Introduction

One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. The next final finish 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.

What is OSP?

OSP (Organic Solderability Preservative) 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.

When discussing OSP PCB, it’s important to highlight both its advantages and limitations. For instance, the OSP PCB finish provides a lead-free solution but may pose challenges with handling and shelf life. Comparing the OSP finish to other options, like ENIG vs OSP, helps in selecting the right final finish.

Key Characteristics:

• Thickness: 5.9 to 11.8 µin [0.15 to 0.30 µm]
• Cost: Lower than most surface treatment technologies
• Application: Mixed metal applications, such as electroless nickel immersion gold (ENIG)

Common OSP Finishes:

• ENTEK CU-56: Used for single reflow process assemblies, not widely utilized due to mixed technology boards.
• ENTEK CU-106A: Prevalent version, suitable for multiple thermal assembly operations.

Pros and Cons of OSP

Pros:

• Inexpensive and suitable for fine pitch surface mount designs
• Environmentally friendly
• Provides a coplanar surface
• Lead-free (Pb-free)
• Ideal for mixed metal applications

Cons:

• Sensitive to handling
• Limited shelf life
• Difficulties with in-circuit testing (ICT)
• Exposed copper after assembly
• Issues with multiple reflow cycles and solder paste removal

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.

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