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What is OSP PCB? Meaning, Pros and Cons

OSP PCB enjoys significant dominance over other board types. It uses OSP as the protective coating, an inexpensive, lead-free film with several other benefits. Below, we explain what it means to have your circuit board coated with the material, including the good and bad sides.

What is OSP PCB?

It is a bare circuit board with OSP as its surface finish. OSP full-form in PCB manufacturing is Organic Solderability Preservative. It means a type of coating applied to bare boards. It primarily protects the copper surfaces against oxidation. You can also call it anti-tarnish.

PCBs contain copper layers. Copper is an active metal that readily oxidizes when exposed to the elements. The oxidation reduces solderability and affects the board’s overall reliability.

Applying a coating protects the copper (and the PCB) against this damage. The coating can be many different materials. One of them, and one of the most popular, is organic solderability preservative.

Organic solderability preservative contains no toxins, is inexpensive, and offers application ease. Other coating materials include HASL, ENIG, ENIPIG, and immersion tin or silver: more about the compound and its properties below.

The thickness characteristics of organic solderability preservatives
The thickness characteristics of organic solderability preservatives
Resource: https://www.semanticscholar.org

OSP PCB Material

The preservative is organic and available in three distinct types: Rosin, Active resin, and Azole. Azole is the most popular of them. It belongs to a group of compounds consisting of benzimidazoles, benzotriazoles, and imidazoles.

The material exhibits various characteristics that make it usable as a circuit board protective coating. First, it forms a thin, flat layer typically 0.2 to 0.5 microns thick.

Secondly, it readily adheres to the copper surfaces through the adsorption process. During soldering, it readily disintegrates and evaporates in the presence of soldering flux.

All these qualities make the material easy to apply. Additionally, it’s lead-free and safe to use on printed circuit boards. That also means it complies with RoHS standards.

OSP finish on PCB copper layers
OSP finish on PCB copper layers
Resource: https://www.researchgate.net

OSP PCB Advantages and Disadvantages

If choosing between organic solderability preservatives and other materials for your circuit board project, you might want to understand its benefits first. You also want to know its shortcomings. Using the preservative to coat your bare boards offers these advantages and disadvantages.

Advantages

  • The finish is inexpensive and easy to apply, which reduces the boards’ manufacturing costs
  • PCB OSP finish is lead-free and won’t harm the environment
  • It shows good wettability during the soldering process
  • The coating is thin and flat, which is an advantage for the reflow process and SMT components
  • These boards are easy to rework or repair. Soldering flux easily removes the film
  • The boards require minimal amounts of solder mask than when using most other coatings

Disadvantages

  • The film is less resistant to abrasion damage, making the boards require great care when handling or storing them
  • The preservative is colorless. This property makes it transparent and challenging to inspect
  • The OSP PCB shelf life is comparatively short, below 12 months
  • The boards are demanding to store, and you cannot keep them for too long before assembly
OSP finish on a circuit board transmission line
OSP finish on a circuit board transmission line
Resource: https://www.researchgate.net

OSP Process in PCB Manufacturing

OSP PCB finish application is a multi-stage process that requires careful execution, or problems might occur, causing solderability and other issues. The main application steps are degreasing, micro-etching, film application, and drying.

Step 1: Cleaning

It removes contaminants that may have accumulated or handling oils and other dirt. The cleaning solution must remain within a specified PH value (4.0-7.0). If not, the board may not attain the required cleanliness level.

Step 2: Etching

Next, the board undergoes a micro-etching process. Etching helps remove any remaining oxidation from the copper surface. It also roughens the copper to prep it for the film application. Another rinse is also necessary before forming the coating.

Step 3: Applying Film

The board goes into a reservoir or passes through a conveyor system for film formation. The material sticks to the copper. After application, the PCB goes through another cleaning process to rinse it.

Step 4: Film Drying

The coating goes to the drying stage. The drying is a highly controlled process to ensure film quality. After drying, the board undergoes a cleaning. After that, it is ready for mounting with components. You may also store it for later assembly.

OSP PCB oxidation tests
OSP PCB oxidation tests
Resource: https://www.researchgate.net

OSP PCB Finish Problems

OSP problems occur when applying the film or during handling (transportation) and storage. They include discoloration and degradation. Causes include the following.

  • Exposure to high temperatures
  • High humidity levels
  • Abrasion or friction

Avoid exposing the board to excessive heat levels. For instance, high OSP PCB baking temperatures can lead to discolorations. It may even degrade the film and cause it to lose its protective characteristics.

Most issues are storage-related, so it helps to understand how and where to keep these circuit boards. The following section has more about how to do it.

How to Store OSP PCB

Manufacturers assemble OSP PCB boards shortly after their fabrication. In cases where assembly is not possible, proper storage is necessary. Various rules apply when storing coated circuit boards to avoid film degradation.

Maintain up to 70% relative humidity level in the storage room. It will help prevent oxidation. The storage temperature matters, too. Keep it below 30 °C to avoid damaging the thin film before assembly.

Avoid storing the boards longer than six months before mounting them with components. The film may degrade. Should you exceed the recommended shelf life, consider having it applied again.

   Visual depiction of the two coating types
Visual depiction of the two coating types
Resource: https://www.researchgate.net

PCB OSP vs. ENIG

Your choice of PCB surface finish impacts its quality and longevity. OSP and ENIG are two popular finish types. But while both are RoHS—compliant and exhibit excellent solderability, they have a few differences.

  • Material: OSP stands for Organic Solderability Preservative. ENIG is an abbreviation for Electroless Nickel Immersion Gold.
  • Application Ease: ENIG is also the more expensive option, seeing the material costs more and its application is complex.
  • Shelf Life: OSP PCB surface finish has a shorter shelf life of about 6-12 months, while ENIG can stay for 12 and longer.
  • Cost: It costs more and requires a complex application process.

These differences should help you to choose between the two PCB types. OSP boards are best for high production runs with tight deadlines. On the other hand, ENIG boards are better if the application requires higher tolerance levels.

Conclusion

An OSP-coated PCB uses an organic solderability preservative as the protective coating for the copper surfaces. It helps prevent oxidation, improving solderability and protecting the PCB quality. Various reasons explain its popularity. It costs less, is safe to use, and forms a thin, flat layer for reflow and SMT assembly processes, among other benefits.

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