PCB007 Magazine

PCB-Jan2017

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48 The PCB Magazine • January 2017 In this two-stage technique, one begins with an X-shaped through-hole. In the initial stage of plating, copper is preferentially deposited in the middle of the through-holes until the grow- ing copper deposits meet to form a bridge. The resulting double-blind vias then fill to complete the copper filled through-hole. Terms to de- scribe this process are bridge and fill (Figure 2). The use of a single copper plating solution in a single step is the ideal process for filling through-holes with copper. For thin core ma- terials of approximately 100 µm in thickness with through-holes of 100 µm diameter at the outsides and 50−70 µm at the middle, a plated surface copper thickness of less than 25 µm can be expected. DC copper plating for through-hole filling is limited by the thickness of the substrate. As substrate thicknesses approach 200 µm, the pro- pensity for the formation of cavities and inclu- sions increases as well as the necessity to plate much higher thicknesses of surface copper. This is due to extended plating times necessary to completely fill the through-hole (Figure 3). This effect is exacerbated for boards with mechani- cally drilled, straight walled, through-holes, as the "dogboning" tendencies of electroplating will tend to close the openings of the through- holes more quickly. Background Two-Step Through-Hole Fill Technology In the two-step through-hole fill process, the bridging and filling steps are split into two ELECTROPLATED COPPER FILLING OF THROUGH-HOLES: INFLUENCE ON HOLE GEOMETRY Figure 1: Paste-plugged vias and copper-filled vias. Figure 2: Stages of DC copper filling. Figure 3: Cavity defects and surface copper in thicker substrates.

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