Issues to be noted during the etching process

1. Reduce side erosion and protrusions, and increase etching coefficient

Side erosion produces protrusions. Usually, the longer the printed circuit board is in the etching solution, the more severe the side etching. Side erosion seriously affects the accuracy of printed conductors, and severe side erosion will make it impossible to produce fine conductors. When the side etching and protrusion decrease, the etching coefficient increases. A high etching coefficient indicates the ability to maintain thin wires, making the etched wires closer to the original drawing size. Electroplating etching resist, whether it is tin lead alloy, tin, tin nickel alloy or nickel, excessive protrusion can cause wire short circuit. Because the protruding edge is prone to breakage, it forms an electrical bridge between two points of the wire.

There are many factors that affect lateral erosion, and the following are summarized:

1) Etching method: Immersion and bubble etching can cause significant side erosion, while splash and spray etching have smaller side erosion, especially spray etching has the best effect.

2) Types of etching solutions: Different etching solutions have different chemical compositions, resulting in different etching rates and etching coefficients. For example, the etching coefficient of acidic copper chloride etching solution is usually 3, while the etching coefficient of alkaline copper chloride etching solution can reach 4. Recent studies have shown that etching systems based on nitric acid can achieve almost no side etching, resulting in etched lines with nearly vertical sidewalls. This etching system is yet to be developed.

3) Etching rate: Slow etching rate can cause severe side etching. The improvement of etching quality is closely related to the acceleration of etching rate. The faster the etching speed, the shorter the time the board stays in the etching solution, the smaller the amount of side etching, and the clearer and neater the etched pattern.

4) The pH value of the etching solution: When the pH value of the alkaline etching solution is high, the side etching increases. To reduce lateral erosion, the pH value should generally be controlled below 8.5, as shown in Figure 10-3.

5) Density of etching solution: If the density of alkaline etching solution is too low, it will aggravate side etching, as shown in Figure 10-4. Choosing etching solution with high copper concentration is beneficial for reducing side etching

6) Copper foil thickness: To achieve minimal etching of thin wires, it is best to use (ultra) thin copper foil. And the thinner the line width,

The thinner the copper foil thickness, the thinner it should be. Because the thinner the copper foil, the shorter the time it stays in the etching solution, and the smaller the amount of side etching.

2. Improve the consistency of etching rate between boards

In continuous etching of boards, the more consistent the etching rate, the more uniformly etched boards can be obtained. To achieve this requirement, it is necessary to ensure that the etching solution remains in the optimal etching state throughout the entire etching process. This requires the selection of etching solutions that are easy to regenerate and compensate for, and have a controllable etching rate. Select processes and equipment that can provide constant operating conditions and automatically control various solution parameters. This is achieved by controlling the amount of copper dissolved, pH value, concentration of the solution, temperature, and uniformity of the solution flow rate (spray system or nozzle and nozzle oscillation).

3. High uniformity of etching rate on the entire surface of the board

The uniformity of etching on both sides of the board and on various parts of the board surface is determined by the uniformity of the etchant flow rate on the board surface. During the etching process, the etching rates of the upper and lower surfaces are often inconsistent. Generally speaking, the etching rate of the lower surface is higher than that of the upper surface. Due to the accumulation of solution on the upper surface, the etching reaction is weakened. The uneven etching of the upper and lower surfaces can be solved by adjusting the spray pressure of the upper and lower nozzles. A common problem with etching printed boards is that it is difficult to completely etch the entire board surface in the same amount of time, as the edges of the board are etched faster than the center of the board. Using a sprinkler system and swinging the nozzle is an effective measure. Further improvement can be achieved by making the spray pressure at the center and edge of the board different, and intermittently etching the front and rear ends of the board to achieve uniform etching of the entire board surface.

4. Improve the ability to safely handle and corrode sharp copper foils and thin laminates

When etching the inner layer of multi-layer boards, such as thin pressed plates, the boards are prone to winding around the rollers and conveyor wheels, resulting in waste. Therefore, the equipment for etching inner laminates must ensure smooth and reliable processing of thin laminates. Many equipment manufacturers attach gears or rollers to etching machines to prevent such phenomena from occurring. A better method is to use an additional left and right swinging polytetrafluoroethylene coated wire as a support for transporting the thin layer pressure plate. For the etching of thin copper foil (such as 1/2 or 1/4 ounce), it is necessary to ensure that it is not scratched or scratched. Thin copper foil cannot withstand the mechanical drawbacks of etching 1 ounce of copper foil, and sometimes severe vibrations may scratch the copper foil.

5. Reducing pollution issues

Copper pollution of water is a common problem in printed circuit production, and the use of ammonia alkali etching solution exacerbates this issue. Because copper is complexed with ammonia, it is not easy to remove by ion exchange or alkaline precipitation methods. So, using the second spray operation method and rinsing the board with copper free additive solution greatly reduces the amount of copper discharged. Then, the excess solution on the surface of the board was removed using an air knife before rinsing with water, thereby reducing the rinsing burden of water on copper and etched salts.