Meaning
Thin organic coating applied to the copper pads of printed circuit boards is designed to prevent copper oxidation during storage and transport prior to assembly. This surface treatment, known as OSP surface finish, is typically a water-based compound containing azole-based organic molecules that selectively bond with the copper atoms to form a protective, monomolecular film. The coating is applied using a simple conveyorized spray or dip process that is more cost-effective and environmentally friendly than metallic finishes like ENIG or HASL.
It provides a flat surface that is ideal for mounting fine-pitch surface mount devices, but its protective properties degrade under the high temperatures of sequential reflow soldering. The application of this finish is restricted to boards that will be assembled within a defined storage window, usually six to twelve months, and it does not provide a durable surface for electrical testing or wire bonding.
Chemical Coating
Deposition of the protective organic layer relies on a chemical reaction between the azole compound and the clean copper surface of the board pads. The process starts with a series of pre-treatment steps, including acid cleaning, micro-etching, and rinsing, to remove any existing oxides or contaminants and create a clean, uniform copper surface. The board is then exposed to the organic solderability preservative solution, which is maintained at a specific temperature and pH to ensure consistent film formation.
The organic molecules react with the copper to form a self-limiting, continuous layer with a thickness of typically 0.2 to 0.5 micrometers. The board is then dried and packaged in protective bags to prevent exposure to moisture and light, which can degrade the coating.
Assembly Handling
Handling of boards with this surface finish during the assembly process requires care to avoid damaging the delicate organic film and causing solderability issues. Operators must wear gloves to prevent oils and moisture from their hands from dissolving the coating, and the boards should be processed immediately after being removed from their vacuum-sealed packaging. Because the organic layer is thin and can be damaged by mechanical abrasion, scraper blades on stencil printers and pick-and-place nozzles must be calibrated to minimize physical contact with the coated pads.
These precautions ensure that the protective layer remains intact until the board enters the reflow oven, where the solder paste will displace the organic film and form a direct metallurgical bond with the copper.
Solder Joint
Formation of the solder joint on an organic solderability preservative coated pad occurs as the flux in the solder paste dissolves the organic film during the heating cycle of the reflow process. Once the coating is removed, the clean copper is exposed to the liquid solder, allowing the tin in the solder to react with the copper to form a strong, uniform Cu6Sn5 intermetallic compound. This direct bonding results in solder joints with excellent mechanical and electrical properties, without the risk of defects like gold embrittlement or black pad that can affect metallic finishes.
However, if the board is subjected to multiple reflow cycles, the organic coating on unpopulated pads can break down, leading to copper oxidation and subsequent wetting issues during the second-side assembly, which requires careful optimization of the reflow profile and flux activity.
