what is osp in PCB

[OSP (Organic Solderability Preservative)] is a film that chemically grows a layer of organic copper complex compound on the surface of copper. This organic film protects the clean bare copper on the board from rust (vulcanization or oxidation) in contact with air in a normal storage environment, and can be easily soldered and diluted during PCBA board assembly. The acid quickly removes and exposes the clean copper surface to form a weld with the molten solder.