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What is OSP Surface Finish?
OSP or Organic Solderability Preservative is an affordable surface finish that provides temporary protection of copper traces on printed circuit boards from oxidation to maintain solderability.
Printed circuit boards serve as the foundation of electronics, providing the mechanical structure and electrical connections between components. For these connections to function properly, the copper traces on PCBs require protective coatings known as surface finishes. There are various options for PCB surface finishes, each with its advantages and tradeoffs. OSP is one common type of affordable PCB finish that provides temporary protection against oxidation and enhances solderability. Compared to other finishes like Electroless Nickel Immersion Gold (ENIG) or Immersion Silver, OSP is low-cost but also has a shorter usable lifespan. This article will examine what OSP surface finish is, its benefits and limitations, and when it may be the best choice for a PCB application. With the right understanding of OSP properties, PCB designers and manufacturers can utilize it effectively as part of their surface finish process.
What is OSP Surface Finish?
OSP stands for Organic Solderability Preservative, and it is a widely used surface finish for printed circuit boards. As the name implies, it is an organic compound applied as a dry film onto the copper traces of a PCB to preserve solderability and prevent oxidation. OSP belongs to a class of preservative coatings known as azole-based organic compounds, and the main materials are rosin, active resin, and azole. These compounds bond effectively with copper to create a temporary barrier against oxygen in the air, which helps keep the copper from oxidizing and tarnishing. The OSP coating provides a protective layer measured in microinches that allows the underlying copper to remain solderable. It is typically applied after PCB fabrication using an inexpensive process of dipping boards in an OSP solution. Compared to metallic finishes like ENIG or Immersion Silver, OSP can be applied easily without plating equipment. It serves as an affordable and straightforward surface finish option to maintain copper solderability. The tradeoff is that OSP does not last as long as other finishes before needing to be soldered.
Benefits of OSP surface finish
OSP offers several advantages that make it an appealing surface finish option for many PCB applications:
- Low Cost – The OSP application process is relatively simple and does not require expensive plating equipment. OSP chemicals are affordable, especially compared to processes like ENIG or Immersion Silver plating. This allows OSP to be a low-cost surface finish, which is ideal for high-volume consumer electronics where cost control is critical.
- Easy Application – OSP can be applied through a simple dipping process, without the need for complex plating steps. This means small PCB shops can easily apply OSP in-house without significant investment in equipment. The process is also fast, making it well-suited for quick-turn PCB production.
- Temporary Oxidation Protection – The OSP coating prevents oxygen from reaching the copper traces, effectively protecting against oxidation and tarnish for a period of time. This preserves solderability and allows the PCB to be stored and shipped. While not as robust as other finishes, it provides useful temporary protection.
- Good Solderability – When fresh, OSP allows for excellent wetting of solder on the PCB pads during assembly. This ensures good solder joints and reliable connections between components. The organic coating promotes solder flow.
Overall, the low cost, simple application, temporary oxidation resistance, and decent solderability make OSP an advantageous choice for many cost-driven PCB applications.
Drawbacks of OSP surface finish
While having some clear benefits, OSP also has some downsides to consider:
- Limited Durability – OSP only protects copper from oxidation for a relatively short period of time, typically around 6 months. The organic coating has a tendency to get degraded, especially in humid environments. This limited lifespan means OSP is not suitable for long-term storage or applications with a long shelf life.
- Potential Copper Oxidation – Once the OSP finish expires, the copper traces become vulnerable to oxidation, which can interfere with soldering and create quality issues. Any handling or damage of the PCB after the OSP wears off can accelerate oxidation.
- More Handling Considerations – Since the OSP coating is relatively thin and fragile compared to metallic finishes, the PCBs require careful handling to avoid damaging the traces. This can add complexity to PCB assembly and soldering processes.
- Lower Temperature Tolerance – The organic compounds used in OSP can break down at high soldering temperatures beyond 250°C. This imposes some limits on assembly methods.
While the short protective effect can be acceptable for quick-turn assemblies, it does present a risk of oxidation and potential solderability issues later if the boards are not handled properly. The thin organic coating also demands gentler handling than with sturdier finishes.
OSP vs. ENIG vs. Immersion Silver: Which is Better?
OSP stands apart from other common finishes like ENIG (Electroless Nickel Immersion Gold) and Immersion Silver when it comes to cost, lifespan, and performance.
ENIG provides a highly durable nickel and gold metallic coating with a long shelf life exceeding 12 months. However, the multi-step plating process requires substantial investment in equipment. This makes ENIG an expensive premium finish.
Immersion Silver involves dipping boards in a silver solution to deposit a layer of silver on the copper traces. It lasts longer than OSP at 12-18 months and provides excellent solderability. But it is still pricier than OSP and requires more controlled plating equipment.
OSP differs as an organic coating applied through a simple and affordable dipping process. It cannot match the durability and shelf life of ENIG or Immersion Silver, lasting about 6 months. But for short term use, OSP provides a cost-effective temporary preservative.
For low-cost consumer electronics with quick assembly times, OSP is often the ideal fit. For industrial, automotive, or military applications requiring long-term reliability, ENIG, and Immersion Silver are better suited, despite their higher costs. OSP fills an important niche as a budget-friendly finish for rapid, high-volume PCB production.
When to Use OSP Finish?
With its distinct properties, OSP is best suited for certain PCB applications:
- High-volume consumer electronics – The low cost of OSP makes it a great fit for consumer electronics like phones, TVs, and gaming devices that require lots of PCBs. The quick assembly time frames match well with OSP’s 6 month lifespan.
- Prototyping and short product cycles – For prototypes and new products that may see design changes, OSP provides an affordable temporary finish. ENIG and Immersion Silver would be overkill for prototyping runs.
- Budget concerns – For any cost-sensitive application where the PCB finish needs to be kept inexpensive, OSP is an excellent choice to save money.
- Simple board designs – OSP can be used effectively on simple single-sided and double-sided boards without fine pitch components. More complex boards are better suited for durable finishes.
- Quick soldering after fabrication – If the PCBs will be assembled almost immediately after fabrication, OSP provides enough temporary protection. But shelf life should not exceed 6 months.
In general, OSP works best on high volume, cost-focused PCBs that will get assembled and soldered promptly, without the need for long term storage and operation. For complex, expensive boards with longer shelf lives, other finishes like ENIG or Immersion Silver may be preferable.
Manufacturing Process of OSP
Here is an overview of the typical OSP manufacturing process:
Step 1: Surface Preparation – The bare copper traces on the PCB are first cleaned to remove any dust, oils or residues. A microetch step then roughens the top layer of copper which improves adhesion of the OSP coating.
Step 2: OSP Application – Boards are immersed in a solution containing OSP chemistry, usually by dipping in tanks. The organic compounds in the OSP solution bond to the copper surfaces. Immersion time is controlled to apply the desired OSP thickness, typically ranging from 0.1 to 0.5 microns.
Step 3: Drying/Curing – After immersion, boards are rinsed and then dried either at room temperature or in an oven at low temperatures around 80-100°C. This evaporates any solvents and fully cures the OSP onto the copper traces.
Step 4: Testing – Quality testing validates the OSP coatings meet requirements for thickness, coverage uniformity and proper application. Solderability testing through methods like rosin flux also verifies effectiveness.
The OSP application itself is fast and straightforward compared to plating processes. But surface preparation and rinsing are critical steps for removing contaminants and achieving uniform, high quality OSP coatings. The curing process also helps maximize coating durability and product shelf life after finishing. Overall, the OSP process takes full advantage of the solderability benefits of OSP while minimizing potential limitations.
How to store OSP PCB?
Here are some tips for properly storing PCBs with OSP surface finish:
- Use moisture barrier bags – OSP coating can degrade faster when exposed to humidity and moisture. Store boards in sealed ESD/moisture barrier bags. Include desiccant packs to absorb excess moisture.
- Control storage temperature – Ideal storage temps are 10-25°C. Avoid temperature extremes that could soften or embrittle the OSP coating. Storing in climate controlled conditions is best.
- Manage storage time – OSP shelf life is typically under 6 months. First-in-first-out (FIFO) inventory management helps avoid exceeding the coating’s lifespan.
- Avoid direct sunlight – UV exposure can break down OSP over time. Keep boards away from windows and direct light.
- Protect from physical damage – Careful handling reduces risks of scratching the coating. Use trays, racks or packaging to avoid touching trace areas.
- Keep clean and dry – Prevent contamination from dirt, dust or liquids which could compromise SMT assembly. Clean boards using isopropyl alcohol only.
- Test solderability – Spot check solderability periodically with nondestructive tests. This verifies the OSP is still effective.
With careful storage and handling, PCBs with OSP finish can be kept solderable and protected from oxidation for their intended usage period. Proper moisture control and temperature are especially key for maximizing OSP shelf life.
In summary, OSP or Organic Solderability Preservative provides a low-cost surface finish option to temporarily protect copper PCB traces against oxidation. While not as durable as finishes like ENIG or Immersion Silver, OSP serves an important role for quick-turn, high volume consumer PCBs thanks to its solderability and cost-effectiveness. By understanding where OSP is most appropriate, PCB designers and manufacturers can utilize it strategically as part of their surface finish process.
As a leading professional PCB manufacturer, JHYPCB provides reliable OSP finishing along with services like prototyping, mass production, and turnkey PCB assembly. Contact us today to discuss your PCB OSP needs and get advanced PCB solutions tailored to your application’s requirements. JHYPCB delivers both technology expertise and manufacturing excellence for all your circuit board needs.