The Use of Insoluble Anodes in Acid Copper Plating

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Introduction

Electrolytic acid copper is the process that builds the traces that carry the current throughout a PCB. How to optimize your electrolytic acid copper plating for today's designs—aspect ratios >20:1 for through-holes and >1:1 aspect ratios for blind via fill—is the challenge. The use of insoluble (mixed metal oxide-coated, or MMO-coated, titanium mesh) produces a consistent and reproducible plated product, is environmentally friendly (eliminates waste), and eliminates anode maintenance, thus increasing the productivity of the plating line.

Vertical acid copper plating remains a very common way for plating PCBs. For best results, the equipment should be optimized with proper rectification and connectivity. The electrolyte and the additives used, along with the plating current density, all play a role in the copper thickness distribution on the plated panel. The anodes have a direct impact on copper thickness distribution. The anode shape, size, and location play a critical role in the thickness distribution of the plated copper in vertical plating of panels in a tank. The vertical plating tank is challenging in its own way, unlike horizontal conveyorized plating where all the panels are exposed to the identical set of anodes, as the part is conveyed through the plating module. In horizontal plating, if the anode setup is not optimum, the thickness distribution within the panel may vary; however, the variation from panel to panel is eliminated.

Soluble Anodes

Soluble anodes need to be filmed for proper dissolution. This is achieved by dummying a fresh copper anode at low current density for 2–3 hours. Once filmed, the film is renewed as the dissolution proceeds. As a byproduct of anode film formation, this film (copper oxide) will sluff off the anodic copper, and if left unattended, will create nodules on the surface of the plated panel. To keep the sluffed copper oxide—referred to as sludge—from contaminating the bath, the anodes are bagged. The bags should be replaced during anode maintenance.

In vertical plating tanks, panels are plated in different cells and various locations within the cell. To minimize variation in copper thickness distribution from the panel racked on the outside edge of the tank relative to the panel in the center of the flight bar and from cell to cell in the tank and from tank to tank requires a good understanding of the role of the anode.

Anode Placement

Proper placement relative to the cathode window of the anode baskets or slabs has a direct impact on the copper thickness distribution. In the case of panel plating, the copper thickness will always be higher toward the edges versus the center of the panel. The outside 2–3", top, bottom, left, and right edges will exhibit a much higher thickness as compared to the inside area. The thickness increases as the measuring location goes further out away from the center. The increase could be >50%; as an example, the area away from the edges could average 1.0 mil. And as you move toward the outside 2–3 inches of edge, the thickness will gradually increase up to 1.5–2.0 mils at the extreme end of the edge (Figure 1).

Figure 1: Copper thickness distribution.

Ideally, the length of the anode should be 3–4 inches short of the bottom of the panel. This will minimize the increased thickness at the bottom edge of the panel. Butting the vertical edges (of panels) together eliminates the extra thickness along the vertical edges, virtually making the cathode one large panel with only the extreme outside edges needing special attention. The easiest way to reduce over-plating on the outside vertical edges is to tuck the anodes inside the cathode window by 3–4 inches. This leaves the top horizontal edge plated with thicker copper. The remedy here is a lot simpler. Rack the panels within 1 inch of solution level. This will cut off the lines of flux that would cause excessive plating at the top edge of the panel (Figure 2).

Figure 2: Ideal anode placement.

Anodes that are excessively short of the bottom of the panel will favor the top of the panel and plate less on the lower part (Figure 3). If the anode is too long, exceeding the length of the panel; this will favor plating on the lower half of the panel as compared to the top half (Figure 4).

Figure 3: Effect of a short anode on distribution.

Figure 4: Effect of a long anode on distribution.

In short, the length and placement of the anodes plays a very important role in the copper thickness distribution on the surface. If properly maintained this anode configuration relative to the cathode (panels) will yield good, consistent copper thickness distribution.