What is OSP (Organic Solderability Preservative) PCB Surface Finish?

Organic Solderability Preservatives (OSPs) are a type of surface finish applied to the copper of a printed circuit board (PCB). The OSP process involves coating the copper surface with a thin layer of organic material that bonds with the copper and protects it from oxidation and other environmental contaminants. The organic layer acts as a barrier against corrosion while providing a conductive surface for electrical connections.

PCB surface finishes protect exposed copper on a PCB from oxidation and ensure a reliable connection between components and the board. They are crucial for the performance, reliability, and lifespan of the PCB, and impact manufacturing quality. They also impact the manufacturing process, including the quality of the soldering and assembly.

In this article, we will explore the concept of Organic Solderability Preservatives (OSPs) as a PCB surface finish, what is it, and the benefits and applications of using it in your PCB.

Organic Solderability Preservatives (OSP) Surface Finish

The thickness of the organic coating is typically just a few nanometers. Although it is almost transparent, it effectively prevents the copper from reacting with other contaminants. Compared to immersion silver, OSPs are generally considered to be more durable and have a lower risk of developing shorting problems.

OSPs are suitable for a variety of applications, including those that involve very fine traces or flat surfaces like membrane switches and EMI shielding. They are also commonly used in aluminum wire bonding applications due to their ability to provide a clean, oxidation-free surface for bonding.

The Process of Coating Organic Solderability Preservatives (OSPs)

The process of coating Organic Solderability Preservatives (OSPs) onto a printed circuit board (PCB) typically involves the following steps:

1. Cleaning the PCB: The first step is to clean the PCB thoroughly to remove any contaminants that could interfere with the OSP coating process. This typically involves a combination of chemical and mechanical cleaning processes.
2. Applying the OSP Coating: Once the PCB has been cleaned, the OSP coating is applied to the surface of the copper using a variety of methods. This can include spraying, dipping, or brushing the coating onto the surface.
3. Curing the Coating: After the OSP coating has been applied, the PCB is typically subjected to a curing process to ensure that the coating adheres properly to the surface of the copper. The curing process can vary depending on the type of OSP coating being used but often involves heating the PCB to a specific temperature for a set amount of time.
4. Testing the Coating: Once the OSP coating has been applied and cured, the PCB is typically subjected to a series of tests to ensure that the coating is performing as expected. This can include testing the surface tension, solderability, and adhesion of the coating.

Overall, the process of coating Organic Solderability Preservatives (OSPs) onto a PCB is a critical step in the manufacturing process.

Advantages of OSP Surface Finish

Advantages of Using Organic Solderability Preservatives (OSPs) as a PCB Surface Finish:

1. Cost-Effective: OSP is generally a cost-effective surface finish option compared to other alternatives such as gold plating or immersion tin. It requires fewer processing steps and materials, resulting in lower overall production costs.
2. Environment Friendly: OSP is a water-based surface finish, making it more environmentally friendly than some other finishes that may contain heavy metals or hazardous chemicals. It is also easier to dispose of, reducing the environmental impact.
3. Good Solderability: OSP provides a flat, uniform surface that enhances solderability. It forms a protective layer on the surface of the PCB that prevents oxidation and provides a clean and solderable surface for component attachment.
4. Excellent for SMT Applications: OSP is well-suited for surface mount technology (SMT) applications. It offers good planarity, allowing for the accurate placement of components during assembly. It also provides a relatively flat surface, facilitating proper solder paste deposition and reflow soldering.
5. Shelf Life: PCBs with OSP finish generally have a longer shelf life compared to finishes like bare copper. The OSP layer protects the copper traces from oxidation during storage, ensuring good solderability even after an extended period.

Disadvantages of OSP PCB Surface Finish

1. Limited Shelf Life: While OSP offers good shelf life compared to some other finishes, it is still susceptible to degradation over time. The protective layer can deteriorate when exposed to heat, humidity, or harsh environmental conditions. Therefore, PCBs with an OSP finish should be used within a reasonable timeframe.
2. Sensitivity to Contamination: OSP surfaces are sensitive to contamination, such as fingerprints, oils, and other organic substances. Even small amounts of contamination can affect solderability and lead to solder joint defects. Special handling precautions are required during assembly to prevent contamination.
3. Thickness Limitation: OSP is a relatively thin surface finish, typically ranging from 0.2 to 0.5 μm. This limited thickness may not provide sufficient protection against harsh environments or high-frequency applications that require thicker coatings for better performance and durability.
4. Limited Solderability Window: OSP surfaces have a narrow solderability window, meaning that the PCB must be soldered within a specific timeframe after applying the finish. If the soldering process is delayed, the OSP layer may lose its solderability, resulting in poor solder joints.

It's important to consider these advantages and disadvantages while selecting a surface finish for a PCB. The specific requirements of the project, such as environmental conditions, solder joint reliability, assembly processes, and cost considerations, should be carefully evaluated to determine if OSP is the most suitable choice.

Industry Applications where OSPs are used as a Surface Finish

Organic Solderability Preservatives (OSPs) are widely used as a surface finish option for printed circuit boards (PCBs) in various applications. Here are some of the common applications of OSP as a PCB surface finish:

• Consumer Electronics: OSP is extensively used in consumer electronic devices such as smartphones, tablets, smartwatches, laptops, and gaming consoles. Its cost-effectiveness, good solderability, and compatibility with surface mount technology (SMT) make it suitable for high-volume consumer electronics manufacturing.
• Telecommunications: OSP is employed in telecommunications equipment, including routers, modems, telecommunication switches, and base stations. The good solderability and flat surface provided by OSP are advantageous for reliable solder joint formation in these devices.
• Automotive Electronics: OSP is used in automotive electronics, including vehicle control units, infotainment systems, sensors, and lighting systems. It offers a cost-effective surface finish option for these applications while providing good solderability and protecting the PCBs during the manufacturing and operational stages.
• Industrial Equipment: OSP finds application in various industrial equipment such as control systems, industrial automation devices, power supplies, and monitoring equipment. Its solderability and cost-effectiveness make it suitable for these rugged and often high-volume applications.
• Medical Devices: OSP is utilized in medical devices and equipment, including patient monitoring systems, diagnostic devices, imaging equipment, and laboratory instruments. It provides a reliable and cost-effective surface finish option for these critical applications.
• Aerospace and Defense: OSP is used in the aerospace and defense industries for PCBs used in avionics, radar systems, communication equipment, and military-grade electronics. Its good solderability and environmental friendliness make it suitable for these applications.
• Industrial Control Systems: OSP is employed in industrial control systems, including programmable logic controllers (PLCs), motor control systems, and industrial automation equipment. Its cost-effectiveness and compatibility with SMT processes make it a popular choice for these applications.

It is worth noting that the specific surface finish requirements may vary depending on the application, environmental conditions, industry standards, and customer preferences. While OSP is suitable for many applications, certain specialized applications may require alternative surface finishes such as gold plating, immersion tin, or ENIG (Electroless Nickel Immersion Gold) for enhanced performance and reliability.

Conclusion

In conclusion, Organic Solderability Preservatives (OSPs) offer a cost-effective and environmentally friendly solution as a PCB surface finish. They provide good solderability, protection against oxidation, and compatibility with surface mount technology (SMT) applications. While OSPs have limitations such as a limited shelf life, sensitivity to contamination, and a narrow solderability window, they are widely used in consumer electronics, telecommunications, automotive, medical devices, aerospace, and industrial control systems. When considering OSP as a surface finish, it is essential to evaluate specific application requirements and factors such as environmental conditions, solder joint reliability, assembly processes, and cost considerations.

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.