PCB Fabrication

Details that should be known during the etching process during PCB production

by:A-TECH      2021-03-16
1. Reduce undercutting and sharp edges, and increase the etching coefficient. Undercutting produces sharp edges. Generally, the longer the printed board is in the etching solution, the more serious the side etching. Side erosion seriously affects the accuracy of printed wires, and severe side erosion will make it impossible to make fine wires. When the undercut and the edge are reduced, the etching coefficient increases. A high etching coefficient indicates the ability to maintain thin wires and make the etched wires close to the original image size. Whether the electroplating etching resist is tin-lead alloy, tin, tin-nickel alloy or nickel, excessive overhang will cause a short circuit of the wire. Because the protruding edge is easily broken, an electrical bridge is formed between the two points of the wire. There are many factors that affect side etching. The following points are summarized:) Etching method: immersion and bubbling etching will cause larger side etching, splash and spray etching are small, especially spray etching has the best effect . ) Type of etching solution: Different etching solutions have different chemical components, their etching rates are different, and the etching coefficients are also different. For example, the etching coefficient of acid copper chloride etching solution is usually 3, and the etching coefficient of alkaline copper chloride etching solution can reach 4. Recent studies have shown that an etching system based on nitric acid can achieve almost no side etching, and the sidewalls of the etched lines are nearly vertical. This etching system is yet to be developed. ) Etching rate: slow etching rate will cause severe undercutting. The improvement of the etching quality has a great relationship with the acceleration of the 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 etched patterns are clear and neat. ) PH value of the etching solution: when the pH value of the alkaline etching solution is higher, the side etching increases. ) Density of the etching solution: Too low density of the alkaline etching solution will aggravate the side etching, as shown in Figure 1. It is advantageous to select an etching solution with a high copper concentration to reduce the side etching. Fig. 1 The influence of the ph value of the alkaline etching solution on the undercutting Fig. 1 The influence of the density of the alkaline etching solution on the undercutting The side etching diagram is advantageous. Copper foil thickness: To etch thin wires with minimal side erosion, it is best to use (ultra) thin copper foil. And the thinner the line width, the thinner the thickness of the copper foil should be. Because the thinner the copper foil, the shorter the time in the etching solution, the smaller the amount of side etching. See Table 1 Table 1 The relationship between the thickness of copper foil and the amount of side etching. The reduction of the thickness of copper foil in various etching solutions (%) copper chloride ammonium persulfate ammonia base 17μm compared with 35μm 46% 50% 33% 10μm Compared with 35μm, 65% 70% 45% 2. Improve the consistency of the etching rate between the board and the board. In continuous board etching, the more consistent the etching rate, the more uniformly etched board can be obtained. To meet this requirement, it is necessary to ensure that the etching solution is always kept in the best etching state during the entire etching process. This requires the selection of an etching solution that is easy to regenerate and compensate, and the etching rate is easy to control. Select technology and equipment that can provide constant operating conditions and can automatically control various solution parameters. It is achieved by controlling the amount of dissolved copper, PH value, concentration of the solution, temperature, uniformity of the solution flow (spray system or nozzle and nozzle swing). 3. High uniformity of etching rate on the entire surface of the board. The uniformity of etching on the upper and lower sides of the board and each part of the board is determined by the uniformity of the etchant flow rate on the surface of the board. During the etching process, the etching rates of the upper and lower plates are often inconsistent. Generally speaking, the etching rate of the lower plate surface is higher than that of the upper plate surface. Because of the accumulation of solution on the upper plate surface, the etching reaction is weakened. The uneven etching of the upper and lower plates can be solved by adjusting the spray pressure of the upper and lower nozzles. A common problem of etching printed boards is that it is difficult to etch all the surface of the board at the same time. The edge of the board is etched faster than the center of the board. Using a spray system and swinging the nozzle is an effective measure. A further improvement can be achieved by making the spray pressure at the center and the edge of the board different, and intermittently etching the front and back ends of the board to achieve the uniformity of the etching on the entire board. 4. Improve the ability to safely handle and etch thin copper foils and thin laminates. When etching thin laminates such as the inner layer of a multilayer board, the board is easy to wind up on the rollers and transfer wheels and cause waste. Therefore, the equipment for etching the inner layer board must ensure that it can handle the thin laminate board smoothly and reliably. Many equipment manufacturers add gears or rollers to the etching machine to prevent this phenomenon from happening. A better method is to use additional swaying PTFE coated wire as a support for the transfer of thin laminates. For the etching of thin copper foil (such as 1/2 or 1/4 ounce), it must be ensured that it is not scratched or scratched. Thin copper foil cannot withstand the mechanical disadvantages of etching 1 ounce of copper foil, and sometimes severe vibrations may scratch the copper foil. 5. The problem of reducing pollution The pollution of copper to water is a common problem in the production of printed circuits, and the use of ammonia alkali etching solution makes this problem even more serious. Because copper is complexed with ammonia, it is not easy to remove it by ion exchange or alkaline precipitation. Therefore, the second spraying operation method is used to rinse the board with a copper-free additive solution, which greatly reduces the amount of copper discharged. Then, an air knife is used to remove the excess solution on the board before the water rinsing, thereby reducing the rinsing burden of the water on the copper and etched salts.
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