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.
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.
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.
Here are some advantages that place Organic Solderability Preservative above other surface finishes applied during PCB Assembly process.
While its pros are impressive, here are specific drawbacks to applying an OSP surface finish on your copper Printed Circuit Board.
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.
Let's work you through these processes.
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.
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.
After completion of topography enhancement, the printed Circuit Board undergoes an acid rinse called sulfuric acid.
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.
After the OSP application and deionization process, the PCB is left to dry.
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.
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 15oC to 30oC 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.
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.
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.