What is OSP - Organic Solderability Preservative?

PCB Basics 

What is Organic Solderability Preservative or Anti-Tarnish Surface Finish in a PCB?

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Editorial Team - PCB Directory

Jan 15, 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, and creates problems when soldering during component placement. OSP or Organic Solderability Preservative does two things: temporarily protect exposed copper from being oxidized and improves solderability before component fixation (assembly).

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

Advantages of OSP:

  • 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
  • It provides a coplanar surface, well suited for tight-pitch pads (BGA, QFP).

Disadvantages of OSP:

  • 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 Finish Process:

The OSP finishing process is composed of three major steps, including pre-cleaning and making the PCB board ready for smooth OSP coating application:

  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 final OSP application, the OSP solution is populated by ions so that it can be eliminated easily during soldering.

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