ANS:B - prevent oxide formation.

The use of flux during soldering is primarily done to prevent oxide formation. Flux serves several purposes in the soldering process, but its primary function is to clean and prepare the surfaces to be soldered by removing oxides and contaminants. Oxides can form on the surface of metals being soldered due to exposure to air or other environmental factors, and these oxides can inhibit the formation of a strong bond between the solder and the metal surfaces. Here are some key functions of flux in soldering:

1. Preventing Oxide Formation: Flux removes existing oxides from the metal surfaces and prevents new oxides from forming during the soldering process. This ensures good wetting and bonding between the solder and the surfaces being joined.
2. Promoting Wetting: Flux helps the solder flow and spread evenly over the surfaces to be joined, a process known as wetting. By reducing surface tension, flux improves the solder's ability to form strong, uniform bonds with the metal surfaces.
3. Removing Contaminants: Flux can also remove other contaminants such as grease, dirt, and residues from the surfaces, ensuring clean and well-prepared surfaces for soldering.
4. Increasing Fluidity: While flux can contribute to the fluidity of the solder by cleaning the surfaces and reducing surface tension, its primary purpose is not to lower the melting temperature of the solder. However, some flux formulations may contain additives that can help reduce the solder's melting temperature and improve flowability.
5. Washing Away Surplus Solder: Flux does not wash away surplus solder. Instead, excess solder is typically removed by wiping or using a desoldering tool after the joint has been soldered.
6. Filling up Joint Gaps: Flux does not fill up joint gaps. Proper joint preparation and fit-up are essential for achieving strong and reliable solder joints. Flux helps improve the wetting and bonding of the solder within the joint, but it does not fill gaps or voids between the surfaces being joined.