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Selection of etching solution

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

1. Physical and chemical aspects

1）Concentration of etching solution: the concentration of etching solution shall be determined by test method according to the principle of metal corrosion and the structural type of copper foil. It shall have a large choice, that is, it refers to a wide process range.

2）Chemical composition of etching solution: different chemical components of etching solution will lead to different etching rates and etching coefficients.For example, the etching coefficient of the commonly used acidic copper chloride etching solution is usually &;The coefficient of alkaline copper chloride etching solution can reach 3.5-4.The nitric acid based etching solution in the development stage can achieve almost no side corrosion problem, and the side wall of the etched wire is close to vertical.

3）Temperature: temperature has a great impact on the characteristics of the etching solution. Generally, in the process of chemical reaction, temperature plays a very important role in accelerating the fluidity of the solution, reducing the viscosity of the etching solution and improving the etching rate.However, if the temperature is too high, it is also easy to cause the volatilization of some chemical components in the etching solution, resulting in the imbalance of the proportion of chemical components in the etching solution. At the same time, if the temperature is too high, it may cause the damage of the polymer resist layer and affect the service life of the etching equipment.Therefore, the etching solution temperature is generally controlled within a certain process range.

4）Adopted copper foil thickness: the thickness of copper foil has an important impact on the conductor density of circuit graphics.The copper foil is thin, the etching time is short, and the side corrosion is very small;On the contrary, lateral erosion is very large.Therefore, the thickness of copper foil must be selected according to the design technical requirements, conductor density and conductor accuracy requirements of circuit graphics.At the same time, the elongation and surface crystalline structure of copper will have a direct impact on the characteristics of etching solution.

5）Geometry of circuit: if the distribution position of circuit graphic wires in X direction and Y direction is uneven, it will directly affect the flow speed of etching solution on the board surface.Similarly, if the wires with wide spacing are distributed in the wires with narrow spacing and wide spacing on the same board, the etching will be excessive.Therefore, when designing the circuit, the designer should first understand the feasibility of the process, try to achieve the uniform distribution of the circuit graphics on the whole board, and the thickness of the conductor should be consistent as far as possible.Especially when making multilayer printed circuit board, large area copper foil as grounding layer has a great impact on the etching quality, so it is recommended to design a mesh pattern.

2. Mechanical aspect

1）Type of equipment: the structural form of equipment is also one of the important factors affecting the characteristics of etching solution.In the initial stage, the immersion tank immersion method is adopted to etch the printed circuit board with wide conductor. The accuracy requirement is not high. It is an available equipment structure.For printed circuit boards with thin wires, narrow spacing, high precision and high density, the immersion etching equipment structure is no longer suitable. The etching equipment in the form of horizontal mechanical transmission structure and swing nozzle device are adopted to make the printed circuit etching on the copper surface of the substrate more uniform, but the horizontal equipment structure will cause over corrosion on the board surface,Therefore, the vertical spray technology is 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 roller and conveyor wheel during etching, and 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 to meet the requirements of fast etching rate, uniform etching and high etching quality.

2）Spray technology:

①　Spray shape: the conditions and structural form of the current general spray system are that the interlocking and conical structure is adopted in the spray system. The etching solution sprayed by all nozzles is fan-shaped and alternating with each other, so that all transmitted printed circuit boards are covered by the etching solution and can flow evenly.The process test results show that:
·Fixed spray: the average etching depth is 0.20mm and the standard deviation is 0.01mm.
·Swing spray: the average etching depth is 0.21mm and the standard deviation is 0.004.

②　Swing mode: the current actual production experience shows that the arc swing is ideal, which can make the etching solution reach the whole plate surface, improve the consistency of etching rate, and provide a reliable guarantee for the production of high-density thin wires.

③　Distance: the so-called distance refers to the distance from the nozzle to the plate surface, that is, the distance from the etching solution to the substrate surface, which is very important.When considering the distance from the nozzle to the substrate surface, it must also be combined with the spray pressure for research and design, that is, in order to achieve high etching quality, it must also comply with economy, adaptability, manufacturability, maintainability and replaceability.

④　Pressure: in the design, the effect of pressure on the spraying of etching solution, whether uniform etching solution flow can be formed on the substrate surface and the balance of the flow amount of etching solution shall be considered.Therefore, too large or too small spray pressure will affect the etching quality.

3. Hydrodynamics

1）Surface tension of etching solution: because any object has a certain surface area, the liquid surface is like a tight film. This film has a molecular inward attraction, which makes it tend to shrink. In order to maintain this tight surface balance, an appropriate tangential force must be added to the surface perimeter to maintain a certain surface area,No longer shrink, this force tangent to the surface is called surface tension.Symbols for tension per unit length acting on a surfaceσexpress.The unit is dyne / cm.The influence of surface tension of etching solution on etching rate and etching quality is related to the wetting degree of solid surface (referring to copper foil surface).The so-called wetting is the adhesion of liquid on the solid surface.That is, the shape and contact angle of the liquid on the solid surface（θ）The larger the contact angle, the worse the wettability of the solid surface, that is, the worse the hydrophilicity. The contact angle should be maintained（θ）In order to minimize the acute angle, the surface properties of the solid must be changed. That is to say, the smaller the liquid surface tension, the better the wettability of the solid surface. However, if the solid surface is contaminated, even if the liquid surface tension is small, the wettability of the solid surface will not be improved. Therefore, in order to obtain the best etching quality, the cleaning treatment of the copper foil surface of the substrate must be strengthened,Improve the surface properties so that it is better wetted 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 with the solid, and the better the treatment effect. This is because the increase of temperature causes the expansion of substances, increases the distance between molecules, and reduces the attraction between molecules. As the temperature of the solution increases, the surface tension decreases gradually. Therefore, strictly controlling the process conditions can better improve the contact state between the solution and the copper surface of the substrate.

2）Viscosity: in 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 copper foil surface of the substrate is poor, which directly affects the etching effect. In order to achieve the best state of the ideal etching solution, it is necessary to make full use of the function of the etching machine to ensure the fluidity of the solution.