Semi-Additive Method for Making Precise Fine Wires:
The main process flow of the semi-additive method is: electroless copper on the whole board (generally, the thickness of the copper layer is less than 0.1 mil, dry film, exposure, and development processes are carried out, and then the lines and holes are electroplated on the line, and then faded after electroplating. The dry film is removed for stenciling to form lines. The advantage of this process is that only the bottom copper (usually <0.1 mil) before pattern plating needs to be etched off during etching, so the stenciling process will not bring obvious side etching to the lines. The lines are distorted. Therefore, with the semi-additive method, we can produce finer and smaller pitch lines. To master this technique, you must first master the following key techniques:
A. Electroless Thick Copper Technology
Electroless thick copper is different from ordinary electroless copper. It requires that the thickness of the copper to be deposited is greater than 0.05mil. If the copper layer is too thin, it will be difficult to pre-treat the surface of the reel film in the next step. Because after grinding, the copper layer will be ground off to expose the substrate. Without grinding, the adhesion between the dry film and the copper may be affected. Therefore, it is necessary to select a suitable copper sinking liquid and match with specific process conditions in order to obtain a satisfactory copper layer.
B. Graphic Plating Technology
Pattern plating technology is the most critical technology in semi-additive technology. Because each type of board has its own line layout, and there are inevitably independent lines, large areas, thick lines, thin lines when wiring. For pattern plating, uneven wiring will inevitably lead to uneven current distribution. The electric charge is most concentrated on the independent line, so the plating thickness of the independent line and the thick line at the same position on the same PCB board will be significantly different. The uneven thickness of this coating will affect the uniformity of the green oil for silk-screen solder mask in the subsequent process.
The commonly used additive method is to form a circuit after full-board copper electroplating, and the uniformity of the coating of full-board electroplating will be much higher than that of pattern electroplating. Therefore, to make thin wires by the additive method, the distribution problem of the patterned electroplating layer must be solved first. Using forward and reverse pulse electroplating power supply, after adjusting the parameters of forward and reverse pulse time, forward and reverse pulse current, a more uniform coating distribution can be obtained after pattern plating.
Blind Hole Filling Technology by Electroplating:
The conventional HDI laser blind via process faces the following problems: There are voids in the blind vias of the SBU layer, in which air may remain, which affects reliability after thermal shock. The conventional method to solve this problem is to fill the blind hole with resin or fill the blind hole with resin ink plugging process by pressing the plate. However, the reliability of the boards produced by these two methods is difficult to guarantee and the production efficiency is low. In order to improve the process capability and improve the HDI process, the process of filling blind holes by electroplating is adopted. The advantage is that the blind holes can be filled with electroplated copper, which greatly improves the reliability. Making circuit patterns or superimposing blind vias greatly improves the process capability to adapt to customers’ increasingly complex and flexible designs.
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