Etching is one of the most important processes in PCB manufacturing, to put it simply, wet PCB etching is the process of controlling corrosion. Under normal circumstances, corrosion will damage the metal, but through the effect of the process, you can control corrosion, this process is called etching. This article we will explain what is pcb etching, pcb etching solutions and other issues.

What is PCB etching?

PCB etching is the process of removing unwanted copper (cu) from a circuit board. When it is not needed, it is nothing more than non-circuit copper that is removed from the board based on the PCB design and the desired circuit pattern is obtained.

In other words, etching is like chiseling a circuit board; let’s say the board is a rock and etching chisels the rock into a beautiful sculpture. During the process, the substrate copper or starting copper is removed from the circuit board, and both rolled and annealed copper are easily etched away compared to electroplated copper.

Layout in advance of the etching process. There are two different methods for inner and outer layer etching. In the outer layer etching process, a nephew layer is plated as a resist. In the inner layer, the photoresist is the resist.

PCB Etching Process

Dry Etching

Dry etching removes unwanted copper by stimulating a chemical reaction through plasma or laser.
▶ Plasma etching has been introduced since the 1960s and has been widely used since the 1970s. Advantages include environmental friendliness, cleanliness, precision, and reduced risk of contamination, but it is costly and suitable for high-end manufacturing.
▶ Laser etching, on the other hand, utilizes a high-energy laser to accurately engrave the copper layer, omitting redundant processes and avoiding the use of chemicals, but with challenges in large-area uniformity and residue disposal, as well as high costs.

Wet Etching

Wet etching uses a chemical solution to dissolve copper, which is divided into two types: acid and alkaline.
▶ Acid etching is mostly used for the inner layer, often using copper chloride, and has the advantage of high etching precision and a small undercut, but is slower and not suitable for mass production.
▶ Alkaline etching is commonly used for the outer layer, which is fast and low cost, but requires strict time control to avoid damage. It is usually done in a conveyorized spray chamber to improve efficiency and accuracy.

PCB etching

Solutions for PCB etching

PCB etching solutions mainly include the following aspects:

Side-etching control: In the process of thick copper PCB etching, the side-etching phenomenon will lead to a decrease in line width accuracy, affect the line impedance and may lead to a short circuit. In order to control side-etching, the etchant formulation can be optimized, using ammonia etchant and dynamically adjusting the pH value and copper ion concentration to improve etching uniformity. In addition, laser direct imaging (LDI) technology can replace the traditional exposure process to improve the graphic accuracy to ±10 μm and reduce the “copper root” problem caused by the residual edge of the resist layer.

Resist layer residue and copper tumor defects processing: After the graphic plating, the lead and tin resist layer is easy to form “edges” due to uneven thickness, resulting in incomplete removal of the film, triggering incomplete etching. It is possible to use a segmented depaneling process, combined with high-pressure spraying technology to completely remove the edge of the resist layer residue. At the same time, online AOI inspection can be used immediately after the etching of automatic optical inspection equipment to identify the copper tumor and line gap, real-time adjustment of etching parameters.

Etching fluid life and environmental management: the traditional sulfuric acid – hydrogen peroxide system has been eliminated due to the difficulty of recycling waste liquid. A closed loop system can be used, integrated copper ion electrolytic recycling device, the copper content in the waste liquid will be reduced to less than 50ppm, and the reuse rate of etching solution reuse up to 80%. Chlorine-free etching processes such as sulfuric acid amine etching solution can not only reduce environmental pollution, but also meet the demand for high-frequency high-speed PCB fine etching.

Micro-etching treatment: through chemical or electrochemical methods in the circuit board surface to form a layer of uniform microscopic roughness, increase the surface area, improve the adhesion of subsequent plating or coating layer. Commonly used micro-etching solutions are sulfuric acid – hydrogen peroxide system and persulfate system. Micro-etching treatment needs to strictly control the processing time and temperature to avoid excessive corrosion.

Roughening treatment: for the need to increase the bonding force of the plating and the substrate of the circuit board, you can use mechanical roughening or chemical roughening methods. Mechanical roughening through sandblasting, sanding and other ways to increase the surface roughness; chemical roughening is the use of chemical reagents and circuit board surface reaction, the formation of rough structure.

Activation treatment: the formation of a layer of catalytically active metal film on the surface of the board, for subsequent plating or chemical plating to provide an active center. Commonly used activation methods are sensitization – activation of two-step method and one-step colloidal palladium activation method. Activation treatment can significantly improve the bonding force between the plating layer and the substrate to ensure the quality of the plating layer.

Summarize

PCB etching is a key process to accurately remove unwanted copper foils from copper-clad boards through chemical or electrochemical methods to form the required circuit graphics, which requires strict control of equipment parameters, chemical formulations and process conditions, combined with on-line monitoring and quality testing to ensure high precision and yield, and focus on environmental protection and cost optimization to meet the needs of high-end electronics manufacturing.