I. Introduction
When discussing the development trends in PCBs, many people often associate them with advancements in precision, density, and reliability. While these are indeed key trends, it is essential to recognize that users are also imposing stricter demands regarding the appearance of PCBs. The solder resist serves as the “coat” of the PCB. Beyond requiring specific thickness and hardness, it must pass solvent resistance and adhesion tests to meet industry standards. Furthermore, the surface color should be uniform and glossy (with domestic customers generally preferring a brighter finish), and it must be free from debris and extraneous markings. The aesthetic quality of PCB solder resist not only reflects an enterprise’s technical and management capabilities but also has a direct impact on its “orders.” Thus, improving the appearance quality of PCB solder resist has become a critical challenge for every PCB manufacturer. Drawing from my practical experience, I will discuss how to enhance the appearance quality of PCB solder resist through four key aspects: screen printing, exposure, development, and post-curing.
II. Factors Affecting the Appearance Quality of PCB Solder Resist
1. Silk Screen:
During the screen printing of photosensitive solder resist ink, several factors can impact appearance quality, including the flatness of the squeegee, the cleanliness of the environment during printing, the type of sealing tape used, the ink pressure during application, and the surface preparation prior to printing. Based on actual production experiences, the most significant influences are the first three factors. An uneven squeegee can easily create marks on the surface of the PCB solder resist; insufficient cleanliness in the workspace can lead to debris on the surface; and improper sealing tape may dissolve adhesive in the ink solvent, resulting in surface particles.
**2. Exposure:**
During the solder mask ink exposure process, the PCB solder resist may not be fully cured. Consequently, when the solder mask film adheres to the PCB solder resist, imprints can easily occur, which significantly affects the aesthetic quality of the PCB solder resist.
**3. Development:**
Currently, the development of solder resist ink typically employs a horizontal transfer method. Given that the PCB solder resist is not entirely cured, the driving wheel and pressure wheel of the developing machine may damage the surface, leading to roller marks that detract from the appearance of the PCB solder resist. Additionally, improper exposure energy can impact the gloss of the PCB solder resist; however, this can be managed using a wedge meter.
**4. Post-Curing:**
During the curing of PCB solder resist, uneven temperatures can result in inconsistent coloring. Excessive heat may cause localized yellowing or blackening, adversely affecting the visual quality of the PCB solder resist.
**3. Enhancing the Appearance Quality of PCB Solder Resist from Four Aspects:**
**1. Silk Screen:**
1.1 When screen printing solder mask ink, the surface irregularities of the screen printing surface can lead to unevenness over time. This can result in squeegee marks on the PCB solder resist. Operators must consistently monitor the condition of the squeegee; if marks are detected, it should be re-ground immediately to maintain its flatness.
1.2 To address the issue of plastic particles on the surface of the PCB solder resist, parallel tests were conducted. Two different tapes were used for sealing, alongside two solder mask inks for screen printing, to observe the presence of rubber particles on the surface.
**Ink and Tape Compatibility:**
| Tape | Ink A | Ink B |
|——–|——————————————|——————————————|
| A | Rubber particles appear after 2 minutes. | Virtually no rubber particles observed. |
| B | Rubber particles appear after 30 minutes. | Virtually no rubber particles observed. |
From these test results, it is evident that the combination of Tape A and Ink B yields the best results, while Tape B works well with both Ink A and Ink B, although Tape B is five times more expensive than Tape A. Thus, in actual production, attention must be given to the compatibility of solder mask ink and sealing tape to prevent surface rubber particles.
1.3 Achieving a PCB with high aesthetic quality requires a clean environment during the silk screening process. All surfaces in contact with the PCB—including countertops, screen frames, blotting paper, and sealing tape—must be cleaned with dust rollers. Transport vehicles should be designated for cleanroom use and kept spotless. Operators must wear specialized cleanroom attire, including caps, and adhere to strict wind-bathing protocols. It’s also vital to maintain air purification throughout the facility; if feasible, regular water spraying can help minimize dust accumulation.
**2. Exposure:**
Addressing the issue of negative film adhesion during solder mask exposure is crucial for enhancing the aesthetic quality of the PCB. This primarily involves equipment considerations. First, ensure the surface temperature of the glass plate on the frame does not exceed 30°C during prolonged exposure. In the case of air-cooled low-power exposure machines (below 7KW), longer exposure times can rapidly raise the glass plate’s temperature; therefore, cooling measures (like air conditioning or heat insulation) should be implemented. Second, it’s essential to maintain the correct vacuum level. Excessive vacuum can cause the film to stick to the PCB solder resist, leading to imprinting. Experiments indicate that a vacuum range of 70-80% yields optimal results.
**Effect of Vacuum on Film Adhesion:**
| Vacuum Level | 60% | 70% | 80% | 90% |
|————–|————–|————–|————–|————–|
| Adhesion | Ghost images during development | No sticking, normal development | No sticking, normal development | No sticking, normal development |
Lastly, the MYLAR film on the exposure frame should be dedicated to the exposure machine, preferably using lithographic film rather than convex or concave films during dry film exposure to minimize negative film effects on the PCB solder resist.
**3. Development:**
During development, the PCB solder resist is not fully cured, making it susceptible to roller marks. This can be mitigated through equipment adjustments. First, ensure that the transmission roller is made from a soft material or fitted with a soft PVC “O” ring, while the pressure and squeeze rollers should also be soft rubber. Second, the stability of the entire transmission system must be assured. Finally, regular cleaning of the pressure and squeeze rollers is necessary to remove any dirt that could cause roller marks.
**4. Post-Curing:**
The post-curing phase primarily focuses on maintaining uniform oven temperature. The temperature uniformity should be regularly tested by measuring nine points (eight corners and one center point) under operational conditions, ensuring the variance does not exceed 5°C. Additionally, guidelines should be established regarding the loading of each oven and the orientation of the boards to prevent uneven heating and damage to the PCB solder resist, which could lead to yellowing and poor appearance.
**Conclusion:**
To enhance the aesthetic quality of PCB solder resist, a comprehensive approach is necessary, encompassing process methodologies, raw materials, equipment, and operator discipline. It is crucial to strictly monitor various parameters across the screen printing, exposure, development, and post-curing stages. By doing so, the visual quality of PCB solder masks can meet and exceed customer expectations.
