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80 PCB007 MAGAZINE I NOVEMBER 2020 pled with protecting the outside edges of the cathode window through thieving and/or shielding. 5. Agitation Agitation is achieved through the solution as well as part agitation. It replenishes the chem- istry at the plating interface and must be op- timized for the type of product and the cur- rent density of plating. Part agitation is either through-hole or knife edge. Both are effective, but through-hole is more common for hole plating. Solution agitation may be achieved with air sparging or eductor mixing. Air sparging initi- ates from the bottom of the cell and is intend- ed to move solution across the surface of the cathode. Eductors may be placed horizontally in the bottom of the tank or on vertical mani- folds. In the former, the flow is laminar to the panel, and the latter offers direct vertical im- pingement. The number of nozzles and their location must be designed to achieve the de- sired outcome. 6. Rectification Plating occurs when current is applied to the plating cell through a rectifier. The amount (weight) of copper plated is directly propor- tional to the current and the time. In DC plat- ing, the current (amps per square foot, or ASF) and time must be set to achieve the desired amount of copper to be plated. For throwing power, lower ASF for extended time gives the best results. An alternative to DC plating is pulse plating. Pulse plating requires a special rectifier that can switch modes from forward to reverse. Al- though pulse has shown good results in im- proving throwing power, it is more complex in setting up, as a specific pulse wave may work well for a certain part number but may need to be modified for a different part. Setting up a pulse wave is fairly involved and requires engineering intervention. In addition, the fin- ished grain structure (coarser than DC plat- ing) on the surface may not be ideal for sub- sequent surface finishing and may create sig- 1. Pretreatment Pretreatment is part of the plating process. It ensures that no air entrapment occurs in high aspect ratio holes and removes organ- ic residues and oxidation. Pretreatment was discussed in detail in last month's column titled "The Critical Role of Pretreatment for Plating." 2. Pattern vs. Panel Plate Pattern plate plates the via and the traces af- ter imaging. This makes the etching for circu- itization much less demanding, as it only in- volves etching a thin uniform layer of the original laminate copper. However, the thick- ness of the plated copper varies with the pat- tern. Dense pattern areas plate less than iso- lated areas. In panel plate, uniformity of thick- ness is easily achieved. Etching for circuitiza- tion is more challenging, particularly in fine patterns, as it involves etching a much thick- er layer combining the laminate copper as well as the plated copper. Manufacturers make a choice between panel and pattern plate based on their product mix and their type of equip- ment/process capability. 3. Plating Chemistry Choosing the chemical system plays a ma- jor role in the quality of plating—namely the throwing power and the grain structure. Chem- istries low in copper and high in acid have bet- ter conductivity and better throwing power. The physical properties of the copper—name- ly tensile strength and elongation (T & E) — are a function of grain structure. Fine equiaxed grain structure produces the desired T & E, in contrast with a columnar structure that will al- ways fail T & E testing. Grain structure is con- trolled by the organic additive package (bright- ener, carrier, and leveler). 4. Plating Cell Setup The plating cell setup must be optimized. This includes anode/cathode spacing, as well as the number and placement of the anodes. This has a direct impact on thickness distri- bution and uniformity, particularly when cou-

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