Factors affecting the characteristics of etching liquid in printed PCB

Factors affecting the characteristics of etching liquid in printed PCB

The choice of etchant is very important, so it is important because it directly affects the accuracy and quality of high-density thin-wire images in the printed circuit board 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 Physical and chemical aspects

1) Concentration of etching solution: According to the metal corrosion principle and the structure type of copper foil, the concentration of the etching liquid should be determined by the test method. It should have a large choice, that is, the process range is wide.

2) The composition of the chemical composition of the etching solution: the chemical composition of the etching liquid is different, the etching rate is different, and the etching coefficient is also different. For example, the etching coefficient of the commonly used acidic copper chloride etching solution is usually &; the alkaline copper chloride etching liquid coefficient can reach 3.5-4. The nitric acid-based etchant, which is in the development stage, can achieve almost no side-etching problems, and the etched wire sidewalls are nearly vertical.

3) Temperature: The influence of temperature on the characteristics of the etching solution is relatively large. Generally, during the chemical reaction, the temperature plays an important role in accelerating the fluidity of the solution and reducing the viscosity of the etching solution and increasing the etching rate. However, if the temperature is too high, it will easily cause some chemical components in the etching solution to volatilize, resulting in an imbalance in the chemical composition of the etching solution. At the same time, the temperature is too high, which may cause high polymer resistance.

The etched layer is destroyed and affects the service life of the etching equipment. 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; on the contrary, the side etching is large. Therefore, the thickness of the copper foil 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., all constitute a direct influence on the characteristics of the etching solution.

5) Circuit geometry: If the distribution position of the circuit pattern wire in the X direction and the Y direction is not balanced, it will directly affect the flow speed of the etching liquid on the plate surface. Similarly, in the state of the narrow-conducting wire portion and the wide-distance wire portion on the same plate surface, the portion where the wire is spaced apart is excessively etched. Therefore, this requires the designer to first understand the feasibility of the process in the circuit design, as far as possible to achieve a uniform distribution of the entire board circuit pattern, the thickness of the wire should be as consistent as possible. Especially in the production of multi-layer printed circuit boards, large-area copper foil as a grounding layer has a great influence on the quality of etching, so it is recommended to design a mesh pattern.

2 mechanical aspects

1) Type of equipment: The structural form of the equipment is also one of the important factors affecting the characteristics of the etching liquid. In the initial stage, the immersion tank immersion method is used to etch the printed circuit board with a wide wire width, and the precision requirement is not high, and the device structure form can be adopted. For thin wires, narrow pitch, high precision, high density printed circuit boards, the immersion etching equipment structure is not suitable, and the etching equipment in the form of horizontal mechanical transmission structure is used and the oscillating nozzle device is used to make the copper of the substrate. The surface printed circuit is more evenly etched, but the horizontal device structure causes over-corrosion of the board surface, so the vertical spray technology has been developed and developed. At the same time, the etching apparatus must also have means for preventing the thin copper clad laminate from being easily wound on the rollers and the transfer wheel during etching to cause waste, and to ensure that the surface of the wire pattern is not scratched or scratched. Therefore, when selecting an etching device, special attention should be paid to the structural form, and whether the etching rate is fast, the etching is uniform, and the etching quality is high.

2) Spray technology:

1 Spray shape: The current general spray system should have the conditions and structure, which is a chained, cone-shaped structure in the spray system. The etchant sprayed from all the nozzles is fan-shaped and alternates with each other. The printed circuit board is completely covered by the etchant and can flow uniformly. From the process test results show that:

Fixed spray: The average etching depth is 0.20 mm and the standard deviation is 0.01 mm.

Swing type spray: The average etching depth is 0.21 mm and the standard deviation is 0.004.

2 Swing mode: Through the current actual production experience, the arc swing is ideal, which can make the etchant reach the whole board surface, improve the consistency of the etch rate, and provide a reliable guarantee for making high-density thin wires.

3 Distance: The so-called distance refers to the distance from the nozzle to the surface of the board, that is, the distance that the etching liquid is sprayed onto the surface of the substrate. This 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 for research and design. To achieve the high quality of etching, it must also be economical, adaptable, manufacturable, and Maintainability and replaceability.

4 Pressure: The design should take into account the effect of pressure on the etchant spray, and whether the surface of the substrate can form a uniform etchant flow and the flow of the etchant is balanced. Therefore, too much or too little spray pressure will have an effect on the quality of the etch.

3 Fluid mechanics

1) Surface tension of the etchant: Since any object has a certain surface area, the liquid surface appears to have a tight film. This film has a molecular-level inward attraction, which causes it to shrink. In order to maintain this For a tight surface balance, a proper and surface tangential force must be added to the surface perimeter to maintain a certain area of the surface, no longer shrinking. This force tangent to the surface is called surface tension. The tension acting on the unit length on the surface is indicated by the symbol σ. The unit is dyne/cm. The effect of the surface tension of the etchant on the etch rate and the etch quality is related to the degree of wetting of the solid surface (referred to as the copper foil surface). The so-called wetting is called the attachment of liquid on a solid surface. That is, the shape of the liquid on the solid surface is related to the contact angle (θ). The larger the contact angle, the worse the wetting of the solid surface, that is, the poorer the hydrophilicity. To maintain the contact angle (θ) to the minimum acute angle, the surface properties of the solid must be changed. That is to say, the smaller the surface tension of the liquid, the better the wetting property to the solid surface, but if the solid surface is stained, even if the surface tension of the liquid is small, the degree of wetting of the solid surface is not improved. Therefore, in order to obtain the most satisfactory etching quality, it is necessary to strengthen the cleaning treatment of the surface of the substrate copper foil, and to improve the surface properties to make the etching liquid better in a wet state. 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, and the attachment to the solid is more desirable, and the treatment effect is good. This is due to the expansion of the substance caused by the increase in temperature, which increases the distance between molecules and reduces the attraction between molecules. Therefore, as the temperature of the solution increases, the surface tension gradually decreases. For this reason, strict control of the process conditions can better improve the contact state of the solution with the copper surface of the substrate.

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