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52 The PCB Magazine • January 2017 towards either side of the substrate surface. At a certain point, the thickness of the bridge lev- els out, increasing only slightly with time. The via depth decreased greatly in the early stages of plating and leveled out at a certain point. This occurred in the region where the bridge thick- ness also leveled out indicating that little fur- ther change in via dimensions can be expected after this point. The plated surface copper increased steadily during the entire process. Minimization of plat- ed surface copper from the bridge step is depen- dent upon the total plating time necessary to form an optimum via. The change in the aspect ratio of the vias formed on either side of the plated copper bridge are presented in Figure 8. The results showed that the aspect ratio of the vias formed on either side of the bridge decreased with time. In this case, the desired aspect ratio of 0.75 was obtained in three hours for this particular hole size and substrate thickness. If the aspect ratio of the vias is too great, there will be a tendency to form cavities in the fill process, as the tops of holes will close faster than the via can fill (Figure 9). If the aspect ratios of the vias are too low or the diameters of the vias are too great, one will then get conformal plating due to ELECTROPLATED COPPER FILLING OF THROUGH-HOLES: INFLUENCE ON HOLE GEOMETRY Figure 7: Bridge characteristics vs. plating time. Figure 8: Via aspect ratio vs. plating time. Figure 9: Cavities in high aspect-ratio vias.