Analysis of Etching Technology of PCB Outer Circuit Board

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The typical process used in printed circuit board (PCB) manufacturing employs the "graphic plating method." In this process, a layer of lead-tin corrosion-resistant coating is pre-applied to the copper foil areas that need to be preserved on the outer layer of the board—essentially marking the areas of the circuit. Following this, the remaining copper foil is removed through chemical etching.

At this stage, there are two layers of copper on the board. In the outer etching process, it is crucial to ensure that only one layer of copper is completely etched away, leaving the desired circuit intact. The graphic plating method is characterized by a layer of copper that remains only beneath the lead-tin resist layer.

An alternative method is the "full plate copper plating process," where the entire board is initially coated with copper. In this case, the areas outside the sensitive film are covered with a layer of tin or lead-tin resist. The primary drawback of full plate copper plating compared to graphic plating is that it requires the surface of the board to go through plating and etching processes twice. This can lead to various problems, especially when dealing with very fine wire widths. Additionally, side corrosion can significantly impact the uniformity of the lines.

Analysis of Etching Technology of PCB Outer Circuit Board

During the processing of the outer circuit of the printed board, another option is to use a sensitive film in place of a metal coating as the corrosion-resistant layer. This technique resembles the etching process used for the inner layers of the board.

Presently, tin or lead-tin is the most commonly used corrosion-resistant layer during the etching process, which often involves ammonia-based etching agents. Ammonia etching agents, including solutions like ammonia water and ammonia chloride, do not react with tin or lead-tin. Furthermore, ammonia/ammonium sulfate etching solutions are also available in the market.

Sulfate-based etching solutions can have their copper content separated and reused through electrolysis after use. Due to their low corrosion rate, these solutions are rarely used in actual production. However, they may find applications in chlorine-free etching methods. Some tests have been conducted using sulfuric acid and hydrogen peroxide as etching agents to remove the outer pattern. For various reasons—economic and related to liquid waste treatment—this process has not gained widespread commercial acceptance. Additionally, sulfuric acid and hydrogen peroxide cannot be used to etch lead-tin corrosion-resistant layers, and thus this approach is not commonly employed for the production of PCB outer layers.

The fundamental requirement for etching quality is to completely remove all copper layers except for those protected by the resist layer. More specifically, etching quality must also consider the consistency of wire widths and the degree of side erosion. Due to the nature of the current etching fluids, etching occurs not only downward but also in all directions, making side erosion almost unavoidable.

Side erosion is a significant topic of discussion regarding etch parameters. It is defined as the ratio of the side erosion width to the etching depth, referred to as the etching factor. In the printed circuit industry, this ratio can vary widely, ranging from 1:1 to 1:5. A smaller degree of side etching, or a lower etching factor, is generally more desirable.

The design of the etching equipment and the composition of the etching fluid can influence the etching factor or the degree of side erosion, which can potentially be controlled. Some additives are known to reduce side corrosion; however, the specific chemical compositions of these additives are typically kept confidential by their developers.

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