Detailed explanation of the technical principle of PCB double-sided tinning (chemical deposition tin)

　　Chemical tin deposition is a surface treatment process that deposits a layer of pure tin on the surface of PCB copper pads through chemical displacement reactions. Its technical principle is mainly based on electrochemical displacement reactions and self catalytic deposition processes.

　　1、 Core Chemical Principles

　　1. Substitution reaction mechanism

　　When the PCB is immersed in chemical tin plating solution, copper atoms undergo redox reactions with tin ions. Copper atoms lose electrons and are oxidized into copper ions entering the solution, while tin ions gain electrons and are reduced to metallic tin deposited on the copper surface. This reaction is self limiting, and it automatically stops when the tin layer completely covers the copper surface.

　　2. Reaction process control

　　Thickness self-adjusting: After the deposited tin layer forms a dense barrier, it blocks the contact between copper and the solution, allowing the reaction to naturally terminate. Typically, a uniform tin layer of 0.8-1.2 μ m can be obtained

　　Temperature effect: The reaction temperature (usually 55-65 ℃) directly affects the deposition rate and the crystal morphology of the tin layer

　　PH control: Maintain an appropriate pH level (approximately 1.5-3.0) to ensure stable reaction

　　2、 Process flow mechanism

　　1. Pre processing stage

　　Cleaning and micro etching: Remove copper surface oxides and pollutants, and form micro rough surfaces through micro etching to increase deposition adhesion

　　Pre immersion protection: prevents copper surface oxidation and maintains copper surface activity before acidic activation

　　2. Catalytic deposition stage

　　Acidic activation: The organic acid components in the solution clean the copper surface and maintain its active state

　　Tin ion supply: Tin salts (such as stannous sulfate) in the solution provide a stable source of tin ions

　　Function of chelating agents: Thiourea and other chelating agents control the reaction rate to ensure a uniform and dense tin layer

　　3. Post processing stage

　　Antioxidant treatment: Form an organic protective film on the surface of the tin layer to prevent oxidation during storage

　　Hot air leveling enhancement (if necessary): By briefly heating to reflow the tin layer, a smoother surface is obtained

　　3、 Technical characteristics and mechanisms

　　1. Advantages of surface flatness

　　Due to its pure chemical deposition, the thickness of the tin layer is uniform, and there is no "tin bump" phenomenon caused by tin spraying process, making it suitable for high-density and fine solder pads.

　　2. Welding compatibility

　　Wetting mechanism: The fresh tin layer directly melts and participates in the formation of solder joints during welding, with excellent wetting properties

　　Interface reaction: During welding, the tin layer and solder quickly form intermetallic compounds such as Cu ₆ Sn ₅, ensuring reliable connection

　　3. Protective mechanism

　　The deposited tin layer completely covers the copper surface, isolating oxygen and moisture, effectively preventing copper oxidation, and maintaining solderability for more than 12 months.

　　4. Process adaptability

　　Through hole coverage: The medicine enters the through hole through capillary action, achieving uniform coverage of the hole wall

　　Lead free compatibility: The deposited pure tin itself complies with RoHS lead-free requirements

　　4、 Key points of quality control

　　Thickness control: precise control through solution concentration, temperature, and immersion time

　　Crystal morphology: Optimize additives to obtain a fine tin crystal structure and prevent whisker growth

　　Impurity control: Strictly control the accumulation of copper ions and regularly analyze the composition of the solution

　　Chemical tin deposition technology forms a dense, uniform, and weldable pure tin protective layer on the copper surface through precise controlled chemical reactions. It is an environmentally friendly and refined surface treatment solution, particularly suitable for the manufacturing of high-density interconnects and fine pitch electronic products.