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1. Flux components
 In a general way, the flux is made with:

Activators :
In general and currently, the principal activators are carboxylic acids, which are added to rosin
in the case of resinous flux.  The main organic acids used in flux are adipic acid, citric acid and
glutaric acid, whose boiling points exceed 300 °C. Thus these are the activators which attack
oxides according to the following reaction :
       RCOOH + MX    RCOOM+HX
Where M: Sn, PB, Cu
       X : Oxide
The activity degree is directly related to acid index of flux. This information is always given on the technical file.  This index corresponds to the mass of KOH to neutralize 1 gram acid present in the residues of flux. The higher index acid is, the  flux activation is stronger. However, aggressive fluxes contain strong acids, mineral acids, amines and halides, but of course due to ecological reasons flux manufacturer are going toward others solutions ( halide free…) .
Vehicle consist in solid or/and non volatile liquid.
Their functions are multiple:
At the soldering temperature,  they dissolve, residues of de-oxidation reaction.
They protect cleaned surface and prevent re-oxidation.
They create a heat exchange and allow a fast and homogeneous heat transfer towards surface being soldered.
Their role is to fluidize the activators and the vehicles  at the ambient temperature and allow to spread uniformly the flux to the precise place of soldering. The solvents evaporate during the preheat.
They allow to improve an existing function of flux. They merge with the vehicles. They are surfactants, rheological agents, thermal stabilizers …

Types of fluxes
As a rule a flux is a solid which has been dissolved by a solvent.
Fluxes soluble in organic liquids
Resinous flux
The most used resin is the rosin, which is made with resin of pine ( 90% resin acids and 10% ester ). The melting point of rosin is located between 125 and 130 °C. To improve the activity of these flows, activators are added.  Three activators categories are:
R : pure rosin
RMA: activated mildly rosin
RA: activated rosin
It is necessary to know the activation temperature of the activators and their aggressiveness degree which will influence the cleaning process after soldering.
Synthetic fluxes
There isn’t any resin. These flux are made with some organic or carboxylic.  They are obtained by organic synthesis.
Water-soluble fluxes
The principal solvent are water or a water/alcohol mixture. it is necessary to well control pre-heating to avoid the boiling. They are in general more aggressive than rosin because they are more acid. Thus they generally impose an intense cleaning with hot water.
Flux no clean
Those flux let a controlled amount of residues on the top of the component and theoretically require no post cleaning.. Those residues come primarily from a reaction between the activators and the vehicles. This flux form after heating a varnished layer consequence of polymerization reaction between doil ( vehicle ) and diacid ( activators ). This electrically insulating varnished layer  prevent the flux to spread, contaminate and corrode the electronic board and components. That is the reason why in theory it is not required to clean. In addition  Dry extract is less than 5% compared to more than 15%usually, as a consequence it is barely visible. On another hand  the time/temperature process window is very sharp. If temperature ramp up is too slow because
of the product thermal inertia a complete polymerization can occur before liquidus temperature is reached. This varnished layer can be eliminated by heated organic solvents baths. No clean flux can be used with organic solvent or water.