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The purpose of flux is to enable the solder to attach itself to the job in hand. This sounds easy but if the wrong flux is chosen then there is the serious possibility of the flux corroding the joint and thereby causing it to fail (come apart). It will therefore be seen that the type of flux chosen is very important. If you feel you need a strong flux then you are probably not cleaning the work sufficiently before soldering.
Inert Fluxes: This type of flux is the most common and is the type that should be used for most electronic and modeling work. Its main property is that it is only active when it is heated and then tends to burn off. One variant of this type, often called "rosin flux", tends to be very sticky.
Active Fluxes: This type of flux is useful materials such as lead but should never be used on thin material such as copper or brass as it will corrode the material very quickly. Before using any type of flux it is a good idea to apply a small amount to an unimportant part of the work.
If the area is brightened then this is an active flux and use should be thought about very carefully and, if possible, avoided.
Most active fluxes are either very acidic or very alkaline.
They do not burn off like the inert type but remain active for a very long time. It can therefore be seen that if this type is used on a printed circuit board or a precious model then it will not take the flux long to destroy the work. I have indeed seen a very nice model locomotive hit the buffers too hard and become a construction kit again. The flux under the joints had remained active and gradually etched away under the joints.
A good example of this is available in the house. Check the joints on your copper water pipes. You will almost certainly find several of them coated with a green material.
This is in fact copper sulphate caused by a reaction between the copper pipe and the flux. You will also note that how ever many times you try to wash it off it always comes back.
In the early days of electronics a flux called "Arax" was in constant use. Arax is still available but tends to be used for soldering stainless steel or other materials which are hard to solder. It was found, after continuous failures of the printed circuit board, that the copper track was etched away under the solder joint thus causing early failure of the board. This can still happen today if the wrong flux is used.
Advice on choosing a suitable solder and flux for a particular job will be given in later Sections.
One final word of warning regarding flux. There are many concoctions on the market purporting to be flux. Most of these are not marked as to their hazard level. I have come across many that are lethal and which, if used in an industrial environment, would need respiratory apparatus and protective clothing. If the bottle or can is not marked as to its contents then do not buy it. In extreme circumstances it could be your life that you are playing with.
I have come across one such bottle whose contents, if used in a confined space without ventilation and with a full temperature soldering iron, would almost certainly cause the user to be overcome by toxic fumes. Beware of innocent looking bottles or cans. If you are doubtful about the flux you already have, or are about to buy, then ask the retailer or better still contact the manufacturer and demand to know the contents.
Under the Consumer Protection Regulations, manufacturers and suppliers of this type of material are legally bound, firstly to clearly mark the hazard level of their product on the bottle or can and, secondly, if required, to supply technical data on the product. If you are using these fluxes to manufacture models or other parts for resale then of course the regulations of the Health and Safety at Work Act apply. In this case it is a requirement of the Act that you know what you are using. All reputable manufacturers will be more than happy to supply you with this Health and Safety information, the others will not, which will enable you to make up your own mind as to whose fluxes to use! Your health may be at risk.