Electroplating of high aspect ratio through holes is a key in the multi-layer board manufacturing process. Since the ratio of board thickness to aperture is higher than 5:1, It is very difficult to make the copper plating layer cover the hole wall evenly.Due to the small pore size and high depth, it is difficult for the through hole to meet the process requirements during the entire processing process. The process step that is most prone to quality problems is the removal of epoxy drilling contamination, that is, concave etching, which makes it difficult to control the micro-etching depth of the concave etching. Because the pore size is small and deep, it is difficult for the concave etchant to pass through the entire hole smoothly, and some contact it first. The epoxy resin part of the etched part is etched, and when it is completely soaked by the etched etchant, the etched depth of the earlier part exceeds the standard, exposing the glass fiber. A void is formed so that the later copper sinking cannot cover it all, and a void phenomenon occurs.
The second is that during the copper precipitation process, the exchange of the solution is blocked, and it is difficult to replace the fresh copper precipitation solution, which greatly reduces the coverage of the copper precipitation. The third is that the dispersion ability of electroplating does not meet the process requirements, and it is easy to dissolve part of the copper sink layer, resulting in the formation of voids or no copper plating layer. In this case, how to use the existing process equipment to improve the reliability of the plated-through hole and the complete compliance of the hole plating layer is the focus of this article.
1. Analysis of The Cause of Electroplating Defects in High Aspect Ratio Through Holes
In order to ensure the high reliability and high stability of the quality of the multi-layer board, it is necessary to fully understand the key points in the whole process of multi-layer board manufacturing, that is, the key control points. To be more specific, it is the process that is prone to quality problems. It is necessary to know not only the location of the problem, but also the root cause of the defect and the factors that directly affect it. Through years of production experience in the production of multi-layer boards, the part where quality problems often occur is to remove epoxy drilling contamination (ie, etchback). Because this type of multi-layer board is thick and the hole diameter is small and deep, it is difficult for the etchant to pass through the entire hole smoothly, just like a water pipe. Make the tap water flow smoothly. Also, because the flow of the pore solution flows into the hole by its own power, if there is no pressure, it will flow through the pore wall. Some parts of the epoxy resin contacted first are etched, and when all of them are immersed in the etched solution, the depth of the etched part of the first part exceeds the standard, exposing the glass fiber, forming a cavity so that the subsequent copper sinking cannot cover it all. A void appears. In the process of copper precipitation, the fluidity of the solution in the hole is poor. The main reason is that the hole diameter is small, and the resistance of the two sides of the hole deep to the hole is too large so that the reacted solution in the hole cannot be replaced with fresh copper precipitation liquid in time. The parts where copper ions are lacking and copper has been deposited will also be etched by the solution. In addition, after the boards treated with other treatment solutions are cleaned, the water in the holes is not exhausted, and air bubbles are formed when the copper precipitation solution is carried out, which hinders the reduction of copper ions and the conductive layer will be lacking in this part. During electroplating, due to the limited dispersing ability of bright acid copper plating, it is difficult for the current to reach the central part when an inrush current is applied, and it is often due to incomplete deposition of copper, and the part where the copper has been immersed cannot be drawn due to insufficient current. The deposited layer is obtained, and thus is etched and dissolved by the acid solution, forming a cavity. If the impulse current taken is too large, the copper-sinking parts, especially both ends of the orifice, will be ablated.
2. Control and Countermeasures for High Aspect Ratio Through Hole Plating
According to the analysis of the reasons discussed above, for the actual situation of copper sinking and electroplating of high aspect ratio multilayer boards, corresponding process countermeasures must be taken, and key control should be strengthened for the most prone to problems. Especially in the removal of epoxy drilling contamination, on the one hand, we should choose a concave etching solution with good wetting performance. The bubbles are driven away, and the swollen and loose epoxy resin is removed from the surface of the inner copper ring, showing an ideal metallic luster and increasing the bonding force with the copper sink layer and the electroplating layer. Because the inner layer circuit of the multi-layer board relies on the complete hole plating layer of the hole to achieve reliable electrical connection with the outer layer or the required inner layer, if the hole plating layer is disconnected from the inner layer circuit or has serious defects in connection, it will be complete circuit breakage after assembly. If the inner layer of epoxy is not firmly adhered to the hole wall, even if the copper layer is plated, the bonding hole is very poor, and it will fall off due to thermal shock during electrical installation or it will also come off after the pull-off test. Therefore, improving the etching process, increasing the flow of the etching solution in the hole, constantly replacing the fresh etching solution, and ensuring that the epoxy drilling contamination of the inner copper ring is cleaned are the fundamentals to ensure the integrity of the coating in the hole.
In the second aspect, to solve the quality problem of copper sinking, it is necessary to analyze the key points of copper sinking on small holes and hole depth. The important thing is to ensure that the copper sinking solution flows smoothly in the hole and is constantly replaced, so that the copper sinking can form a dense conductive layer on the surface of the hole wall. The specific method is that according to the characteristics of the thickness of the PCB multi-layer board and the aperture ratio, in addition to selecting the formula and process conditions of the copper sinking series with relatively high activity, the main purpose is to increase the fluidity of the small hole solution. Auxiliary equipment, vibration devices should be added in the key process of plated-through holes, The purpose is to escape the air bubbles in all the holes on the multilayer board, and more importantly, to ensure the free flow of the copper sinking liquid in the holes, to fully contact the hole wall to achieve a complete reaction, and the deposition layer to be complete and defect-free. But it is also very important to consider how to match the mounting method of this device. If a basket is used to hang the copper sink, the plate must be inclined at an angle, and the swing device can improve the fluidity of the copper sink liquid in the hole.
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