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Welding material doesn't always adhere well to components, resulting in bad welding joints, bridged pins, or no welding joints. Use flux and the right temperature to overcome these problems.
When welding melts and forms a joint between two metal surfaces, it forms a metallurgical bond by chemically reacting with the other metal surfaces.A good bond requires two things: a welding that is metallurgically compatible with the metals being bonded, and a good metal surface free of oxides, dust, and grime that would prevent a good bond. Dust and dirt removal is prevented by cleaning the surface or using good storage techniques. Oxides, on the other hand, require another approach.
Oxides form on nearly all metals when oxygen in the air reacts with the metal.On iron, oxidation is often called rust, but oxidation affects tin, aluminum, copper, silver, and nearly every metal used in electronics. Oxides make welding more difficult or even impossible, preventing a metallurgical bond with the welding. Oxidation occurs all the time, but it occurs much faster at higher temperatures, such as when flux washes the metal surface and reacts with the oxide layer, leaving a primed surface for a good welding result.
When welding, flux is left on the metal surface to prevent additional oxide from forming due to the high temperatures during the welding process.Like welding, there are several types of flux, each with a primary use and some limitations.
For many applications, the flux included in the welding core is sufficient. However, there are some applications where the additional flux is beneficial, such as surface mount welding and dewelding. In all cases, the best flux to use is the least acidic (most aggressive) flux that will act on the oxides on the components and produce a good welding bond.
Some of the oldest types of flux are rosins based on pine sap (which has been refined and purified). Rosin flux is still used, but modern rosin fluxes blend different fluxes to optimize their properties.
Ideally, the flux flows easily (especially at high temperatures), removes oxides quickly, and helps remove foreign matter from the surface of the metals being welded. Rosin flux is acidic in liquid form, but becomes solid and inert when cooled. Since rosin solids are inert, they can be left on the PCB without damaging the circuit, unless the circuit gets hot enough that the rosin may become liquid and begin to eat away at the connections. Therefore, it is always a good policy to remove rosin flux residue from the PCB. Likewise, if you are applying conformal coatings or important PCB cosmetics, you should use alcohol to remove flux residue.
One of the more common fluxes is water-soluble organic acid flux. Common weak acids in organic acid fluxes include citric acid, lactic acid, and stearic acid. Weak organic acids are mixed with solvents such as isopropyl alcohol and water.
Organic acid flux is stronger than rosin flux and can remove oxides faster. In addition, the water-soluble nature of organic acid flux allows the PCB to be easily cleaned with plain water, just protecting those components that don't get wet. Since OA residues are conductive and can affect the operation and performance of the circuit, remove flux residue after you are done welding.
Inorganic acid flux works better with stronger metals such as copper, brass, and stainless steel. It is a mixture of strong acids such as hydrochloric acid, zinc chloride, and ammonium chloride. Inorganic acid flux requires thorough cleaning after use to remove any corrosive residue on the surface that can weaken or destroy welding joints if left in place. Inorganic acid flux should not be used for electronic assembly or electrical work.
The fumes released when welding includes several chemical compounds from the acid and its reaction with the oxide layer. Other compounds such as formaldehyde, toluene, alcohol, and acid fumes are often present in welding fumes. These fumes can cause asthma and increase sensitivity to welding fumes. Ensure adequate ventilation and use a respirator if necessary.
Highqualitypcb SMT: Since the boiling point of welding is several times higher than the boiling temperature of the flux and the melting temperature of welding, the cancer and lead risks from welding fumes are very low. The greatest lead risk is from the handling of the welding itself. Special care should be taken when using welding, wash your hands carefully, and avoid eating, drinking, and smoking in areas with welding to prevent welding from entering the human body.Flux residues may cause harm to PCB.