1. The PCB tin sinking process is specifically designed to enhance SMT and chip packaging by chemically depositing a tin metal coating onto the copper surface.

2. This innovative, eco-friendly process replaces the traditional Pb-Sn alloy coating method and is widely employed in electronic products, hardware, decorations, and more.

3. Two common processes used for printed circuit boards are tin spraying and tin sinking.

4. Tin spraying involves directly immersing the PCB into molten tin paste. After leveling with hot air, a dense tin layer, typically ranging from 1μm to 40μm, forms on the copper surface.

5. In contrast, immersion tin primarily utilizes a displacement reaction to create a very thin tin layer on the PCB surface, usually measuring about 0.8μm to 1.2μm.

6. The tin immersion process is more frequently applied in the surface treatment of circuit boards.

7. Chemical tin deposition is a widely used PCB tin deposition method.

8. Its operating principle involves altering the chemical potential of copper ions, which induces a chemical substitution reaction with stannous ions in the plating solution, fundamentally an electrochemical reaction.

9. The reduced tin metal is deposited onto the copper substrate, forming a tin plating layer, while the metal complex adsorbed on the immersion tin layer catalyzes the reduction of tin ions to metallic tin, enabling continuous reduction of tin ions.

10. The chemical reaction equation is 2Cu + 4TU + Sn²⁻ → 2Cu + (TU)₂ + Sn.

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1. The thickness of the sprayed tin layer ranges from approximately 1 µm to 40 µm. The surface structure is relatively dense, exhibiting high hardness and resistance to scratching. The sprayed tin consists solely of pure tin during production, making the surface easy to clean. It can be stored at room temperature for up to one year without significant risk of surface discoloration during the welding process. In contrast, the tin thickness for immersion tin is about 0.8 µm to 1.2 µm, resulting in a looser surface structure, lower hardness, and increased susceptibility to scratching. The tin immersion process involves complex chemical reactions, which complicate cleaning. The surface is prone to residue, leading to discoloration during welding, and it has a shorter storage life, lasting only three months at room temperature. Prolonged storage can result in color changes.

2. The primary defects observed in chemical tin immersion plates include a darkened tin surface and poor solderability, both caused by surface contamination. Extensive data analysis and on-site investigations have led to the conclusion that the causes are primarily due to several factors. First, the consumption of liquid in the production process: the high viscosity of the tin bath liquid results in substantial usage, leading to quick contamination. Moreover, the large volumes of tin bath liquid entering the thiourea washing tank rapidly increase the copper content, adversely affecting the cleaning effectiveness of the production board and causing defects such as darkening of the tin surface and waste accumulation of solid thiourea. Second, improper baking time is a contributing factor. Third, at the conclusion of the chemical tin process, inadequate cleaning due to significant drag from the bath liquid can lead to poor solderability.

3. Similarities and differences between spray tin and sink tin:

– Similarities:

Both the tin spraying and tin immersion processes serve as surface treatment methods to fulfill the requirements for lead-free soldering.

– Differences:

– Process Flow: In tin spraying, the sequence is pre-treatment, tin spraying, testing, molding, and appearance inspection. In tin immersion, the order is testing, chemical treatment, tin immersion, molding, and appearance inspection.

– Process Principle: The tin spraying method involves directly immersing the PCB board in molten tin paste, where hot air levels it out to form a dense tin layer on the copper surface. The immersion tin process relies on a displacement reaction to create a very thin tin layer on the PCB surface.

– Physical Characteristics: The tin layer from spray tin typically measures between 1 µm and 40 µm in thickness, with a dense surface structure and higher hardness, making it less prone to scratches. As it uses pure tin during production, its surface is easy to clean and can be stored for one year at normal temperatures with minimal risk of discoloration during welding. Conversely, immersion tin features a thickness of about 0.8 µm to 1.2 µm, a looser surface structure, lower hardness, and higher susceptibility to scratches. Due to complex chemical reactions, cleaning is difficult, and residues can lead to color variations during welding. Its storage duration is limited to three months at room temperature, with potential discoloration over extended periods.

– Appearance Characteristics: Spray tin has a bright and attractive surface, while sink tin tends to have a light white, dull surface that is more prone to discoloration.