Why do PCB circuit boards need to undergo tin deposition treatment

　　The main purpose of immersion tin treatment on PCB circuit boards is to form a flat and solderable metal protective layer on the surface of copper pads to solve specific problems and meet application requirements. The core reasons are as follows:

　　1、 Core purpose: As a weldable surface treatment

　　This is the main reason. Bare copper is highly prone to oxidation in air, forming difficult to weld copper oxide. The deposited tin layer can:

　　Isolate the air and protect the copper surface: provide a temporary protective layer for PCB pads and vias, maintaining their good solderability during the effective storage period (usually 6-12 months).

　　Provide welding interface: Molten solder can quickly form intermetallic compounds with the deposited solder layer, achieving reliable welding connections.

　　2、 Addressing the key pain points of traditional craftsmanship: suitable for high-precision, high-density design

　　Compared to other surface treatment processes, tin deposition has outstanding advantages in handling fine spacing and high-density designs:

　　Ultra flat surface: The deposited tin layer is very thin (usually 1-1.5 μ m) and extremely flat, with almost no thickness fluctuations. This is crucial for the mounting of components such as BGA, QFN, and fine pitch SMT, as it can avoid poor soldering caused by uneven pad surfaces (such as standing stones and virtual soldering).

　　No "black plate" risk: Unlike the "black plate" problem that may occur due to improper process control in chemical nickel palladium, the tin deposition process is relatively simple and does not have this risk.

　　Excellent secondary soldering performance: The deposited tin plate can maintain good soldering performance even after multiple reflow soldering processes.

　　3、 Meet special application and process requirements

　　Compatible with lead-free soldering: Modern tin deposition processes are lead-free and meet environmental requirements.

　　Suitable for crimping connections: The tin plated surface is relatively soft and flat, providing better insertion and contact reliability than gold or silver plated connectors for connectors that require crimping, such as some backplane connectors.

　　Beneficial for ICT testing: The flat surface makes the probe contact for online testing more stable and reliable.

　　Cost benefit considerations: In applications that require a flat surface but are more sensitive to cost than gold deposition, tin deposition is a cost-effective option with lower cost than gold deposition and much better flatness than tin spraying.

　　4、 Technical limitations and precautions

　　Choosing tin deposition also requires understanding its limitations:

　　Tin whisker risk: Pure tin layers may grow "tin whiskers" after long-term storage or temperature cycling, which poses a risk of short circuits. It can be suppressed by adding a small amount of other metals (such as bismuth) to tin or by heat treatment, but it cannot be completely eradicated. Therefore, products with ultra-high reliability, long lifespan, and extremely small spacing need to be carefully evaluated.

　　Limited storage life: usually shorter than immersion gold. Poor storage conditions (temperature, humidity) can accelerate its aging.

　　Not suitable for prolonged exposure to high temperatures: The deposited tin layer will quickly form copper tin intermetallic compounds with the underlying copper at high temperatures, consuming the solderable layer.

　　The core of PCB tinning treatment is to obtain an extremely flat, solderable, lead-free, and relatively low-cost protective surface. It is particularly suitable for electronic products that require high-density assembly, good coplanarity, involve crimping processes, but have cost control requirements. But before making a choice, it is necessary to carefully weigh the potential tin whisker risk and the compatibility with the application scenario.