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80 PCB007 MAGAZINE I FEBRUARY 2023 e shape of the via is critical to enable uni- form copper plating. e fluid dynamics of the plating operation require the constant replen- ishment of key plating additives to ensure that fresh electrolyte is exchanged to reduce con- centration polarization. With concentration polarization, the diffusion layer is starved of copper ions and other additives. A non-ideal via formation is shown in Fig- ure 2. Note that the diameter of the blind via is slightly narrower at the top. In addition, the quality of the overall via is compromised, as evidenced by the excessive removal of the adhesive material. With a situation such as this, laminar flow of the electrolyte to the blind via is disrupted, further impacting uniform plating. Other important considerations and condi- tions to be mastered include: 1. Ensure consistent lamination thicknesses for outer layers, otherwise laser drilling will be seriously affected. 2. Use caution on energy levels so that delamination or epoxy residue is not produced at the bottom of the blind vias. 3. Make careful selection of dielectrics to be laser drilled (laser-drillable prepreg). 4. Check the depth of field of the laser drill to verify the thickest board that can be laser drilled. 5. Fabricators must invest in latest registra- tion and via formation equipment. › Hole and via positional accuracy is a possible issue › Systems available to predict material movement Figure 2: Non-ideal via formation.