, etc. Even if you get a good product now, you have no idea whether the next batch will be the same.

Just order proper electronics flux from a reputable reseller like Digi-Key or Mouser. If you share your location (for example, by setting it in your forum profile) we can much better suggest where to buy.


Indeed, despite some people's claims that rosin in alcohol makes a "great" flux, rosin without added activators is a very mild flux that is good only for parts/boards that are very clean already. It can't cut through serious oxides the way activated fluxes can.

"Soldering grease" is plumbing flux. Do not use it on electronics, as it will cause corrosion.

And how exactly does wetting work I wonder; I know it seems to split the solder when there is the plastic of the PCB between the different metal contacts; but why does the solder not just keep connecting to itself between those metal contacts since it has already connected to itself before flux and it seems to separate instead into different parts between the PCB contacts after adding flux?

Wetting is name for the solvent action of molten solder on the metal of the components/boards. The surface tension of solder is very high, so it wants to pull back. Solder mask (the colored lacquer on PCBs) is specifically designed to make it hard for solder to bridge across it.

[tooki]

If you haven't watched it already, the following series (only about an hour in total) is hands-down the best soldering tutorial out there. Even though it's old and doesn't cover very much SMD, understanding the soldering physics and chemistry as demonstrated in it will help you massively with any other tutorials you find.

https://www.youtube.com/playlist?list=PL926EC0F1F93C1837


For SMD tutorials, look at John Gammel's videos:
https://www.youtube.com/@jkgamm041

[tooki]

In general, you do not need additional flux, use cored solder which contains flux.

I don't think that's the best advice, since especially with SMD work, there are situations where external flux is essential. (Even when THT was king, there were still situations where external flux was useful.)

[Solder_Junkie]

In general, you do not need additional flux, use cored solder which contains flux.

I don't think that's the best advice, since especially with SMD work, there are situations where external flux is essential. (Even when THT was king, there were still situations where external flux was useful.)

In close to 40 years in the service industry working for a multinational manufacturer on  their products, mostly through hole boards (SMD was a board swap), we never used additional flux.

Don’t forget the more flux you add, the more you end up having to clean off afterwards. Adding flux is easy and it does no harm, it’s not easy to clean off. Those who say there is no need to clean flux off should look at commercial boards in new equipment, they are spotlessly clean.

SJ

[tooki]

I figured out I'm in need of taking flux seriously;

Yes, indeed, flux is essential for soldering, but good solder has the flux already included.

No. The quality of the solder has nothing to do with whether it has a flux core or not. There is terrible quality flux-core solder, and there's top-quality solder with no flux core.

Frankly, when looking at solder made of good-quality metal, the type and quality of the flux core is what makes all the difference. A well-designed, high-activity flux core (like Kester 44) runs circles around a solder with inferior flux (like most Stannol I've tried).

In electronics, we basically never use wire solder without a flux core. But it's common in plumbing, for example.

Lots of SMT soldering techniques do require adding extra flux though.

Indeed. It's essential to certain techniques. (Heck, that's even true in THT soldering, for example in mass production. It also helps in manual THT work if your components are a bit more tarnished.)

Solder wick also contains (powdered) flux, but adding more flux often helps there too.

I thought it was made by coating it in liquid flux and drying it. I always assumed the powdery appearance was because of the wick getting flexed after the flux was dry, causing it to fracture.

But I totally agree, most wicks can be improved by adding more flux.

There are lots of good (and also bad and ugly) video's on youtube about the use of (extra) flux during soldering. Go watch them to see what a good soldering process looks like. (Hint: "GreatScott" is terrible at soldering. Avoid him at all costs when it's the soldering you are interested in.)

Absolutely! I love GreatScott's projects and explanations, but his soldering is awful!

Wetability is the same principle as with water.

If you pour a glass of water over a T-shirt it gets wet (and sometimes other nice side effects).
If you pour a glass of water over a raincoat it rolls of.

It's actually not like that at all!

A much better analogy would be a block of salt: where water comes into direct contact with the salt, it dissolves the surface. But where the block is greasy, the water cannot come into contact with the salt.

Soldering involves the solvent action of the molten solder. See the first video in the Pace tutorial series I linked above.

With soldering, there is always a bit of dirt and oxidized stuff on everything you want to solder.

Which is why it's always a good idea to clean your boards (and to the extent possible, components) with solvent (like alcohol) to remove contaminants. Then the flux can take care of oxidation.

In practice, the solder sticks to the clean other metals, but not to the dirt and rust, and a part of the job of the flux is to remove the dirt.

Solder doesn't "stick" like glue. It dissolves the substrate, creating an intermetallic layer between the substrate and the solder itself.

FYI, "rust" means red iron oxides specifically. We rarely deal with that in electronics. We deal with oxidation all the time in electronics soldering, but it's practically never rust.

While flux can remove some contaminants, its primary job is to deal with oxides. It's best to remove contaminants prior to soldering.

I think the flux also lowers the surface tension of the solder (but I'm not entirely sure). But if it does, it is (a bit) comparable to mixing in a bit of soap with the water before you pour it over the T-shirt. (Although the flux does not dissolve into the solder, so it's a mediocre analogy).

I think it lowers the surface tension, but I think more in the sense that oxides (which form on the solder itself, too) have extremely high surface tension, and by eliminating the oxides, the flux allows the solder itself to contact the joint.

[tooki]

In general, you do not need additional flux, use cored solder which contains flux.

I don't think that's the best advice, since especially with SMD work, there are situations where external flux is essential. (Even when THT was king, there were still situations where external flux was useful.)

In close to 40 years in the service industry working for a multinational manufacturer on  their products, mostly through hole boards (SMD was a board swap), we never used additional flux.

And? Just because you never used it doesn't mean it might not have made some things easier. (It is possible to do something the wrong way for decades, after all! )

SMD is unavoidable these days, and SMD rework practically always requires external flux.

My point is simply that a blanket statement that "in general, you don't need additional flux" is, IMHO, very outdated advice. In SMD rework, you may not even be using wire solder at all...

Don’t forget the more flux you add, the more you end up having to clean off afterwards.

Very true.

Adding flux is easy and it does no harm, it’s not easy to clean off.

Depends on the flux (e.g. no-clean fluxes tend to be much harder to remove than "traditional" rosin fluxes), and how the flux was handled after application (e.g. overcooked flux is far harder to remove than flux heated briefly).

Those who say there is no need to clean flux off should look at commercial boards in new equipment, they are spotlessly clean.

I don't think you can make that claim, as a blanket statement. Many boards are cleaned, and many are not. Some use carefully controlled reflow soldering processes using no-clean flux, such that the amount of flux residues is so small you don't notice with the naked eye. Some use water-soluble fluxes that are then removed in hot water. And, frankly, it's not uncommon to see boards that have been cleaned, but not spotlessly.

I suspect that a lot of the boards you think are spotlessly clean are in fact ones soldered carefully with no-clean solder and not cleaned at all. Easy to test: apply isopropyl alcohol and give the board a quick cleaning. If you see white residues once it has fully dried, you've likely got flux on the board.

[helius]

In metallurgy there are two states for metal surfaces: active and passive. An active surface has metal bonds exposed that can react with other substances. A passive surface has its metal bonds protected by a thin layer of non-metallic material, and will not react.
The solvent action of solder is to react with an underlying metal and form bonds to it, as tooki said. This is analogous to the solvency of something like water, which surrounds and bonds to a solute like table salt to form a clear and homogeneous mixture called a solution. Solder surrounds and bonds to metals such as copper and tin to form a tightly adhering, strong joint after the solder cools. The heart of the joint is a solid solution of metal atoms from both the copper and solder mixed homogeneously, around a micron thick. This is called "wetting". If the solder doesn't form this solid solution layer, the joint is a defective "dry joint".
For the solder to react with the metal and bond to it in this way, the surface of the metal must be active. If the metal is passivated with oxides etc, the solder cannot get "close enough" to form metallic bonds. So the reason flux is used is to activate the metal surface: to dissolve and remove the oxides that form the passivating layer on the metal. The activity of flux is a measure of how strong its ability to dissolve oxides is. Pure rosin has some activity but is relatively weak. Most electronic flux has activators added in a few tenths of a percent that increase the flux's activity. The chemical properties of the activators are what determine the suitability of the flux to the application.
There is also a very important aspect which is the disposition of the activators after soldering is complete. If the activators remain in the dried flux and don't continue their chemical reaction, post-process cleaning is not necessary. If the activators continue to react, the flux must be cleaned off or corrosion will occur. Zinc chloride is an example of an activator that continues to react and so it must be cleaned off. In fact it is corrosive at such trace amounts that it should never really be used on electronics at all. Zinc chloride is the activator in plumber's flux. Other activators include halides: organic salts of fluorine, chlorine, or bromine; phosphates; citrates; imides, etc. The J-STD-004 category reveals details about what activators are present. Usually you want a ROL0 type flux if the board won't be cleaned. That means ROsin base, Low activity, less than 0.5% halides.
Some metal surfaces are very strongly protected by passive layers, such as aluminum or stainless steel. In order to solder to these metals very strong specialized activators are needed, such as orthophosphoric acid. Obviously cleaning must be meticulous after they are used.
Once you understand the purpose of flux, you also will see how it should be used: applied thinly to the component leads and the pads of the board. Never apply flux to the soldering iron!

[epigramx]

Indeed, despite some people's claims that rosin in alcohol makes a "great" flux, rosin without added activators is a very mild flux that is good only for parts/boards that are very clean already. It can't cut through serious oxides the way activated fluxes can.

"Soldering grease" is plumbing flux. Do not use it on electronics, as it will cause corrosion.

In metallurgy there are two states for metal surfaces: active and passive. An active surface has metal bonds exposed that can react with other substances. A passive surface has its metal bonds protected by a thin layer of non-metallic material, and will not react.
The solvent action of solder is to react with an underlying metal and form bonds to it, as tooki said. This is analogous to the solvency of something like water, which surrounds and bonds to a solute like table salt to form a clear and homogeneous mixture called a solution. Solder surrounds and bonds to metals such as copper and tin to form a tightly adhering, strong joint after the solder cools. The heart of the joint is a solid solution of metal atoms from both the copper and solder mixed homogeneously, around a micron thick. This is called "wetting". If the solder doesn't form this solid solution layer, the joint is a defective "dry joint".
For the solder to react with the metal and bond to it in this way, the surface of the metal must be active. If the metal is passivated with oxides etc, the solder cannot get "close enough" to form metallic bonds. So the reason flux is used is to activate the metal surface: to dissolve and remove the oxides that form the passivating layer on the metal. The activity of flux is a measure of how strong its ability to dissolve oxides is. Pure rosin has some activity but is relatively weak. Most electronic flux has activators added in a few tenths of a percent that increase the flux's activity. The chemical properties of the activators are what determine the suitability of the flux to the application.
There is also a very important aspect which is the disposition of the activators after soldering is complete. If the activators remain in the dried flux and don't continue their chemical reaction, post-process cleaning is not necessary. If the activators continue to react, the flux must be cleaned off or corrosion will occur. Zinc chloride is an example of an activator that continues to react and so it must be cleaned off. In fact it is corrosive at such trace amounts that it should never really be used on electronics at all. Zinc chloride is the activator in plumber's flux. Other activators include halides: organic salts of fluorine, chlorine, or bromine; phosphates; citrates; imides, etc. The J-STD-004 category reveals details about what activators are present. Usually you want a ROL0 type flux if the board won't be cleaned. That means ROsin base, Low activity, less than 0.5% halides.
Some metal surfaces are very strongly protected by passive layers, such as aluminum or stainless steel. In order to solder to these metals very strong specialized activators are needed, such as orthophosphoric acid. Obviously cleaning must be meticulous after they are used.
Once you understand the purpose of flux, you also will see how it should be used: applied thinly to the component leads and the pads of the board. Never apply flux to the soldering iron!

Yeah pure rosin does not live to its reputation at all: I had to replace the surface-connected oled screen of a ts100 soldering iron and pure rosin-alcohol made a mess; a cheap 'soldering grease' felt almost perfect compared to it; but it triggered me to make this thread because obviously it wasn't the best material for the purpose.

Thanks I'll look for ROL0 type flux I guess. When you say apply it on leads/pads: do you mean it might help to always apply some in situations that solder itself might not have enough (e.g. would it have been good to add a little before connecting new switches to a g502 mouse after I removed the old solder with a pump?)?

[helius]

I apply liquid flux using a cotton bud (Q-Tip™) to the pads in all repair situations. Experience will be a better guide for where and how much to use, but an advantage of flux sometimes overlooked is that it makes a faster thermal bridge between the iron and the work, making it possible to get in and out quickly which is always advised to prevent damage. You should be able to touch the iron to the work and do whatever you are trying to do (solder, desolder, wipe, clear bridges, etc) in less than 2 seconds before removing the heat.

This does not apply to hot air, where heating for over 30 seconds is often required. Fluxes for hot air rework can be rather different as they need to withstand prolonged heat without drying up. I think most of them require post-process cleaning. You can watch Louis Rossmann's videos and see what he uses.

It also applies less when assembling brand new boards, where the pads are usually very clean to begin with. But is the surface of the solder wire itself clean if it has been lying around for a long time? There are aerospace soldering videos that teach cutting off a length of solder wire, cleaning it with alcohol, soldering a single lead, and then throwing the rest of the wire away... Or alternatively, you could use liquid flux which removes oxides from both the work and the solder.