the Advanced Technology of PCB Multilayer Circuit Board (Part 4)

3. Semi-additive method to produce precision fine wire technology   

The main process flow of the semi-additive method is: electroless copper sinking on the whole board (generally copper layer thickness <0.1mil, dry roll film, exposure, development process, and then line and hole positions are electroplated, and then faded after electroplating The dry film is cut off and the circuit is formed. The advantage of this process is that only the bottom copper before the pattern plating (usually <0.1mil) is etched during etching. Therefore, the etching process will not cause obvious side etching to the lines. Lines are distorted. Therefore, the semi-additive method can enable us to produce thinner and smaller pitch lines.

To master this technology, we must first master the following key technologies: 　　

A. Electroless thick copper technology 　　 Electroless thick copper is different from ordinary electroless copper. It requires that the deposited copper thickness is greater than 0.05 mil. If the copper layer is too thin, it will be difficult to perform surface pretreatment for the dry film in the next step. Because after the polishing plate, the copper layer will be worn off and expose the base material, and the adhesion between the dry film and the copper may be affected if the plate is not polished. Therefore, a suitable copper immersion solution must be selected and matched with specific process conditions to obtain a satisfactory copper layer.

B. Graphic plating technology 　　Pattern plating technology is the most critical technology in the semi-additive method. Because each type of board has its own wiring layout, and there are inevitably independent lines, large areas, thick lines, thin lines, etc. when wiring. For pattern plating, uneven wiring will inevitably cause uneven current distribution. The electric charge is the most concentrated on the independent line, so the thickness of the coating of the independent line and the thick line at the same position on the same board will be significantly different. The uneven thickness of the coating will affect the uniformity of the solder mask green oil in the subsequent process.