In the production of PCB circuit boards, many manufacturers continue to rely on wet film imaging technology due to cost considerations. This choice can lead to issues such as “seepage” and “bright edges” (thin tin) during the electroplating process with pure tin. In this article, I will share solutions to common challenges associated with pure tin plating that I have compiled over the years. The electroplating processes for circuit boards can be broadly categorized into: acid bright copper electroplating, nickel/gold electroplating, and tin electroplating. This article outlines the technology and workflow of the electroplating process, as well as specific operational methods involved in PCB circuit board fabrication. Shenzhen Honglijie offers professional services in PCBA, PCB copying, PCB design, SMT chip processing, and both OEM and ODM materials.
**Process Flow:**
Acid pickling – full board copper electroplating – graphic transfer – acid degreasing – secondary countercurrent rinsing – micro-etching – secondary countercurrent rinsing – acid pickling – tinning – secondary countercurrent rinsing – countercurrent rinsing – acid pickling – graphic copper plating – two-stage countercurrent rinsing – nickel plating – secondary water washing – citric acid immersion – gold plating – recycling – 2-3 rinses with pure water – drying.
**2. Analysis of the Causes of “Seepage” in Wet Film Boards (Quality Issues with Non-Pure Tin Solutions)**
1. The copper surface brushed before silk screening must be thoroughly clean to ensure optimal adhesion between the copper surface and the wet film.
2. If the exposure energy for the wet film is insufficient, the wet film may not cure completely, resulting in poor resistance to pure tin electroplating.
3. Unreasonable pre-bake parameters for the wet film can lead to significant temperature variations within the oven. As the thermal curing of photosensitive materials is highly sensitive to temperature, inadequate heat can result in incomplete curing, diminishing the wet film’s ability to resist electroplating with pure tin.
4. The absence of post-treatment or curing treatments diminishes the resistance to electroplating pure tin.
5. Boards intended for electroplating with pure tin must be thoroughly cleaned with water. Additionally, each board should be placed on a rack or dry board, and stacking of boards is not permitted.
6. Issues related to wet film quality.
7. The production and storage environment, along with duration, have significant impacts. A subpar storage environment or prolonged storage can cause the wet film to swell, thereby reducing its resistance to electroplating pure tin.
8. The wet film may be compromised and dissolved by the pure tin brightener and other organic contaminants present in the tin tank. Insufficient anode area in the tin plating tank will inevitably lead to decreased current efficiency and oxygen evolution during the electroplating process (principle of electroplating: oxygen evolution at the anode, hydrogen evolution at the cathode). Excessively high current density and elevated sulfuric acid content can result in hydrogen generation at the cathode, which attacks the wet film and leads to tin permeation (commonly referred to as “permeation”).
9. A high concentration of stripping solution (sodium hydroxide), elevated temperature, or prolonged immersion can result in tin flow or dissolution (known as “dialysis”).
10. Excessive current density during pure tin plating is problematic. Generally, the optimal current density for maintaining wet film quality is between 1.0 to 2.0 A/dm². Exceeding this range can lead to increased susceptibility to “seepage.”
3. Causes of “seepage” related to potion issues and proposed improvements.
1. Reason:
The occurrence of “dipping” due to potion issues primarily stems from the formulation of the pure tin brightener. The brightener’s strong penetrating capability attacks the wet film during the electroplating process, resulting in “dialysis.” Specifically, excessive addition of pure tin brightener or slightly elevated current can trigger “dipping.” Under normal current conditions, “dipping” is linked to uncontrolled solution parameters, such as excessive pure tin brightener, high current levels, or elevated stannous sulfate or sulfuric acid concentrations, all of which enhance the attack on the wet film.
2. PCB design and improvement measures:
The aggressive nature of most pure tin brighteners when influenced by electric current necessitates careful management to avoid “seepage” in the wet film during pure tin plating. It is recommended to consider the following three points for optimal wet film plating:
1. When adding pure tin brightener, it should be done in small increments and monitored closely. The content of pure tin brightener in the plating solution should typically be maintained at the lower limit.
2. Keep the current density within the acceptable range.
3. Maintain control over the syrup composition, ensuring that stannous sulfate and sulfuric acid concentrations remain at the lower limit, which will also help mitigate “dialysis.”
**Process Flow:**
Acid pickling – full board copper electroplating – graphic transfer – acid degreasing – secondary countercurrent rinsing – micro-etching – secondary countercurrent rinsing – acid pickling – tinning – secondary countercurrent rinsing – countercurrent rinsing – acid pickling – graphic copper plating – two-stage countercurrent rinsing – nickel plating – secondary water washing – citric acid immersion – gold plating – recycling – 2-3 rinses with pure water – drying.
**2. Analysis of the Causes of “Seepage” in Wet Film Boards (Quality Issues with Non-Pure Tin Solutions)**
1. The copper surface brushed before silk screening must be thoroughly clean to ensure optimal adhesion between the copper surface and the wet film.
2. If the exposure energy for the wet film is insufficient, the wet film may not cure completely, resulting in poor resistance to pure tin electroplating.
3. Unreasonable pre-bake parameters for the wet film can lead to significant temperature variations within the oven. As the thermal curing of photosensitive materials is highly sensitive to temperature, inadequate heat can result in incomplete curing, diminishing the wet film’s ability to resist electroplating with pure tin.
4. The absence of post-treatment or curing treatments diminishes the resistance to electroplating pure tin.
5. Boards intended for electroplating with pure tin must be thoroughly cleaned with water. Additionally, each board should be placed on a rack or dry board, and stacking of boards is not permitted.
6. Issues related to wet film quality.
7. The production and storage environment, along with duration, have significant impacts. A subpar storage environment or prolonged storage can cause the wet film to swell, thereby reducing its resistance to electroplating pure tin.
8. The wet film may be compromised and dissolved by the pure tin brightener and other organic contaminants present in the tin tank. Insufficient anode area in the tin plating tank will inevitably lead to decreased current efficiency and oxygen evolution during the electroplating process (principle of electroplating: oxygen evolution at the anode, hydrogen evolution at the cathode). Excessively high current density and elevated sulfuric acid content can result in hydrogen generation at the cathode, which attacks the wet film and leads to tin permeation (commonly referred to as “permeation”).
9. A high concentration of stripping solution (sodium hydroxide), elevated temperature, or prolonged immersion can result in tin flow or dissolution (known as “dialysis”).
10. Excessive current density during pure tin plating is problematic. Generally, the optimal current density for maintaining wet film quality is between 1.0 to 2.0 A/dm². Exceeding this range can lead to increased susceptibility to “seepage.”
3. Causes of “seepage” related to potion issues and proposed improvements.
1. Reason:
The occurrence of “dipping” due to potion issues primarily stems from the formulation of the pure tin brightener. The brightener’s strong penetrating capability attacks the wet film during the electroplating process, resulting in “dialysis.” Specifically, excessive addition of pure tin brightener or slightly elevated current can trigger “dipping.” Under normal current conditions, “dipping” is linked to uncontrolled solution parameters, such as excessive pure tin brightener, high current levels, or elevated stannous sulfate or sulfuric acid concentrations, all of which enhance the attack on the wet film.
2. PCB design and improvement measures:
The aggressive nature of most pure tin brighteners when influenced by electric current necessitates careful management to avoid “seepage” in the wet film during pure tin plating. It is recommended to consider the following three points for optimal wet film plating:
1. When adding pure tin brightener, it should be done in small increments and monitored closely. The content of pure tin brightener in the plating solution should typically be maintained at the lower limit.
2. Keep the current density within the acceptable range.
3. Maintain control over the syrup composition, ensuring that stannous sulfate and sulfuric acid concentrations remain at the lower limit, which will also help mitigate “dialysis.”
