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74 DESIGN007 MAGAZINE I NOVEMBER 2019 smoke. Once it gets out, it's really hard to put back in (Figure 1). In design, you can reduce the likelihood of shorts, damaged traces, and exposed copper by keeping adequate distance between traces and mounting holes. Make sure to leave room for screw heads, washers, and other sorts of fasteners. Allow as much annular ring as pos- sible for vias and plated holes. Within reason, more is always better. Vias need a bare min- imum pad 0.010" larger than the drill. Other plated holes need 0.016" larger than the drill size. You can design to reduce the chance of slivers as well; keep widths and spacing above 0.006 inches. Copper: The Key to the PCB Process For PCB manufacturing, copper is integrated into the board in three different ways. The raw laminate material (dielectric) is copper-clad, meaning there is a consistent thickness of copper adhered to both sides of the dielectric material. This copper has ei- ther a plating mask or an etching mask applied that defines all circuitry and pads for that layer. Even- tually, all the cop- per that is between and surrounding the desired circuitry and pads will be chemically etched away, leaving only the needed copper. For the external and other plated layers— where there are plated holes—the second application of copper is needed and added after the holes are drilled. This copper is a very thin layer of catalytically deposited cop - per used primarily to metalize the plated through-holes to the point that they will con- duct current for electroplating. This deposi- tion of copper is a critical step to ensure that all the holes and vias will successfully plate with copper. Once these holes are metalized and the plat- ing mask is applied, it is time to electroplate all the circuitry, pads, and through-holes with copper (Figure 2). On a typical PCB, the elec- troplating process increases the thickness of the base copper foil by approximately 1.0 mil of copper and deposits a total thickness of 0.8 Figure 1: Trace width. Figure 2: Plate.

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