Immersion Silver PCB Finish
In order to protect printed circuits and enhance their characteristics, it is necessary to invest more during the PCB manufacturing process. One way to achieve this is through PCB surface finishes, which aims to protect the copper foil, improve its corrosion resistance, and provide a solid foundation for subsequent circuit component installation and connections. Immersion silver is one of the most commonly used surface treatment processes, as it provides excellent electrical conductivity and reliable soldering characteristics for circuit boards. However, despite its popularity, many experts still struggle with performing this process or comparing it to other available finish types. In this article, FS Technology will discuss the working principle, advantages, disadvantages, and other aspects of PCB immersion silver technology.
Introduction of immersion silver process
What is immersion silver
Immersion silver is a surface finish commonly used in PCB manufacturing. During the process, the PCB is immersed in a solution containing a reducing agent, anti-tarnish agent, and silver salt. The silver salt is reduced to form a thin layer of pure silver on the copper pads and traces of the board, resulting in a surface finish known as immersion Ag or ImAg.
This thin layer of silver serves as a protective coating for the copper and provides excellent electrical conductivity. It also creates a flat and uniform surface that facilitates the assembly of electronic components. Additionally, the immersion silver finish offers good corrosion resistance, protecting the PCB from environmental factors such as humidity and oxidation.
At FS Technology, we understand the importance of selecting the appropriate surface finish for your PCB project. Our experienced team can provide guidance and recommendations on the best surface finish for your specific application. We use advanced manufacturing processes and top-quality materials to ensure that your PCBs are produced to the highest standards of quality and reliability.
The principle of PCB immersion silver
The PCB silver plating process involves several steps. At its core, the process entails immersing a circuit board in a silver solution, typically composed of silver nitrate. During this step, the silver ions undergo a chemical reaction, bonding with the copper-deposited silver layer on the PCB’s surface. The final step involves rinsing and drying the silver layer to produce a uniform and smooth surface with excellent solderability.
The following chemical equation illustrates this process:
The equation demonstrates that the copper partially dissolves, releasing two electrons that are absorbed by the silver solution. The silver in the solution receives these two electrons and precipitates as a metallic layer on the PCB’s surface, ultimately forming the silver layer.
How FS technology applies immersion silver
As a professional PCBA company, we apply the silver plating process after the final copper plating and before the solder mask application. Our process flow consists of the following steps:
- Surface Preparation: The circuit board undergoes complete cleaning to remove all oxidants, dust particles, moisture, and other particles. This process involves chemical etching and mechanical cleaning of the substrate material.
- Activation: The cleaned board is then treated with an activation solution that enhances the nucleation feature of silver ions on the substrate. The activation solution can be a mixture of palladium, tin, and other elements.
- Immersion: The board is then immersed in a solution containing silver ions with a reducing agent that facilitates the application of silver on the board surface. The temperature and duration of immersion are uniformly controlled to ensure the accurate thickness of the layer.
- Rinsing and Drying: After the immersion process, the board is rinsed with deionized water to remove residual chemicals and contaminants on the surface. The board is then dried.
- Post-treatment: To increase the reliability and durability of the immersion silver layer, post-treatment is applied. This process involves the use of organic material to create a protective coating that prevents oxidation corrosion.
Features of Immersion Silver Finishes
- Good Solderability: Immersion silver finishes offer smoother surface areas that avoid oxidation and promote good soldering. The result is a reliable and strengthened bonding between the silver surface and the solder.
- Anti-Tarnish: Silver is prone to tarnish, which decreases conductivity and soldering abilities. However, immersion silver has resistant features to tarnishing, due to the existence of thin oxides that protect the silver surface from oxidation.
- Good Conductivity: Silver is a highly conductive material, with a conductivity value of about 63.0 x 10^6 S/m. A thin silver layer in immersion silver plating offers good conductivity features, making it an excellent option for radio frequency applications.
- Environmentally Friendly: Immersion silver finishes are less environmentally damaging than other finishes, such as ENIG, because they do not use any hazardous materials like gold or nickel.
- Handling Issues: Immersion silver is a soft material that can be easily scratched or damaged during PCB assembly. Careful handling is required to avoid rubbing, pressure, or the application of harsh solvents.
- Limited Supply Chain: Immersion silver finishes are not commonly used for larger applications and are not as easily accessible as other types of finishes.
- Limited Compatibility: Immersion silver finishes are not compatible with all types of inorganic acids, such as chloride and sulfuric.
- Sealing Requirements: After creating immersion silver finish boards, they must be properly sealed in certain types of packaging to avoid damage from environmental conditions.
Challenges of Immersion Silver PCB Manufacturing
While immersion silver PCB is a suitable option for electronic projects, manufacturers must address several challenges to ensure its success.
- Oxidation and Tarnishing: Silver PCBs are prone to oxidation and tarnishing when exposed to air or moisture, resulting in decreased reliability and performance. Applying a protective conformal coating or stable-type finish can mitigate this issue.
- Solderability Issues: The formation of silver oxide on the board surface can hinder proper soldering and reduce bonding accuracy. Using flux can remove the oxide layer and improve wetting.
- Handling Issues: Due to its soft nature, immersion silver is susceptible to scratches and abrasions, necessitating careful handling during assembly. Avoiding excessive rubbing, pressure, and harsh solvents can help prevent damage.
- Compatibility with Lead-Free Soldering: Immersion silver is not compatible with lead-free soldering due to its low melting point and tendency to form intermetallics with soldering material. Pre-treatment is necessary to address these issues.
- Environmental Concerns: The chemicals and processes involved in immersion silver PCB manufacturing can pose health risks and harm the environment by releasing toxic materials and hazardous waste. Proper handling and disposal measures must be implemented to minimize these impacts.
Comparison of PCB Surface Treatments
|Surface Finish||Solderability||Conductivity||Corrosion Resistance||Environmental Impact||Suitable Applications|
|Immersion Silver||Good||Good||Anti-Tarnish||Less environmentally friendly||RF applications|
|ENIG||Good||Good||Excellent||More environmentally friendly||High-density applications|
|HASL||Good||Good||Good||More environmentally friendly||Low-cost applications|
|OSP||Good||Fair||Fair||More environmentally friendly||Low-cost, low-density applications|
Electroless nickel immersion gold (ENIG) for your next PCB project ENIG, or electroless nickel immersion gold, is a surface finishing process applied to the copper