PCB Blog - Factors affecting the characteristics of the etching solution in the PCB board

1. Selection of etching solution
The choice of etching solution is very important, and it is so important because it directly affects the accuracy and quality of high-density thin wire images in the printed circuit boards manufacturing process. Of course, the etching characteristics of the etching solution are affected by many factors, including physical, chemical and mechanical aspects. It is briefly described as follows:

1.1 Physical and chemical aspects
1) The concentration of the etching solution: The concentration of the etching solution should be determined by the test method according to the principle of metal corrosion and the structure type of the copper foil.
2) The composition of the chemical composition of the etching solution: the chemical composition of the etching solution is different, the etching rate is different, and the etching coefficient is also different. For example, the etching coefficient of the commonly used acid cupric chloride etching solution is usually &; the coefficient of alkaline cupric chloride etching solution can reach 3.5-4. The nitric acid-based etching solution in the development stage can achieve almost no side etching problem, and the sidewall of the etched wire is close to vertical.
3) Temperature: The temperature has a great influence on the characteristics of the etching solution. Usually, during the chemical reaction process, the temperature plays an important role in accelerating the fluidity of the solution, reducing the viscosity of the etching solution, and increasing the etching rate. However, if the temperature is too high, it is also easy to cause some chemical components in the etching solution to volatilize, resulting in an imbalance of chemical components in the etching solution. At the same time, if the temperature is too high, it may cause polymer resistance, damage to the etching layer and affect the use of etching equipment. life. Therefore, the temperature of the etching solution is generally controlled within a certain process range.
4) Thickness of copper foil used: The thickness of copper foil has an important influence on the wire density of the circuit pattern. The copper foil is thin, the etching time is short, and the side etching is small; otherwise, the side etching is large. Therefore, the copper foil thickness must be selected according to the design technical requirements and the wire density and wire accuracy requirements of the circuit pattern. At the same time, the elongation of copper, the surface crystal structure, etc., will have a direct impact on the characteristics of the etching solution.
5) The geometry of the circuit: If the distribution position of the circuit pattern wires in the X direction and the Y direction is not balanced, it will directly affect the flow speed of the etching solution on the board surface. Similarly, if the conductive parts with narrow spacing and the conductive parts with wide spacing are on the same board surface, the etching will be excessive at the parts where the conductive lines with wide spacing are distributed. Therefore, this requires the designer to first understand the feasibility of the process when designing the circuit, try to make the circuit pattern evenly distributed on the entire board, and the thickness of the wires should be as consistent as possible. Especially in the production of multi-layer printed circuit boards, large-area copper foil is used as the ground layer, which has a great impact on the quality of etching, so it is recommended to design a mesh pattern.

2.2 Mechanical aspects
2.2.1 Type of equipment: The structure of the equipment is also one of the important factors affecting the characteristics of the etching solution. In the initial stage, the method of immersion in an immersion tank is used to etch the printed circuit board with a wide wire. The precision requirement is not high, and it is an acceptable equipment structure. For printed circuit boards with thin wires, narrow spacing, high precision and high density, the structure of immersion etching equipment is no longer suitable. Etching equipment in the form of horizontal mechanical transmission structure is used and a swing nozzle device is used to make the copper of the substrate. The surface printed circuit is etched more uniformly, but the horizontal device structure will cause over-corrosion of the board surface, so the vertical spray technology is developed and developed. At the same time, the etching equipment must also have a device to prevent the thin copper-clad laminate from being easily wound on the rollers and conveying wheels during etching and cause waste, and to ensure that the metal on the surface of the wire pattern is not scratched or scratched. Therefore, when selecting etching equipment, special attention should be paid to the structural form, whether it can meet the requirements of fast etching rate, uniform etching and high etching quality. The figure below is the outline drawing of the acid etching machine and the alkaline etching machine.

2.2.2 Spray technology:
1) Spray shape: The conditions and structure that the general spray system should have at present is to adopt a chain and cone structure in the spray system. All conveyed printed circuit boards are fully covered with etching solution and can flow evenly. The process test results show that:
2) Fixed spray: the average etching depth is 0.20mm and the standard deviation is 0.01mm.
Oscillating shower: The average etching depth is 0.21mm and the standard deviation is 0.004.
3) Swing mode: It has been proved by current actual production experience that arc swing is ideal, which can make the etching solution reach the entire board surface, improve the consistency of the etching rate, and provide a reliable guarantee for the production of high-density thin wires.

2.2.3 Distance: The so-called distance refers to the distance from the nozzle to the surface of the board, that is, the distance from the etchant sprayed to the surface of the substrate, which is very important. When considering the distance from the nozzle to the surface of the substrate, it must also be combined with the pressure of the spray to conduct research and design, that is, to achieve high quality etching, it must also meet the requirements of economy, adaptability, manufacturability, and availability. Maintainability and replaceability.

2.2.4 Pressure: In the design, it is necessary to consider the effect of pressure on the spray of the etching solution, whether it can form a uniform flow of the etching solution on the surface of the substrate and the balance of the flow of the etching solution. Therefore, if the spray pressure is too large or too small, it will affect the etching quality.

2.3 Fluid mechanics
1) Surface tension of the etching solution: Because any object has a certain surface area, the surface of the liquid is like a thin film, which has a molecular-level inward attraction, making it shrink. In order to maintain this In a tense surface balance, an appropriate tangential force must be added to the surface perimeter so that the surface can maintain a certain area and no longer shrink. This force tangent to the surface is called surface tension. The tension per unit length on a surface is represented by the symbol σ. The unit is dynes/cm. The effect of the surface tension of the etching solution on the etching rate and etching quality is related to the wetting degree of the solid surface (referring to the surface of the copper foil). The so-called wetting is the phenomenon of liquid adherence on the solid surface. That is, the shape of the liquid on the solid surface is related to the size of the contact angle (θ). The larger the contact angle, the worse the wetting of the solid surface, that is, the worse the hydrophilicity. To maintain the contact angle (θ) as an acute angle, the surface properties of the solid must be changed. That is to say, the lower the surface tension of the liquid, the better the wetting performance of the solid surface, but if the solid surface is stained, even if the surface tension of the liquid is small, the wetting of the solid surface will not be improved. Therefore, in order to obtain the etching quality of the scorpion, it is necessary to strengthen the cleaning treatment of the surface of the copper foil of the substrate, and improve the surface properties to make it better wet with the etching solution. To improve the surface tension of the liquid, it is also necessary to increase the operating temperature. The higher the temperature, the smaller the surface tension of the liquid, the more ideal the adhesion to the solid, and the better the treatment effect. This is due to the expansion of the material caused by the increase in temperature, which increases the distance between molecules and reduces the attraction between molecules, so as the temperature of the solution increases, the surface tension gradually decreases. Therefore, by strictly controlling the process conditions, the contact state between the solution and the copper surface of the substrate can be better improved.
2) Viscosity: During the etching process, with the continuous dissolution of copper, the viscosity of the etching solution will increase, so that the fluidity of the etching solution on the surface of the copper foil of the substrate is poor, which directly affects the etching effect. To achieve the ideal state of the etching solution, it is necessary to make full use of the function of the etching machine to ensure the fluidity of the solution on PCB board.