Introduction

One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. The next final finishes to discuss in this series is OSP. As with all surface finishes there are pros and cons with the decision of which to use. It is a combination of application, cost, and the properties of the finish. OSP is RoHS-compliant as there is zero lead content in the finish.

Final Finishes

OSP (Organic Solderability Preservative)

OSP is a thin organic coating, typically 5.9 to 11.8 µin [0.15 to 0.30 µm] thick, designed to prevent the oxidation of copper to maintain solderability over an extended period. Compared with other surface finish technologies, OSP is somewhat different. It uses a chemical process to produce an organic film on the bare copper surface which acts as a barrier to copper oxidation. OSP is organic, not metallic, and its cost is lower than most surface treatment technologies.

As with all surface finishes, the primary purpose is to protect the solderable surfaces on the PCB from oxidation and to aid in assembly soldering. This process coats a very thin coating of an organic material that inhibits copper oxidation. It is so thin that it is nearly impossible to see and measure. The organic material is removed by the assembly flux. Boards that have been OSP coated will have bright copper pad coloration. OSP is specially designed for mixed metal applications, such as electroless nickel immersion gold (ENIG). The OSP selectively deposits on copper while leaving gold connectors or metallic heat sinks free of contamination.

There are a couple of common OSP finishes widely used in the industry:

ENTEK CU-56: This is used for assemblies that will only go through a single reflow process. This finish is not being utilized much anymore because of the use of mixed technology boards (SMT and through-hole).

ENTEK CU-106A: This is the most prevalent version primarily due to the ability to survive multiple thermal assembly operations. PCBs that have multiple surface finishes can use the CU-106A(X) finish.

Pros and Cons of OSP

Pros

• Inexpensive and typically used in fine pitch surface mount designs
• Environmentally friendly
• Provides a coplanar surface
• Lead-free (Pb-free)
• Mixed metal applications

Cons

• Press fit pin insertion
• Handling sensitive
• Limited shelf life
• In-circuit test (ICT) difficulties
• Exposed copper after assembly
• Multiple reflow cycles and solder paste removal are problematic

Understanding the cost drivers in PCB fabrication and early engagement between the designer and the fabricator are crucial elements that lead to cost-effective design success. Following your fabricator’s DFM guidelines is the first place to start.