1. Types of PCB etching
2. It should be noted that during the etching process, PCBs typically have two layers of copper. In outer layer etching, one layer of copper needs complete removal, leaving behind the final circuit configuration. This method involves pattern electroplating, where copper plating exists solely beneath the lead-tin resist layer.
3. Another method involves copper plating the entire board, with only the areas not covered by photosensitive film coated in tin or lead-tin resist. This process is known as the “full board copper plating process.”
4. Compared to pattern electroplating, full board copper plating requires copper plating over the entire board surface, leading to increased copper thickness and subsequent etching of all areas. This approach becomes disadvantageous particularly when dealing with fine wire widths, as it can lead to a host of issues.
5. Additionally, side corrosion can significantly impact line uniformity during the etching process.
1. There is another method in the processing technology of the outer circuit of the printed circuit board, which uses photosensitive film instead of metal plating as the resist layer. This method closely resembles the inner layer etching process; you can refer to the etching process used in inner layer manufacturing.
2. Currently, tin or lead-tin is the most commonly used anti-corrosion layer, employed in the etching process using ammonia-based etchants. Ammonia-based etchants are commonly used chemical solutions that do not undergo any chemical reactions with tin or lead-tin. Ammonia etchants typically refer to ammonia/ammonium chloride etching solutions. Additionally, ammonia/ammonium sulfate etching chemicals are also available in the market. After using sulfate-based etching solutions, copper can be separated through electrolysis for reuse. Due to its low corrosion rate, it is generally less common in practical production, but it is expected to be utilized in chlorine-free etching.
3. There have been attempts to use sulfuric acid-hydrogen peroxide as an etchant for corroding outer layer patterns. However, due to factors such as cost and the treatment of waste liquids, this process has not gained widespread commercial adoption. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for etching lead-tin resist, and it is not the primary method for outer layer PCB production, hence receiving minimal attention from most practitioners.
4. **PCB Etching Quality and Historical Issues**
5. The fundamental requirement for etching quality is the complete removal of all copper layers except under the resist layer. Strictly defined, etching quality must also consider wire width consistency and undercutting degree. Current etching solutions inherently cause side etching in addition to downward etching effects, which is nearly unavoidable. Side etching, defined by the ratio of side etch width to etch depth (the etching factor), is a parameter frequently discussed in the printed circuit industry, varying widely from 1:1 to 1:5.
6. A smaller undercut degree or lower etching factor is preferred. The configuration of etching equipment and the composition of etching solutions affect the etching factor and the extent of side etching, which can be managed or controlled optimistically. Some additives can mitigate side erosion, but their chemical composition is typically kept confidential as trade secrets by developers.
7. Etching quality issues often originate before the PCB enters the etching machine, as the processes in PCB fabrication are interconnected. Problems observed in etching quality are often symptomatic of issues occurring earlier in the process, such as during film removal or even before.
8. For outer layer graphics etching, the phenomenon of “inverted stream” is particularly pronounced compared to other PCB processes, highlighting underlying issues. This is exacerbated by the numerous steps from lamination to photosensitive application, culminating in the successful transfer of outer layer patterns. The greater the number of steps, the higher the likelihood of problems—a unique characteristic of PCB production.
9. Ideally, after entering the etching stage, the PCB should maintain a state where the total thickness of copper plus tin or lead-tin from pattern electroplating does not exceed the thickness of the resistive photosensitive film. In practice, however, electroplated patterns globally exhibit greater thickness than photosensitive patterns. During copper and lead-tin electroplating, the excessive plating height leads to lateral buildup, creating edges that extend beyond the photosensitive film’s coverage.
10. These tin or lead-tin edges prevent complete removal of the photosensitive film during stripping, leaving residual adhesive beneath. This residual adhesive, or “residual film,” causes incomplete etching, resulting in copper roots forming on both sides of the lines post-etching. This narrowing of line spacing fails to meet Party A’s requirements, potentially leading to rejection, significantly increasing PCB production costs.
11. Moreover, reactions in the printed circuit industry may lead to the formation and accumulation of residual film and copper in corrosive liquids, clogging etching machine nozzles and acid-resistant pumps. Such occurrences necessitate shutdowns for cleaning and maintenance, impacting overall efficiency.
2. It should be noted that during the etching process, PCBs typically have two layers of copper. In outer layer etching, one layer of copper needs complete removal, leaving behind the final circuit configuration. This method involves pattern electroplating, where copper plating exists solely beneath the lead-tin resist layer.
3. Another method involves copper plating the entire board, with only the areas not covered by photosensitive film coated in tin or lead-tin resist. This process is known as the “full board copper plating process.”
4. Compared to pattern electroplating, full board copper plating requires copper plating over the entire board surface, leading to increased copper thickness and subsequent etching of all areas. This approach becomes disadvantageous particularly when dealing with fine wire widths, as it can lead to a host of issues.
5. Additionally, side corrosion can significantly impact line uniformity during the etching process.
1. There is another method in the processing technology of the outer circuit of the printed circuit board, which uses photosensitive film instead of metal plating as the resist layer. This method closely resembles the inner layer etching process; you can refer to the etching process used in inner layer manufacturing.
2. Currently, tin or lead-tin is the most commonly used anti-corrosion layer, employed in the etching process using ammonia-based etchants. Ammonia-based etchants are commonly used chemical solutions that do not undergo any chemical reactions with tin or lead-tin. Ammonia etchants typically refer to ammonia/ammonium chloride etching solutions. Additionally, ammonia/ammonium sulfate etching chemicals are also available in the market. After using sulfate-based etching solutions, copper can be separated through electrolysis for reuse. Due to its low corrosion rate, it is generally less common in practical production, but it is expected to be utilized in chlorine-free etching.
3. There have been attempts to use sulfuric acid-hydrogen peroxide as an etchant for corroding outer layer patterns. However, due to factors such as cost and the treatment of waste liquids, this process has not gained widespread commercial adoption. Furthermore, sulfuric acid-hydrogen peroxide cannot be used for etching lead-tin resist, and it is not the primary method for outer layer PCB production, hence receiving minimal attention from most practitioners.
4. **PCB Etching Quality and Historical Issues**
5. The fundamental requirement for etching quality is the complete removal of all copper layers except under the resist layer. Strictly defined, etching quality must also consider wire width consistency and undercutting degree. Current etching solutions inherently cause side etching in addition to downward etching effects, which is nearly unavoidable. Side etching, defined by the ratio of side etch width to etch depth (the etching factor), is a parameter frequently discussed in the printed circuit industry, varying widely from 1:1 to 1:5.
6. A smaller undercut degree or lower etching factor is preferred. The configuration of etching equipment and the composition of etching solutions affect the etching factor and the extent of side etching, which can be managed or controlled optimistically. Some additives can mitigate side erosion, but their chemical composition is typically kept confidential as trade secrets by developers.
7. Etching quality issues often originate before the PCB enters the etching machine, as the processes in PCB fabrication are interconnected. Problems observed in etching quality are often symptomatic of issues occurring earlier in the process, such as during film removal or even before.
8. For outer layer graphics etching, the phenomenon of “inverted stream” is particularly pronounced compared to other PCB processes, highlighting underlying issues. This is exacerbated by the numerous steps from lamination to photosensitive application, culminating in the successful transfer of outer layer patterns. The greater the number of steps, the higher the likelihood of problems—a unique characteristic of PCB production.
9. Ideally, after entering the etching stage, the PCB should maintain a state where the total thickness of copper plus tin or lead-tin from pattern electroplating does not exceed the thickness of the resistive photosensitive film. In practice, however, electroplated patterns globally exhibit greater thickness than photosensitive patterns. During copper and lead-tin electroplating, the excessive plating height leads to lateral buildup, creating edges that extend beyond the photosensitive film’s coverage.
10. These tin or lead-tin edges prevent complete removal of the photosensitive film during stripping, leaving residual adhesive beneath. This residual adhesive, or “residual film,” causes incomplete etching, resulting in copper roots forming on both sides of the lines post-etching. This narrowing of line spacing fails to meet Party A’s requirements, potentially leading to rejection, significantly increasing PCB production costs.
11. Moreover, reactions in the printed circuit industry may lead to the formation and accumulation of residual film and copper in corrosive liquids, clogging etching machine nozzles and acid-resistant pumps. Such occurrences necessitate shutdowns for cleaning and maintenance, impacting overall efficiency.
