Surface finishing is an important aspect of a PCB manufacturing process, which takes place after the application of a solder mask on the PCB. This is coating between a plain PCB board and a component. It is a process used to protect the surface of PCBs. It is used primarily for these reasons: To prevent oxidation of the Copper (Cu) traces and to ensure solderability. There are various surface finishes solutions in the PCB industry: Immersion Tin (Sn), Immersion Silver (Ag), Electroless Nickel Immersion Gold, Lead-Free Hot Air Solder Leveling, Tin/Lead Hot Air Solder Level, Electroless Nickel Electroless Palladium Immersion Gold, and Organic Solderability Preservative.
In this article, we will discuss OSP surface finish, its advantages and disadvantages, the manufacturing process of OSP surface finish, and others. Furthermore, let’s explain what PCB surface finish means.
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What is PCB Surface Finish?
PCB Surface finish forms the main interface between a PCB and the components. PCB surface finishing has been common, readily available, and overwhelming to certain electronic designers. A PCB surface finish may be either metallic or organic. Comparing both metallic and organic surface finish and every available option can quickly show you their relative advantages or disadvantages. The PCB surface finish solutions that are mainly used in the industry are Organic Solderability Preservative, Electroless Nickel Immersion Gold, Electroless Nickel Electroless Palladium Immersion Gold, Immersion Tin or Immersion Silver.
PCB surface finish is very essential for the optimum performance of PCB as it protects the copper from oxidation. It is critical in establishing reliable connectivity between the electronic components and the PCB. A PCB surface finishing has two main functions:
- To create a surface that is solderable and sufficient for components to be soldered to the printed circuit board
- To prevent any copper traces that are exposed from oxidation.
PCB surface finish turns out to be as essential in the PCB manufacturing process as choosing the appropriate material required for a plain board. Choosing the appropriate type of surface finish required for your project is essential for its performance and could be challenging. Particularly due to the complexity of surface mounts and industry standards from WEEE and RoHS regulations which enforce and implement environmental protection. It is recommended you seek the assistance of your PCB manufacturer when choosing a PCB surface finish for your project.
What Conditions Influence The Choice Of A PCB Surface Finish?
Typically, in choosing the most appropriate and suitable PCB surface finish, the major and deciding factors are the process of PCB assembly, end application, and design process of the said PCB. However, the following conditions can generally influence the choice of a PCB surface finish.
- The protection of the metal traces on the PCBs especially copper from oxidation
- Surface solderability used for the mechanical and electrical components attached to the PCB
- Surface bondability over chip components mounted with the help of aluminum and gold wire.
- Mechanical applications such as attain and stress
- Environmental conditions like relative humidity, temperature, and others
- Mechanical contacts that require oxidation protection and abrasion resistance.
- A combination of any of surface bondability, solderability, and protection of copper traces from oxidation.
For this article, we will be discussing all you need to know about Organic Solderability Preservative (OSP) surface finish.
What Is OSP Surface Finish?
OSP refers to Organic Solderability Preservative. It is the leading PCB surface finishing coating option used by the PCB industry for protecting Copper surfaces and its features that are exposed from oxidation before the PCB is populated. Organic Solderability Preservative surface finish provides excellent soldering joint strength, selective Gold and Copper capabilities without masks, excellent solderability, lead-free application, simplicity of processing, and minimal costs relative to metallic PCB surface finishing.
OSP surface finish is water-based and environmentally friendly. Before the boards go through the bathing process, they are cleaned. This helps to provide a good flat surface. OSP surface finish is a quick process that doesn’t need much equipment, making it to be cost-effective.
The water-based organic compound used in the OSP surface finish belongs to the Azole family group of chemical compounds such as benzimidazoles, imidazoles, and benzotriazoles. These organic chemical compounds are absorbed on the copper’s surface forming a bond with the copper atoms and producing a protective layer or film.
Organic Solderability Preservative (OSP) surface finish has been designed specifically to create a thin, uniform, protective layer of coating on the copper surface of the printed circuit board. This layer of coating protects the circuit from oxidation in the course of PCB assembly and storage. OSP surface finish is applied on the copper pads for their protection before soldering is applied. This Organic Solderability Preservative surface finishing protects the surface of the copper from oxidation through the creation of a protective organic-metallic layer that protects the copper traces before the process of soldering.
In comparison to other types of PCB surface finishing with high toxicity levels or large consumption of energy, OSP surface finishing is safe and eco-friendly. Its popularity is increasing as a result of an increase in demand for lead-free PCB surface finish processes and fine pitching solutions. OSP surface finish requires minimal maintenance of equipment.
However, OSP surface finish has a restricted shelf life. It could be sensitive during handling. It is frequently used in soldering when there is a dissipation of the protective layer as a result of soldering. Once the protective layer is removed, the copper exposed and oxidation may occur. When several surface finishes are required on the PCB, OSP surface finish can be used or applied on other PCB surface finish such as wire bonding with soldering; mechanic surface contact and soldering; and others.
What Is The Manufacturing Process of OSP Surface Finish?
There are four stages in the manufacturing process of OSP surface finish. These stages are:
Step 1. Cleaning: Cleaning is the first step in the PCB manufacturing process of OSP surface finish. In this stage, organic contaminations like oil remain, fingerprints, and other oxidants are removed. This is essential to ensure the surface of the copper is contaminated and the PCB has the same thickness over the copper foil surface. Ensure this cleaning process is checked often to prevent the performance of the process is not affected negatively.
Step 2. Topography Magnification and Enhancement: For this stage, micro-etching is used to prevent any oxidation process that occurs on the surface of the copper film. This helps to improve the bonding between the Organic Solderability Preservative solution and the copper film. The micro-etching speed should also be considered.
Step 3. De-ionization Washing: This helps to remove various ions that might have contaminated the copper surface.
Step 4. Film Formation: This is the final stage of the manufacturing process of OSP surface finish. In this stage, deionization rinsing is repeated to prevent the contamination of the OSP solution by other ions. Contamination of the OSP solution can cause tarnishing to occur. The de-ionization should be done with preservatives of pH value ranging from 4.0 to 7.0.
What Are The Advantages Of OSP Surface Finish?
The advantages of OSP surface finish compared to other PCB surface finish mentioned earlier in this article are:
- OSP surface finish provides excellent flatness
- It is good for smaller components/BGA/fine pitch SMT
- OSP surface finish isn’t expensive
- OSP PCB surface finish could be reworked and it is suitable for Horizontal Type Production Line.
- It is a clean, environmentally friendly, simple, SMT and lead-free soldering process.
What Are The Disadvantages Of OSP Surface Finish?
OSP surface finish has the following drawbacks:
- OSP surface finish handling process is very sensitive. Gloves should be worn and scratches must be voided.
- It has a minimal window of operation between each stage of PCB assembly
- OSP PCB surface finish has restricted thermal cycles, hence it is not suitable for several soldering processes
- OSP surface finish shelf life is short, hence it is not suitable for certain types of shipment and longer stock holding.
- Inspection of OSP surface finish can be very difficult
- Cleaning solder paste misprinted on the board can have an adverse effect on the PCB OSP surface finish.
- Baking the PCB before it is used can also have an adverse effect.
What Benefits Do PCB OSP surface Finish Offer?
- PCBs with OSP surface finishes and coating usually perform optimally during the process of soldering.
- OSP surface finish PCBs are flexible and can be repaired easily when damage occurs. PCB manufacturers can rework OSP surface finish circuit boards with no problem. This enables PCB assemblers to create a new coating on the PCB whenever it is required.
- OSP surface finish is eco-friendly. It uses water-based chemical compounds to create protective films on the copper layer. These organic compounds are environmentally friendly.
- OSP surface finish is essential for manufacturers of electronics in adhering to environmental standards and regulations by RoHS (Restriction of Hazardous Substances Directive) and WEEE (Waste from Electrical and Electronic Equipment)
- The process involves in OSP surface finish and the application of simple elements and equipment helps in minimizing the cost required in the fabrication and manufacturing of PCBs.
- PCBs produced with OSP surface finish can be stored under the right conditions (vacuum storage and relative humidity) for a long time.
How To Store OSP PCB?
PCBs with OSP surface finish are normally exposed to high temperature and humidity for a longer time, resulting in oxidation which occurs on PCB surfaces, and low solderability eventually. Therefore, prior to storing PCBs with OSP surface furnish, the following principles should be adopted.
- Make use of vacuum storage that has a desiccant and a humidity indicator.
- Place a release paper between the PCBs to the destruction of the said PCB surface by friction.
- Don’t expose the PCBs to the rays of sunlight.
- For optimum storage of OSP PCB, the following are required:
- Relative humidity of between 30% and 70% RH
- The temperature should be between 15°C and 30°C
- The time of storage for OSP PCBs should not be more than 12 months.
What You Should Pay Attention To When Assembling OSP PCB?
When assembling OSP PCB, there are several factors you should pay attention to. This includes but is not limited to the OSP surface finishing process; the layout of the printed circuit board; and your recent outcome value of OSP finish. Below are other factors you should be considering:
- Solderability: Solderability is important to avoid soldering issues that can arise, preventing the OSP printed circuit boards from functioning efficiently.
- Will the OSP surface finish solder efficiently to the copper: It is important you consider this because copper enables OSP to function as the appropriate finish in situations like this.
- Duration of the OSP surface finish process: Some PCB surface finishes may be long or short. Surface finishes such as HASL take longer. Hence, you should pay attention to the duration of the process when choosing an OSP surface finish.
- Reliability: This involves the measurement of how reliable the OSP surface finish you have chosen will be.
- Corrosion: Pay attention to how vulnerable the material is to corrosion prior to choosing a PCB OSP surface finishing. Some surface finishing like Immersion silver oxidizes easily when compared to others.
Why OSP Surface Finishing Is Popular In The PCB Industry?
Prior to 2005, Lead HASL surface finish was used in the industry as it was the main process of most PCBs surface finish. However, this wasn’t environmentally friendly as the lead metal pollutes the surrounding. After 2005, PCB manufacturers were under pressure from environmental institutions and governments to do away with Lead HASL surface finish and adopt Lead-free HASL.
However, the use of a Lead-free HASL surface finish resulted in an increase in PCB manufacturing. Hence, the reason why OSP Surface finish is widely used in the PCB manufacturing industry, is it is cost-effective and environmentally friendly. OSP surface finish is the most widely used surface finishing for all copper pad PCB applications. OSP surface finish is a unique type of PCB surface finish. Despite its disadvantages highlighted earlier, OSP surface finish can be a great choice generally as an organic solution required for your PCB surface finish.