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3. Flux and its requirement

The basic mechanism of soldering is controlled by the solubility of one metal into the other. For the formation of a inter-metallic bond, the metallic surfaces must be free of any contaminants, and particularly metal oxides. In soldering, the function of flux is to chemically react with oxides and quickly produce a fresh, oil-free, and oxide free surface at soldering temperatures so that intermetallic bond can take place.

Fluxes can be broadly classified into 3 types based on their constituents and activity.

            3.1 Inorganic acid
            3.2 Organic acid
            3.3 Rosin
            3.4 No clean

3.1 Inorganic acid: Inorganic acid fluxes are highly corrosive and seldom used in the electronic industry. These fluxes are capable of removing oxide films of ferrous and nonferrous metals. the inorganic fluxes are generally used for noneletronics applications such as brazing of copper pipes. The major disadvantage of inorganic fluxes for use in electronics is that they leave behind chemically active residues that can cause serious field failures.

3.2 Organic acid: The organic acid fluxes are stronger than rosin fluxes but weaker than inorganic fluxes. These fluxes can easily be cleaned by water. Because of their solubility in water, the organic acid fluxes may be environmentally more desirable. OA fluxes are widely used for wave soldering of components and meet military and commercial requirements for cleanliness. However, solder pastes with water soluble flux are not widely used because they are not as tacky as rosin-based fluxes, and tackiness in solder paste is necessary to prevent part movement during placement.

3.3 Rosin: Rosin fluxes are used for both wave soldering and in solder paste for reflow soldering. Rosin is inactive at room temperature but becomes active when heated to soldering temperatures. The melting point of rosin is 172 to 175) C, or just below the melting point of solder (1830C) which is a desirable property. As the rosin fluxes are weak, halide activators are used. Rosin fluxes can be classified into three groups.

            1. Rosin activated (RA
            2. Rosin mildly activated (RMA)
            3. Rosin ( R )

These three categories differ basically in the concentration of activators. As the name suggests, the cleaning action of RA flux more powerful than RMA flux and also requires through cleaning after reflow. The residue after soldering of RMA flux by itself is not very harmful, it may attract dust and other harmful chemicals from atmosphere and therefore it is recommended to clean the same after reflow. RMA flux is most favoured one for surface mount reflow applications. The rosin fluxes can be cleaned by either aqueous or solvent methods.

3.4 No-clean fluxes Eliminating the cleaning of the flux after reflow saves cleaning cost, may improve product reliability, also helps environment. Cleaning with solvent or water-based cleaning agents uses expensive equipment and costly. As the residues left behind by no-clean fluxes are inert, and nontacky, the possibility of corrosion or harmful dust collection at the joints will not arise with rightly selected no_clean. To qualify as a no-clean flux, the material must:

            1. Leave no corrosive residues
            2. Leave a nontacky residue that does not collect dust
            3. Be safe and must not degrade equipment
            4. Allow penetration of probe pins for electrical testing allow visual inspection of joints
            5. Provide excellent solderability

No-clean fluxes typically have solid content varying from 1 to 5% as against a solid content of about 30% for “cleaning-required” fluxes. The activators in no_clean fluxes have changed chloride_containing halides to carboxylic and dicarboxylic acids.

Flux Type Corrosion of residues Residue removal Activity
Rosin Non corrosive Not critical, but required Fair/ good
Organic acid corrosive water wash required Fair/ good
Inorganic fluxes Highly corrosive very critical, requires thorough cleaning very good
no_clean flux Non corrosive Not required Fair
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