Common PCB board surface treatment processes include: OSP, electroless nickel plating/gold immersion, silver immersion, tin immersion, etc.
1. Silver immersion
The silver immersion process falls between OSP and electroless nickel/gold immersion, offering simplicity and speed. Unlike electroless nickel/gold plating, silver immersion does not deposit a thick layer on the PCB. Despite exposure to heat, humidity, and pollutants, it maintains good electrical performance and solderability, albeit losing its luster over time. Since there is no nickel layer beneath the silver coating, it lacks the robust physical strength of electroless nickel plating/gold immersion. Silver immersion involves a displacement reaction, resulting in an almost submicron coating of pure silver. Occasionally, the silver immersion process includes organic substances to prevent silver corrosion and mitigate silver migration issues. Typically, measuring the thickness of this organic layer proves challenging, with analysis indicating that its weight is less than 1%.
2. Tin dipping
As all solders are based on tin, tin layers can match any type of solder. From this perspective, the tin dipping process holds significant developmental potential. However, tin whiskers easily appear in PCBs following the tin dipping process, leading to reliability issues in soldering due to tin whiskers and tin migration. Consequently, the use of the tin dipping process is restricted. To address these concerns, an organic additive is introduced into the tin dipping solution to granularize the tin layer structure. This innovation overcomes previous challenges and offers excellent thermal stability and solderability. The tin dipping process forms a flat copper-tin intermetallic compound, ensuring comparable solderability to hot air leveling without the flatness issues associated with the latter. Additionally, it avoids diffusion issues between electroless nickel plating and gold dipping metals. However, tin dipping plated boards cannot be stored for extended periods.
3. OSP
OSP distinguishes itself from other surface treatment processes by serving as a barrier between copper and air. In essence, OSP chemically cultivates an organic film on the clean bare copper surface. This film provides oxidation, heat shock, and moisture resistance, safeguarding the copper surface from further oxidation or sulfidation in normal environmental conditions. Simultaneously, it must be easily removed by flux during subsequent high-temperature soldering processes. OSP finds wide industrial use due to its straightforward process and cost-effectiveness. Early organic coating molecules like imidazole and benzotriazole played anti-rust roles, whereas the latest, primarily benzimidazole molecules, ensure multiple reflow soldering capabilities. To achieve this, the chemical bath typically requires copper additives to adsorb successive layers of organic coating molecules (20 to 100 layers) onto the copper surface. The standard process includes degreasing, micro-etching, pickling, pure water cleaning, organic coating, and final cleaning. Compared to other processes, OSP offers relatively easy process control.
4. Chemical gold
Gold deposition and gold plating are commonly used in PCB prototyping, often causing confusion due to their similar nature. Electroplated nickel gold, commonly termed “hard gold,” involves electroplating gold particles onto the PCB, enhancing adhesion and durability. This process significantly improves PCB hardness and wear resistance, effectively preventing copper and other metal diffusion, meeting requirements for hot pressing and brazing. The resulting coating is uniform, fine, low-porosity, low-stress, and ductile, making it widely suitable for electronic product PCB prototyping. In contrast, gold deposition involves chemically bonding gold particles to the PCB bonding pad, offering weaker adhesion, hence termed “soft gold.” Electroless nickel plating/gold immersion deposits a thick nickel-gold alloy layer with excellent electrical properties on the copper surface, providing long-term PCB protection and superior electrical performance compared to OSP. Unlike OSP, which acts solely as a rust-preventative barrier, electroless nickel plating/gold immersion offers long-term PCB utility and environmental resilience, thanks to the nickel layer’s ability to prevent copper-gold diffusion. A mere 5 µm thick nickel layer can control Z-direction expansion at high temperatures and inhibit copper dissolution, advantageous for lead-free soldering. The process involves six chemical tanks and nearly 100 chemical types, making it relatively complex.
Key differences between PCB gold plating and gold deposition:
1) Thickness: Gold deposition generally results in a thicker layer than gold plating due to different crystal structures. Consequently, gold deposition tends to exhibit a deeper golden hue compared to the lighter shade of gold plating.
2) Crystal structure: The crystal structures formed by gold plating and electroplated nickel gold differ, impacting weldability. Gold plating, being softer, is easier to weld than electroplated nickel gold, preferred for wear resistance in gold finger plates.
3) Application timing: Nickel gold electroplating occurs before resistance welding, whereas gold deposition follows resistance welding on the PCB during prototyping.
1. Silver immersion
The silver immersion process falls between OSP and electroless nickel/gold immersion, offering simplicity and speed. Unlike electroless nickel/gold plating, silver immersion does not deposit a thick layer on the PCB. Despite exposure to heat, humidity, and pollutants, it maintains good electrical performance and solderability, albeit losing its luster over time. Since there is no nickel layer beneath the silver coating, it lacks the robust physical strength of electroless nickel plating/gold immersion. Silver immersion involves a displacement reaction, resulting in an almost submicron coating of pure silver. Occasionally, the silver immersion process includes organic substances to prevent silver corrosion and mitigate silver migration issues. Typically, measuring the thickness of this organic layer proves challenging, with analysis indicating that its weight is less than 1%.
2. Tin dipping
As all solders are based on tin, tin layers can match any type of solder. From this perspective, the tin dipping process holds significant developmental potential. However, tin whiskers easily appear in PCBs following the tin dipping process, leading to reliability issues in soldering due to tin whiskers and tin migration. Consequently, the use of the tin dipping process is restricted. To address these concerns, an organic additive is introduced into the tin dipping solution to granularize the tin layer structure. This innovation overcomes previous challenges and offers excellent thermal stability and solderability. The tin dipping process forms a flat copper-tin intermetallic compound, ensuring comparable solderability to hot air leveling without the flatness issues associated with the latter. Additionally, it avoids diffusion issues between electroless nickel plating and gold dipping metals. However, tin dipping plated boards cannot be stored for extended periods.
3. OSP
OSP distinguishes itself from other surface treatment processes by serving as a barrier between copper and air. In essence, OSP chemically cultivates an organic film on the clean bare copper surface. This film provides oxidation, heat shock, and moisture resistance, safeguarding the copper surface from further oxidation or sulfidation in normal environmental conditions. Simultaneously, it must be easily removed by flux during subsequent high-temperature soldering processes. OSP finds wide industrial use due to its straightforward process and cost-effectiveness. Early organic coating molecules like imidazole and benzotriazole played anti-rust roles, whereas the latest, primarily benzimidazole molecules, ensure multiple reflow soldering capabilities. To achieve this, the chemical bath typically requires copper additives to adsorb successive layers of organic coating molecules (20 to 100 layers) onto the copper surface. The standard process includes degreasing, micro-etching, pickling, pure water cleaning, organic coating, and final cleaning. Compared to other processes, OSP offers relatively easy process control.
4. Chemical gold
Gold deposition and gold plating are commonly used in PCB prototyping, often causing confusion due to their similar nature. Electroplated nickel gold, commonly termed “hard gold,” involves electroplating gold particles onto the PCB, enhancing adhesion and durability. This process significantly improves PCB hardness and wear resistance, effectively preventing copper and other metal diffusion, meeting requirements for hot pressing and brazing. The resulting coating is uniform, fine, low-porosity, low-stress, and ductile, making it widely suitable for electronic product PCB prototyping. In contrast, gold deposition involves chemically bonding gold particles to the PCB bonding pad, offering weaker adhesion, hence termed “soft gold.” Electroless nickel plating/gold immersion deposits a thick nickel-gold alloy layer with excellent electrical properties on the copper surface, providing long-term PCB protection and superior electrical performance compared to OSP. Unlike OSP, which acts solely as a rust-preventative barrier, electroless nickel plating/gold immersion offers long-term PCB utility and environmental resilience, thanks to the nickel layer’s ability to prevent copper-gold diffusion. A mere 5 µm thick nickel layer can control Z-direction expansion at high temperatures and inhibit copper dissolution, advantageous for lead-free soldering. The process involves six chemical tanks and nearly 100 chemical types, making it relatively complex.
Key differences between PCB gold plating and gold deposition:
1) Thickness: Gold deposition generally results in a thicker layer than gold plating due to different crystal structures. Consequently, gold deposition tends to exhibit a deeper golden hue compared to the lighter shade of gold plating.
2) Crystal structure: The crystal structures formed by gold plating and electroplated nickel gold differ, impacting weldability. Gold plating, being softer, is easier to weld than electroplated nickel gold, preferred for wear resistance in gold finger plates.
3) Application timing: Nickel gold electroplating occurs before resistance welding, whereas gold deposition follows resistance welding on the PCB during prototyping.
