PCB Vacuum Etching Technology

1. PCB etching definition

Etching: the surface of the copper clad foil is etched by a chemical solution to remove the unwanted copper conductor, leaving the copper conductor to form a line pattern. This subtraction process is the mainstream of the current printed circuit board processing.

2. The key to etching is etching solution, etching operating conditions and etching equipment

A. Etch solution

At present, the acid copper chloride etching solution with copper chloride and hydrochloric acid, and the basic copper chloride etching solution with copper chloride and ammonia water as the mainstream

B. Etch operating conditions

The etching operation condition is to control the process parameters such as temperature, pressure, time and solution concentration, so that the etching process is in the best condition.

C. Etching equipment is around the production efficiency, etching speed and etching uniformity and continuous improvement. At present the mainstream is horizontal transmission spray type.

3. etching equipment

Mainly reflected in the transmission mode and spray mode.

A. Transmission mode is divided into: fixed, vertical, horizontal

Stationary: Early equipment was a tank in which the PCB was immersed after holding the etching solution. Disadvantages: low efficiency of putting board and taking board; Low etching efficiency; Cannot do thin lines, line width/spacing above 0.5mm.

Vertical type: PCB board is hung vertically on the fixture into the etching machine, and the surface of the board is sprayed in solution on both sides of the etching room. Due to the complexity of the etching equipment and the lack of excellent etching results, this technology has not been developed.

Horizont-type: use rollers to transfer PCB with spacing between rollers, install nozzles up and down, etch by spray. Advantages: uniform transmission rate, relatively uniform etching; High efficiency, both sides can be etched simultaneously. Horizontal transmission is the dominant mode at present.

B. Spray method

In order to make the solution can be fully sprayed to every part of the PCB board surface, and to accelerate the flow of the solution on the board, it is necessary to move the spray rod/nozzle to achieve this effect;

The structure of the nozzle also has a variety of patterns, according to the shape of the ejected liquid: a. Solid wire harness type, b. Hollow cone, C. solid cone, D. flat fan, etc. From the effect of the different shapes of the jet fluid, the area of the solid wire harness is too small, and the area of the cone is too large, which is blocked out in the case of dense rollers. Therefore, more flat fan nozzle is selected in the etching machine, and the solution is perpendicular to the surface of the PCB board to impact the copper foil, and the effect is better.

4. "pool effect"

At present, the mainstream horizontal transmission spray type, can achieve maximum production automation, cost reduction, but this way is not perfect, will lead to the "pool effect", so that the etching results are different. Ordinary boards have little effect, but for ultra-fine lines, they cannot meet the etching tolerance requirements. The "pool effect" occurs: when the PCB is transmitted horizontally, it is located below the board and near the edge of the board. The etching solution is easy to flow away, and the old and new etching solution is easier to exchange. And the position of the plate center, easy to form a "pool", etching fluid flow is limited; Therefore, the middle line on the PCB board will be less etched than other positions.

Pill E.K., a German company, has unveiled a new process to prevent the "pool effect" by improving the flow of etching solution from the upper portion of a plate surface by absorbing used etching solution. This method is called vacuum etching.

5. vacuum etching

Principle: The nozzles are installed in the etching section, and the air extraction unit is installed between the nozzles relatively close to the circuit board surface. The suction unit creates a negative pressure in the operating area, just low enough to prevent the etching fluid from creating a "pool effect". By comparison, the etching effect of the upper part of the plate is basically the same as that of the lower part.

Effect: Through the uniformity test to confirm the etching, using 35μm copper foil coated laminate in the conventional and vacuum etching machine respectively for the whole plate surface etching, about the middle of the plate copper foil etching to 18μm thickness stop, detection of copper thickness distribution of the whole plate surface. The thickness distribution of copper obtained by conventional etching is usually high mountain in the center, while the thickness distribution of copper obtained by vacuum etching is uniform. The thickness difference between the center and the edge of conventional etching is about ±4μm (18 ~ 10μm) in the length of 600mm, while that between the center and the edge of vacuum etching is only about ±1μm (19 ~ 17μm), which is only one fourth of that of the former.

The line width/line distance ≤30/30μm can be easily realized by vacuum etching machine. In order to get the ideal effect, we need to grasp the factors such as vacuum strength, spray pressure and copper foil thickness. In the etching machine, the suction amount of the vacuum nozzle on the surface solution should be equal to or slightly greater than the spraying amount. If the suction amount is small, it will cause the retention of old solution. The spray pressure will affect the etching speed and the corrosion degree of the side, but the pressure will destroy the etching resistance film, especially on the detail distance line graphics. For line width/line distance =30/30μm, the thickness of copper foil etched by conventional equipment is less than 12μm, while the thickness of copper foil etched by vacuum is 18μm can be processed. Reference data, line width/line distance =30/30μm, 18μm copper foil, spray pressure 0.1 MPa (1.02kgf/cm2) suction 40Hz (about 120L/min), to get a uniform line pattern.