PCB007 Magazine

PCB007-June2019

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74 PCB007 MAGAZINE I JUNE 2019 article on VeCS, we will see multi-depth slots connecting signals, not only connecting the signal from the outside layers to the internal layer but from, for example, layer 3 to layer 10 creating an internal layer transition. Also in future articles, we will address transparent layer transitions where more micro-machining techniques will come into play to define the different depths as well as the sequence of pro- cessing. It would be preferred to have machine codes defined that determined the depths and sequence of the processing. CAM systems need to accommodate the rules and process sequences to minimise the potential for error in engineering and fabrication. The VeCS slot shown in Figure 2 is close to a 2.7 mm depth in a 3.0 mm product. The plat- ing is split at the bottom of the deep slot creat- ing two potentials. This particular slot is part of a test coupon, and therefore, has a connec- tion on every layer. This would not be repre- sentative of a typical design. If we use the VeCS for an internal layer transition (no connection at the top layer) we micro-machine the vertical trace away in the same process step as we do the cross route. Building on part one of this series where I in - troduced the VeCS slot, I will introduce some additional features under the topic of micro- machining. Power VeCS Electronics design applications constantly need more power and current, which opposes the need to make the features smaller, increas- ing resistance. These opposing constraints still apply to VeCS. With Power-VeCS, the vertical copper trace is shaped so that the cross section is larger, giving it a lower conductivity. Figure 3 shows the use of a half round cylinder ex- tending from the side wall of the slot on the top layers (adding this construct on every layer compromises the routing channel). We create the lower resistance connection by a combination of the vertical trace shapes with a stackup where Layer 2 (green) and Layer 3 (red) are a power/ground layer. In this man- ner, we create a lower resistance connection while keeping the connection to all ground and power in the same position. This prevents the Figure 2: Basic high aspect ratio VeCS-2 element. Figure 3: Micro-machined power-VeCS application to be applied to positions where lower resistance is required.

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