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26 The PCB Magazine • May 2015 ELECTrOPLATING THrOUGH-HOLES wITH DIFFErENT GEOMETry continues FEaturE In theory, once the transition from cylin- drical through-hole to double blind via has taken place, a microvia fill system could be used to complete the filling. Although it is pos- sible to use two different chemistries to achieve through-hole fill, most printed wiring board manufacturers have chosen to adopt a single chemistry/one step approach to through-hole fill, regardless of the hole shape. However, man- ufacturers may be forced to modify their ap- proach as new challenges in hole size, planarity and void requirements emerge. In order to appreciate all the aspects of the "single chemistry does it all" challenge, it is use- ful to discuss the effects of different through- hole geometries on through-holes fill. Through-Hole Shape Currently, blind vias, buried vias and though-holes are used in the fabrication of high-density interconnect designs. Mechanical drilling has been the conven- tional way to form holes and this process is still the basis for most printed circuit board manu- facturing. In addition, through-holes can be formed using either one-sided (OSLTH) or double-sided laser drilling (DSLTH). The shape of such holes is not perfectly cylindrical, as shown in Figure 5 below. The tapered hole-wall profiles make such holes somewhat easier to fill with copper than the more cylindrical mechanically drilled holes. As a result of this difference, mechani- cally drilled holes may show a highest tendency for seam void formation than double-sided laser drilled through-holes. One-sided laser drilled holes (OSLTH) can be prone to voiding due to the drilling process, which may cause excessive differences in hole di- ameter between the top and bottom of the panel. Although panel thickness also has a major impact on the ability to achieve good fill with- out voiding, overall DSLTH are easier to fill and less prone to void formation. Figure 3: Butterfly structure leading to subsequent via formation. Figure 4: Mechanically drilled hole (100 μm x 100 μm).