In PCB proofing, a layer of lead-tin resist is pre-plated on the part of the copper foil that needs to be kept on the outer layer of the board, that is, the graphic part of the circuit, and then the rest of the copper foil is etched away chemically, which is called etching.
As the last step from the light board to the display of circuit pattern, what quality problems should be paid attention to in etching?
The quality requirement of etching is to be able to completely remove all copper layers except under the resist layer. Strictly speaking, the etching quality must include the consistency of wire width and the degree of side etching.
Side etching is often discussed in etching. The ratio of etching width to etching depth is called etching factor; In the printed circuit industry, small side etching or low etching factor is the most satisfactory. The structure of the etching equipment and the etching solution with different components will all affect the etching factor or side etching degree.
In many ways, the etching quality existed long before the PCB entered etching machine. Because there is a very close internal relationship between the various processes of PCB proofing, there is no process that is not affected by other processes and does not affect other processes. Many problems identified as etching quality actually existed in the process of removing film or even before.
Theoretically speaking, PCB proofing enters the etching stage. In the pattern electroplating method, the ideal state should be: the sum of the thicknesses of electroplated copper and lead and tin should not exceed the thickness of the electroplating-resistant photosensitive film, so that the electroplated pattern is completely blocked by the “walls” on both sides of the film and embedded in it. However, in actual production, the coating pattern is much thicker than the photosensitive pattern; Because the height of the coating exceeds that of the photosensitive film, it tends to accumulate laterally. The tin or lead-tin resist layer covered above the line extends to both sides, forming a “edge”, covering a small part of the photosensitive film under the “edge”. Or the “edge” formed by tin or lead-tin, so that the photosensitive film can not be completely removed when removing the film, leaving a small amount of “residual glue” under the “edge”, resulting in incomplete etching. After etching the lines, “copper root” is formed on both sides, which narrows the line spacing, resulting in the printed board not meeting the customer’s requirements and even being rejected. Rejection will greatly increase the production cost of PCB.
In PCB proofing, once there is a problem in etching process, it must be a batch problem, which will eventually cause great quality hidden trouble to the products. Therefore, it is particularly important to find a suitable PCB proofing manufacturer.
