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24 DESIGN007 MAGAZINE I APRIL 2022 sure you leave enough room away from the component pin to allow the solder mask to cover the trace and not your pin. at's usually about a 1.6–2 mil (40-50 micron) space to the trace. en you can place the traces down to 1 mil apart (25 micron), impedance depending (Figure 3). Are there signal integrity or impedance advantages and disadvantages with additive? Yes, but it depends on how you look at it. inner dielectrics mean thinner boards. is could be positive or negative, depending on your mechanical needs. Current SI calculators don't want to work with a trace width that is smaller than the thickness of the copper. Speculations are that the high aspect ratio can reduce insertion loss and improve coupling of differential lines, as there is less friction to the material surface. e narrow spacing can improve inductive and capacitive coupling— the only thing between the traces is resin, no glass. e process itself allows very precise Figure 2: While traditional subtractive traces are trapezoidal, additive traces have straight walls. Figure 3: Additive and semi-additive processes permit line and space densities up to 36X that of typical subtractive PCBs.

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