A comprehensive guide to OSP PCB surface finish
The copper used on a printed circuit board (PCB) is important as it defines the connections between components on the board. However, copper is a chemically active material that can oxidize due to atmospheric moisture, causing problems during high-temperature soldering and reducing board reliability.
To address these issues, surface finishing is applied to the circuit board. Surface finishing serves two purposes: protecting the copper from oxidation and providing a surface that is favorable for high solderability during component connection. Different types of finishes are used for PCB boards, such as HASL, immersion Tin/Silver, ENIG, ENEPIG, and OSP.
Among these finishes, Organic Solderability Preservative (OSP) is a popular choice due to its low cost and environmental friendliness. It forms an organic coating that protects the copper and provides a favorable surface for soldering, making it an attractive option for many applications.
What is Organic Solderability Preservative (OSP) ?
Concept explanation of OSP
OSP, or organic solderability preservatives, is a type of surface finish for printed circuit boards (PCBs). It is also known as anti-tarnish. The OSP PCB process involves applying a layer of organic finish onto a clean copper surface through an adsorption process.
The organic finish helps to protect the copper from oxidation and the effects of moisture and environmental pressure. During the soldering process, the finish can be easily removed through the use of PCB flux. This allows the clean copper to mix with the melting solder, producing solder joints in less time.
The compound used in OSP surface treatment is a water-based compound that is part of azole groups, such as benzimidazoles, imidazoles, and benzotriazoles. These compounds are absorbed into the copper surface, creating a film. The thickness of the film generated by benzotriazoles is thinner than that of imidazoles. Typically, the film has a thickness of tens to hundreds of nanometers.
Features of OSP PCB surface finish
- Boards created using OSP are easy to manufacture and can be easily reworked if needed, which is why PCBA manufacturers often use a fresh coating to repair damaged layers.
- Bare board have good wetting properties during soldering on vias and pads.
- This is an inexpensive option and is commonly chosen by electronics manufacturers.
- The lead-free feature of OSP PCB plating makes it suitable for SMD components and PCBA processing through SMT assembly.
- Its flat surface makes it suitable for tight pitch pads such as BGA and QFP.
- Created using a water-based compound that is safe for people and the environment.
- It requires less solder mask ink compared to other surface finishes.
- Susceptible to mechanical damage, so requires more care in handling.
- The colorless and transparent state makes visual inspection difficult for operators, and its thickness is difficult to measure, which may affect welding performance.
- Due to its fragile nature, it requires careful transportation and handling to avoid scratches and damage to the protective layer.
- It is not suitable for plated through holes as it has a short shelf life of around 6 months.
- Is a sensitive finish and even minor water or moisture can affect its performance.
- The OSP process requires certain variations and is not compatible with ICT, which can cause damage to the PCB with ICT probes. It requires careful handling measures due to its reputation with ICT restrictions.
- In the PCB assembly process, PCBA board needs to be processed at high temperature, which may cause changes in OSP during the assembly process.
OSP VS ENIG
- The lifespan of OSP is approximately 6 to 11 months, while ENIG has a lifespan of 12 months.
- OSP has certain limitations in PCB assembly, while ENIG is more complex in terms of handling.
- Despite being more expensive, ENIG is preferred by electronics manufacturers.
- Both technologies ensure a flat surface, are RoHS compliant, and have good solderability.
- Aluminum wire bonding is required for ENIG, but not for OSP.
How OSP Manufactured
Here is a block diagram created to help understand the manufacturing process of OSP PCB. This diagram is divided into different blocks that define each step in the manufacturing process.
- The first step is cleaning, which removes organic contaminants such as oil and oxidation films from the copper foil, the main component of the OSP. Insufficient cleaning can result in an uneven thickness of the created preservative. To achieve high-quality OSP films, the concentration of the cleaning liquid must be within a certain range according to laboratory standards. The cleaning process must be regularly monitored to ensure the required standard is met. If the desired results are not achieved, the cleaning liquid should be changed.
- The second block is Topography Enhancement, where micro-etching is used to remove the oxidation produced on the copper foil that causes strong bonding between the copper foil and organic solderability preservative solution. The film build rate depends on the micro-etching speed. To achieve a smooth film thickness, the speed of micro-etching must be stable. The range for micro-etching speed is about 1.0 to 1.5 micrometers per minute.
- The best option is to use a rinse before creating the preservative, as OSP solution can get polluted by ions, which can cause tarnishing after the completion of the reflow soldering process. In addition to that, DI rinse must be used after the creation of the preservative with a pH value of 4 to 7. If these parameters are not followed, the preservative can be destroyed due to pollution.
- The OSP PCB coating is then applied to the cleaned copper surface through an adsorption process. The OSP solution contains organic compounds such as benzimidazoles, imidazoles, and benzotriazoles that form a thin layer on the copper surface. The thickness of the coating can be controlled by adjusting the concentration and immersion time of the solution.
- After the coating is applied, the PCB is dried and cured in a controlled environment to remove any remaining moisture and to ensure proper adhesion of the OSP layer.
- Once the coating is applied, the PCB is inspected for any defects or irregularities. The OSP-coated circuit boards are then subjected to various PCB tests to ensure their quality, reliability, and performance.
OSP Problems after Soldering
Here are some suggestions that can be used in open nitrogen conditions for secondary reflow to achieve good welding results. The color of OSP boards can be affected during the soldering process, so certain measures need to be taken regarding preservative thickness, soldering duration, etching quantity, etc. There are two factors that can affect OSP PCBA board performance after soldering, which are listed below.
- Uniform color exists
- Color becomes darker
- Tarnish causes
- Colors changes from brown to dark brown
- OSP thickness must be in some limited value.
- Micro etching must be in some range.
- During the PCB manufacturing process, contaminants must be completely removed then affect the soldering process.