I. Overview
Currently, the common process for processing printed circuit boards involves the “pattern plating method.” This method entails pre-plating a lead-tin resist layer on the copper foil section of the outer layer of the board that needs to be preserved, known as the graphic portion of the circuit. Subsequently, the remaining copper foil is chemically corroded through a process called etching. It is important to note that the board consists of two layers of copper at this stage. During the outer layer etching process, only one layer of copper needs to be completely etched away, while the remainder will form the final required circuit. This pattern plating technique is distinguished by the presence of a copper layer solely beneath the lead-tin resist. Another processing approach involves copper-plating the entire board, with only the portion excluding the photosensitive film covered by a tin or lead-tin resist layer. This method is referred to as the “full board copper plating process.” In comparison to pattern plating, the drawback of full board copper plating is that copper is plated twice across the entire board, necessitating etching removal. Consequently, a variety of issues may arise when the wire width is extremely narrow. Additionally, side corrosion (refer to Figure 4) can significantly impact the line uniformity.
1. In the processing technology of the outer circuit of the printed board, there is another method that involves using photosensitive film instead of metal coating as the resist layer. This method closely resembles the inner layer etching process, and you can draw parallels to the etching in the inner layer fabrication process. Currently, tin or lead-tin is commonly employed as the resist layer, utilized in the etching process with ammonia etchant. Ammonia etchant, a widely used chemical liquid, exhibits no chemical reaction with tin or lead-tin. It mainly comprises ammonia water/ammonium chloride etching solution, with ammonia water/ammonium sulfate etching solution also available in the market. The sulfate-based etching solution, allowing for copper separation via electrolysis post-use, facilitates reusability. However, due to its low corrosion rate, it’s relatively uncommon in actual production, albeit anticipated for chlorine-free etching. Attempts have been made to utilize sulfuric acid-hydrogen peroxide as an etchant for corroding the outer layer pattern. Yet, due to various factors including economic considerations and waste liquid treatment, this method hasn’t found widespread commercial adoption. Moreover, sulfuric acid-hydrogen peroxide isn’t suitable for etching lead-tin resist, and it’s not the primary method in PCB outer layer production, thus receiving minimal attention from practitioners.
2. Etching Quality and Early Issues
Ensuring high-quality etching involves completely removing all copper layers except those under the resist layer. Ideally, etching quality should also consider wire width uniformity and side etching extent. Given the characteristics of current etchants, side etching, stemming from their omnidirectional action, is almost unavoidable. The undercut problem, denoting the ratio of undercut width to etching depth (referred to as the etching factor), is frequently discussed in the printed circuit industry, varying widely from 1:1 to 1:5. Achieving a small undercut degree or low etch factor is desirable. The design of etching equipment and variations in etching solution compositions affect the etching factor and side etching extent, which can be controlled to some extent. Certain additives can mitigate side etching, yet their chemical compositions are typically trade secrets. The interconnectedness of processes in printed circuit processing means that etching issues often trace back to earlier stages, such as the stripping process. The outer layer pattern etching process tends to magnify these issues due to its reflective “reverse stream” phenomenon, coupled with the multitude of processes preceding it. Theoretical ideal cross-sectional states during etching may not align with actual production outcomes, often resulting in copper accumulation and other challenges.
3. Equipment Adjustment and Corrosive Solution Interaction
Ammonia etching in PCB processing entails intricate chemical reactions and equipment requirements. Once set up, continuous operation is crucial to maintain efficiency. Equipment condition significantly influences the etching process. High-pressure spraying, essential for neat line sides and high-quality etching, demands careful selection of nozzle structure and spraying method. Various theories exist to optimize side effects, emphasizing the importance of ensuring constant metal-etchant contact for efficient etching. Ammonia content in the etching solution plays a pivotal role in etching rate, necessitating measures to maintain optimal levels. Control systems, such as pH meters, help manage solution parameters. Ongoing research in chemical etching explores innovative structural designs for enhanced performance. Experimental findings suggest design modifications, like fan-shaped nozzles and precise spray angles, can improve etching outcomes by optimizing solution distribution and jet force.
4. Different Etching States on Upper and Lower Boards
Etching quality issues primarily manifest on the upper surface of the board due to colloidal buildup from etchants. Colloidal solids accumulate on the copper surface, affecting ejection force and fresh etchant replenishment, leading to variations in etching degrees between upper and lower patterns. The leading edge of the board, entering the etching machine first, is prone to complete etching or over-corrosion due to faster etching rates before colloidal buildup occurs. Conversely, the rear-entry edge, encountering accumulated buildup, experiences slower etching rates.
5. Maintenance of Etching Equipment
Maintaining clean nozzles is crucial for smooth spraying and uniform etching. Blockages can cause uneven etching, potentially rendering the entire PCB unusable. Regular replacement of worn parts, including nozzles, is essential. Preventing slag accumulation in the etching machine is equally critical, as excessive buildup can disrupt chemical balance and impede the etching process. Proper cleaning with hydrochloric acid or solution replenishment is necessary to address sudden slagging events, often indicating underlying solution balance issues. Residual film remnants can also contribute to slag formation, underscoring the importance of thorough film removal processes to ensure optimal etching outcomes.
Currently, the common process for processing printed circuit boards involves the “pattern plating method.” This method entails pre-plating a lead-tin resist layer on the copper foil section of the outer layer of the board that needs to be preserved, known as the graphic portion of the circuit. Subsequently, the remaining copper foil is chemically corroded through a process called etching. It is important to note that the board consists of two layers of copper at this stage. During the outer layer etching process, only one layer of copper needs to be completely etched away, while the remainder will form the final required circuit. This pattern plating technique is distinguished by the presence of a copper layer solely beneath the lead-tin resist. Another processing approach involves copper-plating the entire board, with only the portion excluding the photosensitive film covered by a tin or lead-tin resist layer. This method is referred to as the “full board copper plating process.” In comparison to pattern plating, the drawback of full board copper plating is that copper is plated twice across the entire board, necessitating etching removal. Consequently, a variety of issues may arise when the wire width is extremely narrow. Additionally, side corrosion (refer to Figure 4) can significantly impact the line uniformity.
1. In the processing technology of the outer circuit of the printed board, there is another method that involves using photosensitive film instead of metal coating as the resist layer. This method closely resembles the inner layer etching process, and you can draw parallels to the etching in the inner layer fabrication process. Currently, tin or lead-tin is commonly employed as the resist layer, utilized in the etching process with ammonia etchant. Ammonia etchant, a widely used chemical liquid, exhibits no chemical reaction with tin or lead-tin. It mainly comprises ammonia water/ammonium chloride etching solution, with ammonia water/ammonium sulfate etching solution also available in the market. The sulfate-based etching solution, allowing for copper separation via electrolysis post-use, facilitates reusability. However, due to its low corrosion rate, it’s relatively uncommon in actual production, albeit anticipated for chlorine-free etching. Attempts have been made to utilize sulfuric acid-hydrogen peroxide as an etchant for corroding the outer layer pattern. Yet, due to various factors including economic considerations and waste liquid treatment, this method hasn’t found widespread commercial adoption. Moreover, sulfuric acid-hydrogen peroxide isn’t suitable for etching lead-tin resist, and it’s not the primary method in PCB outer layer production, thus receiving minimal attention from practitioners.
2. Etching Quality and Early Issues
Ensuring high-quality etching involves completely removing all copper layers except those under the resist layer. Ideally, etching quality should also consider wire width uniformity and side etching extent. Given the characteristics of current etchants, side etching, stemming from their omnidirectional action, is almost unavoidable. The undercut problem, denoting the ratio of undercut width to etching depth (referred to as the etching factor), is frequently discussed in the printed circuit industry, varying widely from 1:1 to 1:5. Achieving a small undercut degree or low etch factor is desirable. The design of etching equipment and variations in etching solution compositions affect the etching factor and side etching extent, which can be controlled to some extent. Certain additives can mitigate side etching, yet their chemical compositions are typically trade secrets. The interconnectedness of processes in printed circuit processing means that etching issues often trace back to earlier stages, such as the stripping process. The outer layer pattern etching process tends to magnify these issues due to its reflective “reverse stream” phenomenon, coupled with the multitude of processes preceding it. Theoretical ideal cross-sectional states during etching may not align with actual production outcomes, often resulting in copper accumulation and other challenges.
3. Equipment Adjustment and Corrosive Solution Interaction
Ammonia etching in PCB processing entails intricate chemical reactions and equipment requirements. Once set up, continuous operation is crucial to maintain efficiency. Equipment condition significantly influences the etching process. High-pressure spraying, essential for neat line sides and high-quality etching, demands careful selection of nozzle structure and spraying method. Various theories exist to optimize side effects, emphasizing the importance of ensuring constant metal-etchant contact for efficient etching. Ammonia content in the etching solution plays a pivotal role in etching rate, necessitating measures to maintain optimal levels. Control systems, such as pH meters, help manage solution parameters. Ongoing research in chemical etching explores innovative structural designs for enhanced performance. Experimental findings suggest design modifications, like fan-shaped nozzles and precise spray angles, can improve etching outcomes by optimizing solution distribution and jet force.
4. Different Etching States on Upper and Lower Boards
Etching quality issues primarily manifest on the upper surface of the board due to colloidal buildup from etchants. Colloidal solids accumulate on the copper surface, affecting ejection force and fresh etchant replenishment, leading to variations in etching degrees between upper and lower patterns. The leading edge of the board, entering the etching machine first, is prone to complete etching or over-corrosion due to faster etching rates before colloidal buildup occurs. Conversely, the rear-entry edge, encountering accumulated buildup, experiences slower etching rates.
5. Maintenance of Etching Equipment
Maintaining clean nozzles is crucial for smooth spraying and uniform etching. Blockages can cause uneven etching, potentially rendering the entire PCB unusable. Regular replacement of worn parts, including nozzles, is essential. Preventing slag accumulation in the etching machine is equally critical, as excessive buildup can disrupt chemical balance and impede the etching process. Proper cleaning with hydrochloric acid or solution replenishment is necessary to address sudden slagging events, often indicating underlying solution balance issues. Residual film remnants can also contribute to slag formation, underscoring the importance of thorough film removal processes to ensure optimal etching outcomes.
