This article provides a detailed overview of the metallographic slicing production process and explores its application in PCB manufacturing, utilizing numerous images and examples, particularly focusing on its role in addressing quality issues during production.
Printed circuit boards are essential components in electronic devices and are extensively used throughout the electronics industry. Their quality must be assessed using specific testing techniques. The PCB manufacturing process is intricate, and any quality issue that arises at any stage can lead to the scrapping of the printed circuit board. Therefore, the inspection of printed circuit boards is categorized into in-process inspection and finished product inspection. Common inspection methods include visual examination with a magnifying glass and backlight inspection. Metallographic slicing technology is favored by PCB manufacturers due to its low investment cost and broad applicability. As a destructive testing method, metallographic slicing allows for the evaluation of multiple properties of printed circuit boards, such as resin contamination, plating cracks, delamination of hole walls, solder coating conditions, interlayer thickness, plating thickness, plating thickness in holes, side erosion, inner layer ring width, interlayer overlap, plating quality, and roughness of hole walls, among others. In essence, much like doctors utilize X-rays to diagnose patients, this technique enables observation of defects and the condition of both the surface and cross-section fine structure of printed circuit boards. I have gained considerable insight into this in my work, and it can be summarized in the following aspects:
1. Microsectioning Production Process
The metallographic slicing production process includes:
Extracting the board to be inspected – taking a sample – precision cutting to fit the mold size – embedding – rough grinding – fine grinding – polishing – micro-etching – observation.
1) Extract the production board that requires metallographic slicing on the production line.
2) Use a shear to cut both the center and edge of the sample intended for metallographic sectioning.
3) Employ a precision cutting machine to trim the sample to fit the mold size, ensuring that the cutting surface remains parallel or perpendicular to the surface intended for observation.
4) Utilize a special mold for metallographic sectioning, placing the sample upright in the mold with the part to be inspected facing upward. Mix the cold curing resin (solid) and curing agent (liquid) in a paper cup at a volume ratio of 2:1, stir thoroughly, and pour into the mold until the sample is fully submerged. Allow the mold to sit for 10-20 minutes until the resin has completely cured.
2. The Role of Metallographic Slicing Technology in the Production Process of Printed Circuit Boards
The quality of printed circuit boards, the identification and resolution of issues, and the enhancement and assessment of processes all rely on metallographic sections as a foundation for objective inspection, research, and judgment.
2.1 The Role in Quality Inspection and Control of the Production Process
The PCB production process is complex, with various processes being interrelated. To ensure the reliability of the final product, the quality of the semi-finished boards at each stage of the intermediate process must be satisfactory. How can we assess the quality of the production board during this process? Metallographic slicing technology provides us with that basis.
2.1.1 Hole Roughness Inspection After Drilling
To ensure the quality of hole metallization in the printed circuit board, the roughness of the hole wall post-drilling must be tested. A test board can be created using drill bits of varying sizes, and a sample can be taken for metallographic sectioning. The roughness can then be measured using a reading microscope. For greater accuracy, the sample may be metallized before sectioning.
2.1.2 Detection of Resin Contamination and Etchback Effects
During drilling, the printed circuit board experiences instant high temperatures, and the epoxy glass substrate is a poor thermal conductor. Heat accumulates during drilling, causing the surface temperature of the hole wall to exceed the glass transition temperature of the epoxy resin, leading to a thin ring of oxygen resin contamination. If multilayer boards are drilled without subsequent etching, the signal lines within the multilayer board may not connect, compromising board quality. Metallographic sections can reveal the effectiveness of resin contamination removal after etchback, aiding in quality control for multilayer boards.
2.2 The Role in Resolving Quality Issues in the Production Process
Quality issues frequently arise during the production of printed circuit boards. If metallographic slicing technology can quickly identify the root cause of these problems, timely corrective measures can be implemented, reducing production costs, ensuring on-time delivery, and enhancing customer satisfaction. Below are some solutions to specific issues:
2.2.1 Plating Void Problem (Plating Voids)
1. Poor Copper Deposition:
a. Slicing Features: Symmetrical ring-shaped voids appear in the holes, with patterned copper plating visible over the entire board, including chemical copper.
b. Cause Analysis: Voids in the symmetrical holes indicate a lack of copper due to trapped bubbles during the copper deposition process, preventing contact between the chemical solution and the hole wall, thus inhibiting the copper deposition reaction.
2. Dry Film Intrusion into Holes:
a. Slicing Feature: No copper present at the air port position, exhibiting asymmetry.
b. Cause Analysis: If the board remains stationary for too long after applying the dry film and is positioned vertically, the dry film may flow into the holes. During pattern plating, copper and tin cannot plate in those areas, resulting in no copper remaining after the film is removed and the copper is etched away during processing.
2.2.2 Undercut
The outer layer pattern of multilayer PCBs is formed through an etching process. When the board passes through the etching solution, unnecessary copper is removed. In the case of thick copper plates with electroplating, the etching solution can attack unprotected copper surfaces on both sides of the circuit, resulting in mushroom-like etching defects. In standard copper plates, the etching solution not only erodes the copper layer vertically but also horizontally, causing the cross-section of the line after etching to resemble a trapezoid, typically wider at the bottom than the top—this phenomenon is known as side etching. The degree of side etching is quantified by the width of the side etching.
In PCB production, excessive side etching can compromise the accuracy of printed traces, making fine wiring impossible. Additionally, severe undercutting can lead to protruding edges, which may cause short circuits. Metallographic sections can help identify significant side etching issues, facilitating the investigation of underlying causes and subsequent improvements. For boards with line widths/spacing of 6 mils or more, controlling etching line width is relatively straightforward, allowing for increased film line width compensation. For boards with line widths/spacing of 4-5 mils, controlling etching line width becomes more challenging, necessitating a target of 90% clean etching for production control. To reduce side etching, strict controls over copper concentration, pH levels, temperature, and spraying methods are typically implemented. For instance, changing from splashing to spraying can enhance etching efficiency and reduce side etching; adjusting the etching rate to a faster pace can mitigate severe side erosion; monitoring the pH level of the etching solution is crucial, as higher pH can increase side corrosion; and utilizing a high copper concentration etching solution can reduce side etching. With targeted improvements, side erosion issues can be effectively resolved.
3. Conclusion
The production process of printed circuit boards is intricate and involves collaborative efforts across multiple stages. The quality control of each process significantly impacts the final product’s quality. Metallographic slicing technology plays a vital role in quality control. Furthermore, various quality issues will invariably arise during PCB production. Effectively addressing these problems requires leveraging metallographic slicing technology’s fast and accurate capabilities to identify the root causes. This understanding allows for the optimization of process parameters, mitigation of human errors, and strict adherence to production standards.
Printed circuit boards are essential components in electronic devices and are extensively used throughout the electronics industry. Their quality must be assessed using specific testing techniques. The PCB manufacturing process is intricate, and any quality issue that arises at any stage can lead to the scrapping of the printed circuit board. Therefore, the inspection of printed circuit boards is categorized into in-process inspection and finished product inspection. Common inspection methods include visual examination with a magnifying glass and backlight inspection. Metallographic slicing technology is favored by PCB manufacturers due to its low investment cost and broad applicability. As a destructive testing method, metallographic slicing allows for the evaluation of multiple properties of printed circuit boards, such as resin contamination, plating cracks, delamination of hole walls, solder coating conditions, interlayer thickness, plating thickness, plating thickness in holes, side erosion, inner layer ring width, interlayer overlap, plating quality, and roughness of hole walls, among others. In essence, much like doctors utilize X-rays to diagnose patients, this technique enables observation of defects and the condition of both the surface and cross-section fine structure of printed circuit boards. I have gained considerable insight into this in my work, and it can be summarized in the following aspects:
1. Microsectioning Production Process
The metallographic slicing production process includes:
Extracting the board to be inspected – taking a sample – precision cutting to fit the mold size – embedding – rough grinding – fine grinding – polishing – micro-etching – observation.
1) Extract the production board that requires metallographic slicing on the production line.
2) Use a shear to cut both the center and edge of the sample intended for metallographic sectioning.
3) Employ a precision cutting machine to trim the sample to fit the mold size, ensuring that the cutting surface remains parallel or perpendicular to the surface intended for observation.
4) Utilize a special mold for metallographic sectioning, placing the sample upright in the mold with the part to be inspected facing upward. Mix the cold curing resin (solid) and curing agent (liquid) in a paper cup at a volume ratio of 2:1, stir thoroughly, and pour into the mold until the sample is fully submerged. Allow the mold to sit for 10-20 minutes until the resin has completely cured.
2. The Role of Metallographic Slicing Technology in the Production Process of Printed Circuit Boards
The quality of printed circuit boards, the identification and resolution of issues, and the enhancement and assessment of processes all rely on metallographic sections as a foundation for objective inspection, research, and judgment.
2.1 The Role in Quality Inspection and Control of the Production Process
The PCB production process is complex, with various processes being interrelated. To ensure the reliability of the final product, the quality of the semi-finished boards at each stage of the intermediate process must be satisfactory. How can we assess the quality of the production board during this process? Metallographic slicing technology provides us with that basis.
2.1.1 Hole Roughness Inspection After Drilling
To ensure the quality of hole metallization in the printed circuit board, the roughness of the hole wall post-drilling must be tested. A test board can be created using drill bits of varying sizes, and a sample can be taken for metallographic sectioning. The roughness can then be measured using a reading microscope. For greater accuracy, the sample may be metallized before sectioning.
2.1.2 Detection of Resin Contamination and Etchback Effects
During drilling, the printed circuit board experiences instant high temperatures, and the epoxy glass substrate is a poor thermal conductor. Heat accumulates during drilling, causing the surface temperature of the hole wall to exceed the glass transition temperature of the epoxy resin, leading to a thin ring of oxygen resin contamination. If multilayer boards are drilled without subsequent etching, the signal lines within the multilayer board may not connect, compromising board quality. Metallographic sections can reveal the effectiveness of resin contamination removal after etchback, aiding in quality control for multilayer boards.
2.2 The Role in Resolving Quality Issues in the Production Process
Quality issues frequently arise during the production of printed circuit boards. If metallographic slicing technology can quickly identify the root cause of these problems, timely corrective measures can be implemented, reducing production costs, ensuring on-time delivery, and enhancing customer satisfaction. Below are some solutions to specific issues:
2.2.1 Plating Void Problem (Plating Voids)
1. Poor Copper Deposition:
a. Slicing Features: Symmetrical ring-shaped voids appear in the holes, with patterned copper plating visible over the entire board, including chemical copper.
b. Cause Analysis: Voids in the symmetrical holes indicate a lack of copper due to trapped bubbles during the copper deposition process, preventing contact between the chemical solution and the hole wall, thus inhibiting the copper deposition reaction.
2. Dry Film Intrusion into Holes:
a. Slicing Feature: No copper present at the air port position, exhibiting asymmetry.
b. Cause Analysis: If the board remains stationary for too long after applying the dry film and is positioned vertically, the dry film may flow into the holes. During pattern plating, copper and tin cannot plate in those areas, resulting in no copper remaining after the film is removed and the copper is etched away during processing.
2.2.2 Undercut
The outer layer pattern of multilayer PCBs is formed through an etching process. When the board passes through the etching solution, unnecessary copper is removed. In the case of thick copper plates with electroplating, the etching solution can attack unprotected copper surfaces on both sides of the circuit, resulting in mushroom-like etching defects. In standard copper plates, the etching solution not only erodes the copper layer vertically but also horizontally, causing the cross-section of the line after etching to resemble a trapezoid, typically wider at the bottom than the top—this phenomenon is known as side etching. The degree of side etching is quantified by the width of the side etching.
In PCB production, excessive side etching can compromise the accuracy of printed traces, making fine wiring impossible. Additionally, severe undercutting can lead to protruding edges, which may cause short circuits. Metallographic sections can help identify significant side etching issues, facilitating the investigation of underlying causes and subsequent improvements. For boards with line widths/spacing of 6 mils or more, controlling etching line width is relatively straightforward, allowing for increased film line width compensation. For boards with line widths/spacing of 4-5 mils, controlling etching line width becomes more challenging, necessitating a target of 90% clean etching for production control. To reduce side etching, strict controls over copper concentration, pH levels, temperature, and spraying methods are typically implemented. For instance, changing from splashing to spraying can enhance etching efficiency and reduce side etching; adjusting the etching rate to a faster pace can mitigate severe side erosion; monitoring the pH level of the etching solution is crucial, as higher pH can increase side corrosion; and utilizing a high copper concentration etching solution can reduce side etching. With targeted improvements, side erosion issues can be effectively resolved.
3. Conclusion
The production process of printed circuit boards is intricate and involves collaborative efforts across multiple stages. The quality control of each process significantly impacts the final product’s quality. Metallographic slicing technology plays a vital role in quality control. Furthermore, various quality issues will invariably arise during PCB production. Effectively addressing these problems requires leveraging metallographic slicing technology’s fast and accurate capabilities to identify the root causes. This understanding allows for the optimization of process parameters, mitigation of human errors, and strict adherence to production standards.
