Surface Finish
Temporary surface finishes applied to exposed copper on printed circuit boards protect the metal from oxidation during storage and transit. An organic solderability preservative, commonly known as OSP, is a thin, carbon-based coating that chemically bonds to the copper pads. This finish prevents the copper from reacting with atmospheric oxygen while remaining compatible with subsequent soldering processes.
Deposition Process
The chemical reaction between the azole compounds in the solution and the copper atoms on the board surface forms a thin organometallic layer. To apply OSP, the clean printed circuit board is passed through a conveyorized chemical bath containing benzotriazole or alkylbenzimidazole. This organic layer is extremely thin, typically ranging from two tenths to five tenths of a micrometer in thickness.
Unlike metallic finishes such as hot air solder leveling, this organic coating does not create a dome-shaped pad, which provides a highly coplanar surface for the placement of fine-pitch components. The solution must be carefully monitored to maintain the correct temperature and pH to ensure a uniform deposition thickness across the entire board.
Assembly Performance
The protective organic layer must degrade cleanly during the solder reflow process to allow the liquid solder to wet the copper pad. When the assembly passes through the reflow oven, the thermal energy breaks down the OSP molecules, and the flux in the solder paste dissolves the remaining organic residue. This allows the tin-lead or lead-free solder to form a reliable intermetallic bond directly with the fresh copper surface.
If the organic coating is too thick, it may not dissolve completely, leading to poor wetting and weak solder joints.
