PCB Manufacturing

OSP PCB Surface Finish: An Introduction to Organic Solderability Preservatives

OSP PCB Surface Finish

Organic Solderability Preservative (OSP) is an organic PCB finish. As the name says, it contains only organic materials which will make this surface environmentally friendly and also compliant with ROHS restrictions. PCBs with this finish are mostly used in environments where there are no harsh conditions and also used when a lead-free solder process is required.

OSP Surface Finish Process

Fig1.0 below shows the step-by-step process of OSP finish. Boards are rinsed after every step in order to maintain a clean surface.

OSP Process

  • Cleaning the PCB Boards
    Boards are cleaned to remove contaminants such as oil, fingerprints and oxidation layer on the copper surface. It is very important to build a quality PCB board surface. If the boards are not cleaned efficiently, it would result in uneven surface coating in the subsequent process due to the chemical formation of contaminates left over. Chemical tubs should be regularly checked and cleaned in order to maintain the concentration of chemicals during the cleaning process.
  • Micro-etch Enhancement
    In this process, a small amount of copper surface is micro-etched to avoid the surface getting oxidized with the surrounding environment. Removing a certain amount of copper binds the copper and OSP solution better. In order to maintain the uniform finish all over the board, the speed of etching should be in control of the coating thickness. This will make the coating uniform and give a smoother surface finish.
  • Acid Rinse
    Boards are then rinsed in an acidic solution like sulphuric acid to ensure the effectiveness and reliability of OSP coating.
  • OSP solution
    OSP solution is now coated on the board followed by the acidic rinse.
  • Deionization(DI) Rinse
    Ions were added to the OSP solution to allow for easy elimination during soldering. Rinsing the board with deionization water helps the surface to prevent the introduction of any other contaminants onto the surface, which will eventually result in the OSP performance.
  • Dry
    Finally, the boards are dried to remove excess moisture and to maintain uniform coating thickness.

Manufacturing Considerations for OSP Finish

Several aspects need to be considered during the manufacturing to get a smooth and finest surface finish. Some of them are listed below for the OSP process: –

Handling the PCB Boards

OSP surface finish is very delicate to the surrounding environment and should be handled with care during manufacturing as it involves several chemical processes and also during assembling the components. Hence, handling the boards is very important to maintain the quality.

Shelf-life

OSP boards have to be stored in a specified environment to maintain the quality of the surface finish. When the boards are exposed to harsh environments, they tend to react with atmospheric elements and cause problems during the soldering process, which in turn reduces the shelf life of the boards.

Compatibility

During the manufacturing of the boards, several processes are involved before and post-coating of the surface finish. Care should be taken to maintain the compatibility between other processes and the OSP coating which might affect the performance of the board in later stages.

Reliability

Unlike other surface finishes, OSP doesn’t provide better reliability in harsh environments with rising temperatures and high levels of humidity factors. Despite using OSP finish in many appliances, proper care is to be taken when boards are used in such conditions.

PCBs with OSP Finish

Characteristics of OSP Finish

  • Cost-effective: The cost of OSP finish is way less than other surface finishes in the market, due to this OSP finish is considered more often by PCB designers and manufacturers when there is a budget constraint.
  • Shelf life: Since the OSP coating is very thin on the copper surface, it tends to degrade over time when exposed to harsh environmental conditions like temperature and humidity variations. A proper storage facility is necessary to obtain the maximum performance of the board during the assembling and soldering of the components.
  • Environmental friendly: Due to the absence of toxic chemicals in the process, this finish is known for ROHS compliance and is considered an eco-friendly finish.
  • Thin Coating thickness: Unlike other finishes, OSP provides a very thin layer of coating on the surface and minimizes the effect on overall PCB dimensional thickness without compromising the performance and solderability of the board.
  • Better solderability: OSP finish provides excellent solderability properties which ensures a strong bond between components and the copper pads during the assembly process. A flat surface of OSP facilitates the paste to accommodate perfectly on the pads which makes the component align exactly on the footprint to ensure durable solder joints during the reflow soldering process.

Advantages of OSP Finish

  • It is Cost-effective and economical.
  • It provides excellent solderability properties to bond strong solder joints.
  • Since it is water-based and doesn’t contain any toxic chemicals, it is an environment-friendly surface finish.
  • The manufacturing process is easy when compared to other surface finishes.
  • Reworking and maintaining is easy compared to other finishes.

Disadvantages of OSP Finish

  • It has a limited shelf life compared to other finishes.
  • Boards are sensitive to handle. Should be stored & handled carefully to avoid damage to the OSP layer.
  • The OSP layer is very thin and is prone to scratches easily during transit.
  • Not suitable for harsh environmental conditions as the protective layer gets degraded under such conditions.

Applications of OSP finish

  • Used in consumer electronic appliances like laptops, mobiles, Televisions and cameras.
  • Due to its environmentally friendly and reliable properties, it is used in automobile electronics like infotainments, ECUs and sensors.
  • Industrial machinery like control systems, instrumentation and many more use OSP boards where performance and cost are the main factors of consideration.
  • Used in telecommunication devices like routers, and network devices due to its reliability, lead-free solder process and compatibility.
  • Due to its lightweight coating, environmentally friendly and durability characteristics, OSP finished boards are used in some medical appliances also.
Conclusion:

To sum up, the OSP process is a cost-effective and sustainable surface finish method used in PCB manufacturing. Due to its excellent solderability, it is compatible with lead-free soldering processes also. However while choosing the OSP finish, factors like limited shelf life, handling sensitivity and environmental conditions have to be taken into consideration.

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