Immersion Tin PCB Surface Finish
Immersion tin is a type of surface Finishes used in printed circuit board (PCB) manufacturing. This process involves coating the copper surface of a circuit board with a thin layer of tin, which helps protect the copper from oxidation and corrosion. Immersion tin also improves the solderability and wire bondability of the PCB, making it a popular choice for electronic components that require high-quality connections. In this article, we’ll dive deeper into the concept of immersion tin PCB and explore its benefits and drawbacks for PCB manufacturing.
Learn about immersion tin process
What's this
Immersion tin, also known as white tin, is a highly sensitive surface treatment. If not applied accurately, it can result in short circuits and poor-quality solder joints that may damage the entire PCBA board. To ensure accuracy, PCBA companies take specific steps during the application process.
However, for boards that require processing by surface mount technology (SMT), immersion tin offers clear advantages that make the process much easier. This type of coating is also less harmful to humans and the environment than older methods, as it has replaced Pb-Sn alloy plating. It provides excellent solderability and protects copper traces from oxidation, safeguarding the copper planes and pads on the board.
What are the characteristics
Thin Coating: The layer created by immersion tin is typically only one to two microns thick, but it provides effective protection against environmental effects like oxidation and corrosion.
High Solderability: The surface resulting from immersion tin is highly solderable, making it easier to apply solder paste during board construction. Compared to other finishes, it also provides better solderability due to its smoother surface.
Flat Surface: Immersion tin provides a flat surface on the board, ensuring good and reliable electrical operation.
Uniformity: The surface finish of immersion tin is uniform all over the board, ensuring consistent operation of the board.
How to implement the process
PCB Cleaning: The board’s surface is spotless to remove any dust particles, grease, or other contaminants.
Micro-etching: The copper surface of the board undergoes a micro-etching process to ensure good adhesion between the copper and tin immersion layer.
Pre-dip: The board is then placed in a pre-dip solution for activation, which strengthens the adhesion between the copper and tin layer.
Immersion Tin Plating: In this step, the board is placed in an electroless plating solution containing tin ions, stabilizers, and reducing agents. The tin ions undergo the reduction process and are deposited onto the copper surface, creating the immersion tin layer on the board.
Post-dip: To remove any excess tin ions and stabilizers, the tin-coated board is placed in a post-dip solution.
Drying and Curing: Finally, the board is dried and cured to ensure that the tin layer is completely formed and stable.
Why Immersion Tin Surface Treatment
Advantage
Quality Solderability: With a thin layer of tin applied to the board, the resulting surface is highly solderable, providing a reliable and accurate configuration between the board and the soldered components. This makes it an ideal finish for electronic projects and devices.
Excellent Electrical Performance: The immersion tin finish offers highly reliable electrical performance. With less surface resistance, there is minimal signal loss during electrical signal transmission.
Cost-Effective Finish: This type of finish is less expensive than other finishes like silver and gold, making it a cost-effective option for budget-conscious projects.
Environmentally Friendly and Less Hazardous: Unlike other finishes like gold and silver, immersion tin is an environmentally friendly and less hazardous option. It does not contain hazardous materials like cyanide, making it a safer option for both people and the environment.
Disadvantage
Tin Whiskers: Immersion tin plating can lead to tin whiskers, which are thin filaments that can cause short circuits and affect the electrical operation of the board. These whiskers are a common issue associated with this type of finish.
Limited Shelf Life: The shelf life is limited due to its susceptibility to oxidation. Over time, boards with this finish can degrade, affecting their performance.
Limited Layer Thickness: The thickness of the tin layer is limited, making it suitable for specific types of projects.
Not Suitable for Harsh Conditions: An immersion tin plating solution is not recommended for use in high-temperature or high-moisture environments, as it can corrode and affect the performance of the board.
Immersion Tin VS Immersion Silver
Immersion tin and immersion silver are both popular surface finishes for PCB construction due to their excellent electrical operation and solderability properties. However, there are some differences between these two finishes that are worth noting.
Immersion tin plating is a more cost-effective process and provides good solderability over multiple thermal cycles. Additionally, it offers excellent resistance to environmental conditions such as high temperature, corrosion, and oxidation, making it a preferred choice for high-condition applications.
On the other hand, immersion silver is a more expensive process but boasts superior resistance to corrosion and excellent electrical conductivity. It is well-suited for high-speed communication projects that require low contact resistance.
The primary difference between these two finishes lies in their layer thickness on the board. Immersion tin has a layer thickness of 0.8 to 1.2 microns, while immersion silver has a thickness of 0.15 to 0.5 microns. This thicker layer of immersion silver makes it a suitable option for applications that require high accuracy.
It is important to note that immersion silver has a shorter shelf life than immersion tin PCB finish and may tarnish or change color over time. Therefore, immersion tin is typically preferred for long-term, reliable operation due to the surface finish’s high stability.
Immersion Tin Thickness Guide
The thickness of the tin layer in immersion tin finishes depends on the specific project and application requirements. Typically, the thickness ranges from 0.8 to 1.2 microns, but thicker layers may be necessary to reduce corrosion and ensure quality solderability in certain applications.
The immersion tin plating process is the main step in producing immersion tin boards, and it regulates the thickness of the tin layer. During this process, the board is immersed in a solution of tin containing a reducing agent that reduces tin ions and deposits a layer of tin on the copper layer.
The thickness of the tin layer is influenced by factors such as the concentration of the tin solution, immersion duration, and solution temperature. Higher tin solution concentrations result in thicker layers, while higher solution temperatures lead to thinner layers.
The thickness of the tin layer plays a crucial role in the performance and reliability of the immersion tin board. A thin layer may result in poor solderability, while a thick layer can increase the use of solder and production costs.
To ensure accurate tin thickness for your PCB, it is essential to seek guidance from experienced PCB manufacturers and experts. They can help you determine the appropriate tin layer thickness based on your specific project requirements.
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