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26 The PCB Magazine • May 2014 deposited until the via is closed, but without ad- ditives less copper is deposited inside the BMV than on the surface and voids may occur. Using organic additives that inhibit the deposition at the surface and increase plating into the BMV will lead to conformal deposition. However, by conformal plating the aspect ratio of the BMV is increased, limiting the solution exchange and mass transport into the BMV. The result can be a BMV with a seam in the centre where copper deposition was not possible due to the increase in the aspect ratio (Figure 2, b). The third way of BMV filling is the bottom-up method where the copper is deposited prefer- entially onto the capture pad, ideally creating a low dimple combined with a minimum of plat- ed surface copper (Figure 2, c). The filling of through holes is a bit different as there is no bottom area where copper plat- ing can be increased. Therefore basically two approaches are possible: The first one is to use slow DC plating and conformal copper growth COPPER FILLING OF BLIND MICROVIAS AND THROUGH-HOLES continues Figure 1: description of technical terms for via filling. Figure 2: schematic of BmV filling process.