Soldering ball on substrates is a chemical process wherein the corrosion reaction between metal and oxygen while soldering at high temperatures. In order to prevent it, you must apply soldering flux during reflow. The purpose of flux paste is to deoxidize the metal surface easily and accelerate the soldering reflow. However, cautious removal of heavy or violent activators became a challenge because they are acidic or corrosive, allowing the solder joints to be affected.
There are typically 3 types of soldering fluxes widely used: rosin core flux, mildly activated rosin & water-soluble flux. Generally, electronic products will select the water-soluble flux, which is easily cleaned with distilled and deionized water with some detergents. However, it has a higher activity, which leaves a residue that must be cleaned thoroughly. The residue is corrosive and can damage the components if not cleaned properly after use. If you can find the neutral and electrically non-conductive flux, it can be completely washed off with water and the residues are neither hygroscopic nor electrically conductive.
There are four constituents of flux that can affect electrical failure: activators, binders, solvents, and addictives. Activators are weak organic acids that pose a short risk concern they may react with metal oxides to form metal salts. After wetting, the salts dissolve and a metallurgical bond or electro-migration is formed. Another one is binders which are insoluble in water with high melting points. Binders can prevent activators from dissolving in water but also form a visible residue. The proper soldering profile must be followed accordingly to ensure the solvent is evaporated away completely. Other addictive such as plasticizers dyes or antioxidants also need to be well considered. What is most important is that the space between traces continues to be smaller, the risk of short failure becomes higher. We need to well understand the chemistry, process, and technologies to avoid defect failures.
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