What is PCB OSP Surface Finish? - The Ultimate Guide is Here

In Printed Circuit Boards, OSP stands for Organic Solderability Preservative, which mainly applies to copper pads. It selectively bonds to copper to prevent it from tarnishing on exposure to moisture. PCB OSP surface finish is environmentally friendly, given its lead-free material, and requires low maintenance.

This post provides an Ultimate Guide to PCB OSP regarding its advantages, disadvantages, applications, and how it compares with other surface finishes.

What is PCB OSP?

OSP surface finish is an organic applied on a bare copper circuit board. Organic Solderability Preservative is an anti-tarnish surface finish whose primary function is to prevent the printed circuit boards' copper layer from oxidation.

OSPs are a water-based chemical compound that belongs to the azole family. The azole family has known organic compounds such as benzotriazoles, benzimidazoles, and imidazoles. These compounds employ coordinate bonding and adsorption upon contact with the copper surface leading to the production of the surface finish film.

The need for a Surface finish in Printed Circuit Boards

In manufacturing printed circuit boards, printed circuit boards are usually laminated with copper. Copper is a highly conductive material on which electrical connections on the printed Circuit Board depend. This conductivity allows PCBs to provide the needed power to run their applications efficiently.

On the flip side, copper has its caveat; it is chemically reactive. When exposed to atmospheric moisture, it oxidizes readily and hence tarnishes. This oxidation reaction reduces the copper PCB's efficiency and reliability.

To that end, there is a rising need to prevent copper PCBs from being exposed to moisture, hence the introduction of surface finishes. The surface finish coating on PCBs serves two primary functions: to protect against oxidation of copper and to maintain standard PCB quality after the final stages of Circuit Card Assembly.

Advantages of OSP

Here are some advantages that place Organic Solderability Preservative above other surface finishes applied during PCB Assembly process.

• Low cost: The use of OSP in PCB is economical and cost-effective, making it a popular surface finish across the PCB industry.
• Simple Manufacturing Process: OSP on Printed Circuit Boards allows for easy reworking and maintainability. It is, therefore, advantageous to PCB engineers to fix OSP coating in less time and cost if damaged.
• Better Board Performance: OSP provides better circuit board performance in solder wetting when flux joins the visas and pads.
• Environment-friendly: Since OSP is a water-based organic compound, it does not harm the environment or negatively impacts human health. Also, the use of Organic Solderability Preservative as surface finishes makes it RoHs compliant as it has no toxic material in its constituents
• Perfect for reflow in double-sided SMT assembly: OSP is widening its application as it is acceptable for both single-sided and double-sided SMT Assembly.

Disadvantages of OSP Surface Finish

While its pros are impressive, here are specific drawbacks to applying an OSP surface finish on your copper Printed Circuit Board.

1. Difficulty in measuring the thickness of surface finish: The Organic Solderability Preservative is an especially transparent thin film, rendering it challenging to ascertain which copper surface is exposed. Also, welding is impossible as its thickness is too narrow.
2. Short Shelf Life: The OSP Shelf-life is not more than six months after the overall process. While users may not immediately use this PCB with an OSP surface finish, they can send it back to their PCB manufacturers for reworking.
3. Not Applicable with Through hole technology: its short shelf life renders it unsuitable for Plated Through-holes
4. Reworking problems: it is challenging to rework as PCB Manufacturers will require more corrosive chemicals to wash off the old OSP. This process could also damage the copper surface rendering the circuit board obsolete.

Processes involved in OSP Surface Finishes

OSP has a long history with Surface Mount Technology; hence its manufacturing process appeals to this Circuit Card Assembly Technology. The following are the step-by-step processes for applying Organic Solderability Preservatives on Printed Circuit Boards.

• Cleaning
• Topography Enhancement
• Acid Rinse
• OSP Application
• Drying

Let's work you through these processes.

Cleaning:

Cleaning the copper circuit board is vital to remove any organic contaminants that would affect the solderability performance of PCB. Removal of such contaminants includes oil, oxidation film, fingerprints, and so on, ensuring the copper surface stays clean and bright.

In addition, proper cleaning helps build preservative quality and ensure even thickness of preservatives. Cleaning should follow a standard process not to affect the copper-printed circuit boards.

Topography Enhancement

In this process, micro etching is applied to eliminate possible oxidation on the copper surface. This process ensures the improvement of bonding forces between the copper foil and OSP solution. Micro etching requires maintaining a certain speed to provide a firm and smooth thickness. PCB Manufacturers keep micro etching speeds in the range of 1.0 to 1.5 um per minute.

Acid rinse.

After completion of topography enhancement, the printed Circuit Board undergoes an acid rinse called sulfuric acid.

OSP Application

The following process is the application of the Organic Solderability Preservative surface finish on the Printed Circuit Board. This process is done at room temperature and with limited exposure to air to prevent oxidation from building up on the Copper surface.

Before the last drying stage, the Printed Circuit Board undergoes deionization. This process ensures no contamination of the OSP solutions by ions that could tarnish the copper circuit board after the reflow.

Drying

After the OSP application and deionization process, the PCB is left to dry.

Drawbacks after OSP Application

When applying OSP surface finishes on Printed Circuit Boards, the PCB often changes color after soldering.

Many PCB manufacturer point to preservative thickness, micro-etching quantity, soldering times, and contaminants as leading causes.

Thankfully, this issue can be detected visually, which makes finding solutions possible. Two situations could occur during visual inspection.

1. During the soldering process, the solder flux can eliminate oxidation. However, this situation does not affect soldering performance; hence, no measures are necessary to prevent this situation.
2. In contrast, if the flux cannot eliminate oxidation, it would affect soldering performance. As such, some measures could be in place when this situation occurs.
   1. The thickness of the OSP layer must be kept in a specified range
   2. Controlling the amount of micro-etching within a specific range
   3. Elimination of all contaminants to prevent poor soldering performance.

Storage Requirements for OSP

There must be great care when storing Printed Circuit Boards with OSP as a surface finish. Organic Solderability Preservative is usually thin and could easily be exposed to the atmosphere during operation or transportation.

Upon exposure to the elements of the atmosphere, oxidation likely damages the printed circuit board's surface, leading to further damage in its assembly process and applications. Hence there is a need to apply strict storage and transportation of OSP Printed Circuit Boards.

Firstly, using vacuum packaging can prevent friction from destroying the surface of the printed circuit board.

Secondly, these PCBs should not be exposed to direct sunlight. There are requirements for an optimal storage environment, which include 30-70% Relative humidity and temperature between 15^oC to 30^oC with a storage span of fewer than 12 months.

While its process is straightforward and cost-effective, it is essential to note that OSP is sensitive to handling. As a result, it can easily retain scratches which would affect its solderability performance.

In contrast to all popular surface finishes, it has a shorter shelf life. Hence, after completing the assembly process, applying these PCBs with an OSP surface finish should take priority over storing them.

Factors to consider when choosing OSP Surface Finish

Choosing an OSP finish depends on several factors. It is imperative that you consider them, especially when you are on the hunt for surface finishes for your PCB.

Solderability: when utilizing OSP as a surface finish for your PCB, you must avoid solderability issues. When there is a problem in solderability, it restricts the PCB performance.

Duration of application: when implementing surface finishes, some surface finishes take longer than others. However, applying OSP surface finish takes less time to apply on the PCB.

Corrosion: another factor to consider is how vulnerable PCB material can be to corrosion before choosing an OSP surface finish. Copper PCBs often corrode upon exposure to air. So, applying the OSP surface finish is important before the atmospheric elements set in on the PCB surface.

Final Thoughts: When should you use OSP surface finish?

OSP surface finish is applicable in the PCB manufacturing process where there is a need to be RoHS compliant. Also, it is instrumental in surface mount technology, where the printed circuit board is flat and needs good wettability to get the best solderability performance.

Organic Solderability Preservative has a main distinguishable characteristic: it is water-based and lead-free, making it environmentally friendly. This guide has examined all the features of Organic Solderability Preservative surface finish.

If you are looking for alternatives to OSP surface finish, consider checking NextPCB blog for available finishes in the PCB market. If you have questions regarding OSP surface finishing or its application, kindly contact us via our homepage, and we shall respond affirmatively.