We have lots of more detailed information on fluxes, but we try to keep it as simple as possible to be able to the reader to understand the topic without having to refer constantly to a dictionary.
Pierre RICHARD
Role of fluxes are mainly in getting rid of oxides on the surface of metals to be soldered, so that those metals can unite creating a proper wetting.
Wetting is only possible when metals are in direct contact, without any other agents (oxides, grease, or other contaminants) in between.
What fluxes do is to break the bond between oxides and metals. It then carry them away in a suspension form. So the fluxes do not neutralize oxides, but just displace them.
The next step will be to get rid of those oxides in suspension and the flux residues.
Fluxes also create a state of optimal spreading of the solder on the conductor cleaned of its oxides and other contaminants.
The most widely used flux since around 5000 years is the pine sap resin. The resin that sweats from those trees is called colophony.
This liquid resin passes through a semi-liquid state before turning into solid crystal in contact with the air.
A solvent is used to transform these crystals into a solution which is then distilled to be used adequately in the soldering processes.
This distillate of the original pine sap is called Rosin. It is normally dissolved in an alcohol base to form a Rosin flux. Resin fluxes are fluxes made-up of natural (Rosin) and synthetic resins.
Elements are sometime added to those fluxes to increase their potential to reduce oxides. Those elements are mostly catalysts which increase the reaction without changing the process involved. Sometimes agents are added to increase the time spend of activity of the flux, thus allowing the flux to be active until the final stage of the soldering process. This is specially the case for the reflow ovens where heat is kept for much longer than in the case of manual touch-up.
Those Rosin fluxes have an organic origin. Their big advantage is that their long organic molecules, once activated by heat of the soldering process, break-up and their residue is totally neutral, which means not at all active and without any risks to unprotected conductors. They are very efficient fluxes with non-corrosive residues. On the other hand, those residues are sticky and should absolutely be cleaned-up to eliminate the risks of absorbing humidity and potentially conductive dusts. Cleaning is done with alcohol base solvents with or without saponifiers (agents capable of transforming residues into soap).
Synthetic Resin Fluxes have been designed to answer to more and more specific demands of the industry. They are mostly made-up of organic long chain molecules which break-down once their soldering temperature reached.
Depending on the need, some halides are sometimes added to increase the duration of action of the flux. These kind of fluxes are sometimes called “No Clean” or “Low Residue” fluxes. The “No Clean” fluxes were designed for applications that don’t need to have the residue be cleaned-off. The “Low Residue” flux can also be left uncleaned on the board depending on the specifications or function of the device. Those “Low Residue” or “No Clean” fluxes contain Rosin solids in smaller quantities than in normal Rosin fluxes such as R (Rosin), RA (Rosin Activated) or RMA (Rosin Mildly Activated). Those fluxes are generally less active than the RA and RMA types. They contain less active agents and they have been chosen mainly because of cleaning cost reduction because it is possible sometimes to leave the residues on the board without being a functional problem on the short and long run. Those residues are non-sticky and non-corrosive.
Depending on the surfaces to remove oxides, it will be necessary to use more or lesser active fluxes. In the case of high activity, we use organic acid type fluxes (OA) with a stronger activity as a reducer as well as a stronger corrosive property. Generally this corrosive factor is still present after soldering, which impedes that those flux residues be cleaned-off thoroughly after the soldering process so that those residues don’t continue to corrode conductors and non-protected metals. The great advantage of those water washable organic fluxes is their great potential of reduction, joined with an easiness of cleaning in deep water.
Not only we have different compositions of fluxes but there are also different forms or states of solvents. We find flux in solid form such as blocks and powder. It is also found in paste or gel form as well as in the liquid state or foam. Powder flux is used in wicking braid. Paste and gel flux are used to elongate their action on a surface, mainly for touch-up wave hand soldering. Liquid flux is mainly used in machine and hand soldering processes. We find gel flux in nearly all solder wire for touch-up. For machine assembly (reflow and wave) we mainly use liquid fluxes which are sometimes foamed-up to be able to control better the quantity of deposition.
It should be known that flux is ESSENTIAL in all soldering processes to guarantee the efficiency of the wetting action. No one can do without it and everyone should know what type to use and how to use it adequately.
Due to adequate measures of protecting the environment, more and more products are banned from the manufacturing sectors. Strict measures have been taken about particular products such as lead, hexavalent chromium, fire retardant products, etc. (RoHS et WEEE). It is also strongly recommended to reduce the volatile organic compounds (VOC) which deplete the ozone layer (Kyoto and others accords). Halides and alcohol used in cleaning PWB are in this category of VOC. To replace all these products, the industry has to be very creative but even with the best ingenuity, nothing will really replace as a drop-in solution most of those banned-to-be products.
As far as fluxes are concerned, the industry is going in the VOC-Free, Halide-Free direction. Will there also be a ban on Rosin in the future?.. What then to replace it with?
Water is becoming the most popular flux solvent, specially in the lead-free technology. For this lead-free technology, organic flux (water-based) is mostly recommended because of its superior reducing and corrosive potential. This, to counteract the problems of reduction of wettability of the new soldering alloys (tin/silver/copper).
Due to the higher temperature and change of alloys in the lead-free processes, fluxes have to be replaced by different ones with specific formulas for those new profiles and specific alloys.
Fluxes are classified with letters such as RA (Rosin), RE (Synthetic Resin), OR (Organic Acid) and IN (Inorganic Acid). Another letter describe the flux activity. “L” represent a low activity flux, “M” a medium activity flux and “H” a high activity flux. Also a third element comes to describe the type of flux. It is a number such as zero or one which describes the content of halides.
“0" (zero) represents a flux containing no halides.
“1" describes a flux containing some halides.
A flux can have then a code such as ROL0, which means a rosin base flux with a low activity and no halides.
ORH0 stands for an organic flux with a strong or high activity without halides.
REM1 will stand for a synthetic resin flux with medium activity and some halides, etc.
There are some other factors to increase the efficiency of fluxes, such as inerting and convection oven profiles.
Inerting consists of using an inert gas such as nitrogen to protect the soldering process from oxydizing with the surrounding oxygen of the air. Inerting also permit a reduction of quantity of flux used in the process and in enhancing the surface appearance of the soldered joint.
Oven profiles for this lead-free technology, should be designed to correspond to the activity of the new fluxes and to temperature or time above liquidus (TAL) or melting point. There should also be an increase of temperature in the pre-heat zone and a decrease in the cooling zone to have acceptable results.
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