Soldering Flux

Flux, what is it and why do we need to use it?  Basically, the purpose of flux is to help facilitate the soldering process by helping the solder to flow and preventing the introduction of oxides.  One problem that we all will encounter when soldering are the introduction of impurities found at the site of the solder joint.  Examples would be dirt, oil, or oxidation.

Example of Flux

Flux is your friend.   Always remember that short blurb when you are trying to solder something.  Attempting to solder without flux by attacking the part with a dry tip is folly.  You will not achieve a solid, clean joint, free of impurities, dirt, and/or oxidation.

Flux helps prevent re-oxidation of the solder at the joint by allowing the solder to flow before re-solidifying.  If you use a dirty or partially cleaned tip and no flux, the solder will melt, “somewhat”.   However, the solder will not flow into the joint very well and the parts will not be connected properly.  This usually results in a “cold” solder joint.  The result of this poor connection will usually be high resistance and low to no current flow.

In a previous article about soldering tips I mentioned something called “wetting”.  I will attempt to explain this phenomenon in more detail.

Basically, wetting allows the surface tension of the melted solder to flow more easily.  This helps the solder to fill the joints between the parts to be soldered and the pad, or wire that you are soldering to.  To precipitate or allow this “wetting” action to take place you need “Flux”.

Flux has been around the soldering industry for many years.  One of the earliest known materials for flux was charcoal.  It was non-corrosive and non-conductive.  It acted like a reducing agent and helped prevent oxidation.

“Pine Tar” was another type of flux used in soldering.  This was used because it had a “rosin-base” and was ideal for soldering with soft solder.  It was non-corrosive and non-conductive at normal temperatures.  But, when you heated the flux for soldering to higher temperatures it was mildly corrosive and reactive.  Some fluxes actually help with the cleaning process as they heat up.  This helps prevent oxidation as the soldering process commences.

You also have the choice of using rosin-based or non-rosin based solder.  Many solder alloys on the market today employ some sort of flux manufactured right into the core of the solder itself.  This helps the wetting action as you solder by supplying the flux right from the core.  No external flux is required.

There are three basic types of flux:

1.      Water Soluble – You can remove the active flux from these from your soldered joint with just water.  (no VOC’s are required)

1.      No-Clean Fluxes – This means that you don’t have to clean the part after using this flux.  The residue left behind is non-corrosive and only needs to be cleaned if it affects connections or causes undue contamination due to excessive wetness in the areas that were soldered.  

1.      Rosin Fluxes – These come in non-activated (R), mildly activated (RMA), and activated (RA) types.  RA and RMA are both forms of activated flux with various levels of a wetting agent, typically some form of acid that increases the wetting action of the parts to be soldered by removing the oxides.

RA flux is corrosive and should be cleaned upon completion of the soldering action.  RMA is not as corrosive as the RA flux.  However, cleaning is preferred to prevent any contamination to the joint after soldering.  You should never use an acid based flux on electronic circuits.  Acid fluxes are best kept for use on brass and copper piping in the plumbing industry.  Damage to the circuit board and components can be irreversible.

I clean all of the soldered joints that I make just so I know that all corrosive fluids or contamination is removed.  Whether I’m using no-clean, RA, or RMA, I always clean my joints with a little isopropyl alcohol.  99% is preferred by most technicians.

Flux also comes in different forms.  There is tacky flux, liquid flux, and flux in paste form.  Some types of fluxes require them to be thinned down with flux thinner.  For this application I suggest referring to the manufactures instructions.

There are many different flux applicators and containers as well.  There are flux pens, flux squeeze dispensers, flux bottles, and tacky flux in syringes.  Flux can also be purchased in gallon containers and larger if your needs dictate a use of flux on a production level.

It is a good practice to get into the habit of “not” mixing fluxes together.  If you are using no-clean flux then you should not mix this with a rosin based solder.  Always use “like agents” with their partner.  i.e. Rosin based fluxes should be used with rosin core solders.  You should use no-clean flux with non-rosin core solder.  Mixing fluxes can cause unwanted results from the interaction of the solder and the flux at the joint.  You may encounter poorly soldered joints, unwarranted corrosion, maybe even overheating or cracking of the metal.

You will find that soldering without flux will make your job much harder.  Quality will suffer and any soldered joints that look fine will most undoubtedly be inferior.

You should also consider utilizing a good flux remover if you’re not satisfied with using isopropyl alcohol.  There are several on the market and each has a specific use.  Check with the manufactures instruction on how to use a particular flux remover.  These special flux removers can be used to take off no-clean, rosin, synthetic, and paste fluxes from most electronic circuit boards. 

Remember to always practice good safety when handling any fluxes, thinners, solvents, and molten solder.  It’s very easy to forget that you can be burned, blinded, or irritated by contact with anything related to your soldering project.

So, get that flux and put it to good use.  “Remember “Flux is your friend”.