About different kinds of solder flux

[IanB]

I'm re-posting some information here from the other Rigol DS1052E thread since I didn't really know the answer before and this information is useful in its own right.

I came across solder with "no-clean" flux, and I didn't know what it was. How is it different from rosin or RA flux? Here's what I found out.

Traditional electrical solder flux is rosin, which is derived from a natural pine tree resin. This has two functions: firstly, it melts and flows over the joint, keeping oxygen out. Secondly it has a very mild cleaning action to dissolve oxide layers on the surfaces and allow the solder to wet and bond to them. Once the joint is cold, the rosin is inactive and does no harm, therefore it can be left in place without cleaning. However, if it is desired to make the board look nice, or if a conformal coating is to be applied, then the solder residues must be removed using a suitable solvent. Isopropyl alcohol alone will work, but other solvents like acetone may be mixed in to improve the cleaning power.

Plain rosin flux is designated as "R" on labels. Since rosin by itself has limited activity to dissolve oxides, some additives may be mixed with it to improve its effectiveness. This is known as activated flux, or "RA". There is also mildly activated flux, "RMA" which lies between R and RA types in activity. The residues of both RMA and RA flux are still safe to leave behind and do not have to be cleaned. However, they may be cleaned for the same reasons as above.

A third kind of flux is "no-clean" flux. This is in fact a specially purified or synthetic kind of rosin flux. It is rather similar in properties to RA or RMA flux and can be used the same way.

All of the rosin fluxes are delivered mixed with some solvent such as IPA to make them be pasty or liquid when cold. If you store the flux for a long time the solvent may evaporate and the flux may go hard.

A last kind of flux is "water soluble" flux. This stuff is a totally different thing from rosin fluxes. There are different types, but all are completely synthetic in nature, and have very aggressive cleaning power. Unlike rosin fluxes, the flux residues from water soluble flux are corrosive when cold and must be completely removed from the board after soldering. Usually this is done by washing with distilled water, maybe including a little detergent.

When soldering SMT parts the parts may have very narrow clearances with the board where flux residues can get trapped and not reached by the washing process. This is bad for reliability when using water soluble fluxes due to the corrosive nature of the flux residues. Therefore the general trend and the best recommendation is to avoid the use of water soluble fluxes and use the rosin type fluxes instead.

[alm]

RMA and RA fluxes may form high-resistance connections, especially as it absorbs moisture. This is especially an issue for high-impedance circuits. They don't have to be cleaned in low-perfomance circuits under mild environmental conditions, but should be cleaned for optimal reliability. I believe most flux manufacturers state this in their data sheets. They won't attack the electronics/PCB though, as opposed to water soluble flux.

[IanB]

On further reading, it appears then that while RA and RMA flux residues may have an undesirable conductivity in some applications if not cleaned, the "no clean" fluxes are specifically formulated to avoid this problem and can be used in all cases without cleaning?

[gregariz]

I personally have had better luck with cleanup on RF circuits using the water soluble flux and a heated ultrasonic cleaner. Any type of flux residue will often damage the performance of a low noise amplifier in the GHz range. As a result I get boards coming out completely shiny and clean with no traces of flux.

I was always under the impression that the move to water based fluxes was largely about the advantage of water based cleanup. I didnt realise anyone was moving back the other way.

I'm also surprised by the idea that water soluble fluxes are more aggressive. The switch to water based fluxes for me was also a switch to the normal use of flux pens to complement the flux as I found I needed more flux not less.

[IanB]

While many people would think it desirable to clean off the flux residues regardless as a matter of good practice, the issue with water soluble fluxes is that you must clean the board afterwards. This mostly would not be an issue, but if you have, say, a large SMT component like a QFP then flux may be trapped under the part where the wash can't reach it. In this case water soluble fluxes (I read) are undesirable.

I also read that military standards only permit the use of rosin type fluxes, and that water soluble fluxes are not compliant.

The aggressive nature of water soluble fluxes is an impression I gained that I may have misremembered. Fluxes can be manufactured with a variety of different compositions and maybe the fluxing properties depend a lot on the particular composition in use?

Water based cleanup might seem like a good plan, but in an industrial setting you still have a lot of contaminated water to dispose of. That may or may not be as easy to deal with as contaminated solvent (which may be purified and reused). In my field (chemical engineering) the environmental considerations in dealing with spent and contaminated waste can be a big headache.

[gregariz]

Water based cleanup might seem like a good plan, but in an industrial setting you still have a lot of contaminated water to dispose of. That may or may not be as easy to deal with as contaminated solvent (which may be purified and reused). In my field (chemical engineering) the environmental considerations in dealing with spent and contaminated waste can be a big headache.

Its my understanding that NASA and sections of the Military only permit the use of lead based R or RA (if cleaned properly).

I don't know for sure, but I'd have a pretty confident guess than alot of board assembly houses that use the new 'organic' water soluble fluxes just flush the contaminated water down the drain. In fact if memory serves correctly some were advertised as such. Just casting an eye at a roll of solder on my bench I see I have Kester 331 organic water soluble solder. I've used the same solder if memory serves in the last few jobs.

[IanB]

I don't know for sure, but I'd have a pretty confident guess than alot of board assembly houses that use the new 'organic' water soluble fluxes just flush the contaminated water down the drain.

While I have no doubt that they might do that, it does not necessarily mean they are permitted to do that. Generally speaking, industrial waste streams will require a permit before they can be discharged to a municipal water treatment plant, and the operator is supposed to conduct regular analysis of the stream to make sure the contaminants are within the terms of the permit. It may of course be the case that the permit is not very difficult to comply with and the general wash water from circuit board operations is dilute enough that it may be discharged to drain without issue. On the other hand, it may be the case that some operators have overlooked the need for a permit...

[gregariz]

The real nasty stuff that I have trouble disposing of I find is board etchant, whether ferric chloride, ammonium persulfate and the like. Ammonium persulfate was supposed to be a clean solution and although I am sure there are plenty of people who have just flushed it down the drain I am uncomfortable doing that. As a result I don't do etching anymore at home.

On the other hand I'm probably responsible for someone flushing it down the drain in China, when I order from there.. so I guess you just can't win.

[A Hellene]

Well, what will stop them from disposing their contaminated byproducts for free into any New Line of Toothpastes with Water Soluble Organic Formula added for Even Cleaner Teeth for the Children than the Fluoride containing toothpastes the Aluminium Production Industry has been generously providing us with, for free also, since its eruption after WWII?
/sarcasm

It is said that, in the fifties, even the Plastics Industry was disposing into the rivers during the nighttime their toxic byproduct, called gasoline... Then, of course, came that unbelievable fleet of those shiny and fuel-wasting commercial vehicles to help the situation out...

I am sorry for the off-topic; I could not resist myself.


-George

[JuKu]

In a previous work, we used organic flux. Basically, it was super strong orange juice, made out of orange peels. Smelled like that, too. Very corrosive (citrus acid), but totally safe to drain when enough water was added.

[KJ6EAD]

Water soluble orangic flux?