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34 DESIGN007 MAGAZINE I APRIL 2022 Subtractive vs. Additive Let's start by taking a brief high-level view of the different fabrication processes. With subtractive fabrication, our PCBs start with a base layer of copper of some thickness already laminated to the substrate. en copper is electrolessly plated onto the board's outer lay- ers, including inside the drill and via holes. A design image is then applied, an etch resist plated onto the exposed traces and holes, aer which etching will occur. is is our subtrac- tive step, where we remove the copper in areas where there was no image applied. is is also the limiting step in the subtractive fabrication process, because as we etch vertically down through the copper, the etching agents also remove copper in a horizontal direction, under the applied design image. e result of this pro- cess is a final copper trace cross-section with a trapezoidal shape. e critical concern here is that if the trace height is half as tall as its width, likely the etching process will remove the trace. With additive fabrication, the process can be imagined as similar to 3D printing. e PCB starts with no copper on the laminate material and is instead "built" up on top of a thin seed layer of electroless copper, or on top of a thin laminated copper foil. is not only allows for trace and gap sizes down to 0.010 mm; it also creates a trace cross-section that has a rectan- gular shape. anks to the manufacturing process, with the formation of these traces now complete, our attention needs to focus on trace imped- ance. Depending on the impedance calcula- tion, our resultant value is based upon the width of the created trace and the height from the trace to its referenced return path. Whether the construction is stripline vs. microstrip, of course, has some impact here. Typical sub- tractive-etch processing provides us with an easy and established method for a 0.075 mm trace with 50 ohms of impedance, by utilizing a 0.050 mm thick dielectric material between the trace and its return path plane. is con- struction becomes significantly more difficult once we move down into sub-0.050 mm traces that you would find using additive fabrication; the dielectric material also needs to decrease in thickness to be able to maintain that 50-ohm impedance, which is where problems exist. Materials that thin, if they are available, are extremely specialized and expensive. While switching to a coplanar waveguide approach for our trace impedance does offer some minor improvements, our dielectric material thickness still plays a major role in our final impedance. Effects on Signal Integrity To complete our overview of additive design, we need to examine its impact on signal integ- rity. e intended use for these micro traces is to be able to increase our density within our design and assist with dense components. With the decrease in the trace size down to micro traces, we improve our ability to route more traces in a smaller area. However, we under- stand from our impedance examination that our return path copper remains the same dis- tance away, due to dielectric material thickness limitations. is means that the resultant field size that is created by signals flowing down our traces has stayed the same; however, we have pulled in our neighboring traces closer within that field to increase our design density. All this comes down to a simple statement: Increasing our routing density also increases the opportunity for crosstalk and other signal integrity issues. To combat possible crosstalk The critical concern here is that if the trace height is half as tall as its width, likely the etching process will remove the trace.