What is Organic Solderability Preservative (OSP) or Anti-Tarnish Surface Finish?

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Editorial Team - PCB Directory

Jun 14, 2020

The overall functionality of a PCB board depends upon the conductivity of copper tracks. These tracks tend to oxidize when exposed to the atmosphere, which creates a problem when soldering components on the board.  Organic Solderability Preservative (OSP) regulates two things; temporary protection of exposed copper from being oxidized and improving the solderability before component fixation (assembly).

OSP is a very thin (100-4000 Angstroms) organic coating on the PCB board, which is a water-based chemical compound of the ‘Azole family’ such as benzotriazoles, imidazoles, and benzimidazoles. This compound gets absorbed by the exposed copper and generates a protected film. 

Advantages of Organic Solderability Preservative:

  • Low cost
  • Environment-friendly
  • Re-workable, but can’t take more than 2-5 rounds of reflow soldering before degradation
  • It is lead-free and can handle SMT components easily
  • Provides a coplanar surface, well suited for tight-pitch pads (BGA, QFP).

Disadvantages of Organic Solderability Preservative:

  • Not Good for PTH (Plated Through Holes)
  • Short Shelf Life, less than 6 month
  • Inspection is difficult as it is transparent and colorless
  • requires careful handling, as it’s susceptible to mechanical damage

OSP Surface Finishing Process:

The OSP finishing process is composed of three major steps that include pre-cleaning and making the PCB board ready to apply a smooth OSP coating:

  1. Clean: Organic contaminants such as oil, fingerprints, oxidation film etc. are removed to get a clean PCB board.
  2. Topography Enhancement: To improve the bonding forces between exposed copper and OSP film, micro-etching is performed to minimize the oxidation generated on copper.
  3. Deionization Rinse: Before the final OSP application, the OSP solution is populated by ions so that it can be eliminated easily during soldering.

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