VOC stands for volatile organic compounds. VOC-Free fluxing agents are those fluxing agents which do contain any volatile organic compounds. In the context of soldering fluxes, it refers to the solvents, mostly alcohols, which evaporate at rather low temperatures.
Why VOC Free?
The need for VOC-free industrial chemicals (which include soldering fluxes also) stems from environmental considerations. Volatile organic compounds (VOCs) are substances which contain carbon and vaporize at low temperatures. Evaporation of carbon dioxide in the atmosphere results in harmful ozone generation which can ultimately contribute to global warming. Closer to the ground, the presence of these volatile organic compounds leads to unhealthy air conditions and smog in the lower atmosphere. Therefore, in the interest of public health and well-being laws were made to restrict the emission of volatile organic compounds to a minimum.
Main ingredients of fluxing agents (fluxes) are (1) vehicles, (2) activators, (3) solvents, and (4) special additives.
Vehicles- are non-volatile liquids or solids that can stand soldering temperatures without decomposing. But they should readily melt at those temperatures. Solid vehicles are natural or synthetic resins natural or modified rosin. They function to carry away products of oxide reduction, protect the reduced metal surface from re-oxidation, and to some extent, a heat conduction medium.
Activators- are chemicals which break the metal oxides from the metal surface. Very active fluxes may contain metal halides like ammonium chloride or zinc chloride hydrochloric acid, citric acid, phosphoric acid, and hydrobromic acid. Milder ones include carboxylic acids, di carboxylic acids, and fatty acids like oleic acid and stearic acid.
Solvents- They facilitate processing and deposition to the metal surfaces eg, through spray for automated plants. They should evaporate before the board touches the solder. Organic solvents, mostly alcohols are inflammable and need precaution in handling and storage. Current non-VOC solvents are based on water and do not pose this problem.
Additives-Additives may be added to fine-tune certain properties like corrosion inhibition.
VOC in the Soldering Context
Most soldering work in electronics assemblies, especially in PV is done by automatic wave soldering plants. Soldering process is facilitated by the application of fluxing agents or simply, fluxes to the print before soldering. No-clean fluxes for wave soldering have to have solvents or thinners for quick and automatic spray application. Traditionally, fluxing agents in wave soldering have contained organic solvents, normally alcohols. These alcohols then act as carriers of the vehicle and activators of the flux to the surfaces being soldered. The Clean Air Act issued by the United States Environmental Protection Agency in 1990 and the subsequent amendments have specified restrictions on the amounts of VOCs specific industrial operations can emit. At that time only one VOC-free fluxing agent was known to exist.
That was known as HF-1189 developed by at Hughes in Fullerton, CA by Ray Turner. This flux was water-based flux with activators derived from lemon juice! Being water based it could be cleaned with water. Later all manufacturers turned their development efforts to VOC free fluxing agents to comply with the regulations.
Current VOC Free Fluxes
The vehicles for these fluxing agents are selected from either rosins, modified natural resins or synthetic resins. Activators for no clean type of fluxes are normally organic acids-carboxylic and dicarboxylic acids. In the new VOC formulations for fluxes, use of carboxylic acids as activators is still retained (It is to be noted that fluxes are classified VOC free if the VOC content is under 1%). Water has replaced the solvents like isopropanol-based compounds. Therefore, suitable surfactants must be added to decrease the surface tension of water. Reduction of surface tension of water in the flux permits better wetting of the PCB by the flux. The replacement of alcohols with water as solvent has made these solvents more environment friendly. They also pose less hazard in storage and handling. However, presence of water did cause technical difficulties in wave soldering processes.
Problems of Water as Solvent
Solvents for no clean wave soldering fluxes have traditionally used volatile organic compounds like isopropanol and methanol. Alcohol solvents have the advantage that they evaporate during the preheat operation. These alcohols boil at temperatures between 18 and 22ᵒ C. By the time the assembly touches the wave, alcohols have already evaporated completely, and only activators and vehicles are left behind. However, if evaporation of the solvent is incomplete it can lead to spattering and tiny solder balls being carried through the holes of the board to the components side. This happens more when water is present. And for this reason, water content used to be minimized in the older flux formations. Now that the VOC-free fluxes are water based, spattering is a noteworthy problem. Spattering occurs when a still-wet board comes in contact with molten solder.
The solution lies in a suitably modified preheat phase. Preheating has two purposes. Evaporating the solvent and bringing the board closer to the solder temperature to avoid thermal shock. Water is said to evaporate twice as slowly as the alcohols. Blowing hot air in the preheat phase helps to achieve both purposes. The heating profile must be controlled carefully. Some manufacturers recommend preheat temperatures between 80 and 150ᵒ C.