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22 The PCB Design Magazine • April 2014 A well-controlled subtractive process repeat- edly produces traces that are approximately trap- ezoid in cross-section, whose sides are at a 25– 45 degree included angle from the base, while the base is at design width. But bath chemistry can vary and traces can be over-etched, further diminishing their cross-section such that their top surface is much narrower than the base. The ratio between the height of the trace post-etch (t) and how far the trace has been eaten away at the top of an edge (x) is the etch factor (F). The higher the etch factor, the better the trace sec- tion resembles an ideal rectangle, provided the base is at design width. Bath chemistry can also vary such that too little copper is removed and traces at their base extend beyond design width. At best with the subtractive process, the width of a trace at the top and at the base can be held to within 0.5 mil of design value. Such a tolerance has little consequence when traces and spaces are 3 mils or wider and signal edge rates are low. However, it has a significant ef- fect on the characteristic impedance of thinner traces and a profound influence if those narrow traces are tightly coupled differential pairs in high-speed digital circuits. Moreover, the small- er the area of a trace cross-section is compared to an ideal rectangle, the worse the IR loss be- comes along the trace. The closer a trace cross- section matches an ideal rectangle, the closer it comes to matching the characteristic imped- ance desired by the PCB designer. If traces could be fabricated at design width with nearly vertical sides, the maximum cir- cuit density could be achieved, signal integrity could be ensured, and PCB yields could be op- timized. That is the case with the semi-additive fabrication process. In contrast to the trace geometries that result from the subtractive process, which are chemis- try- and process-defined, the trace geometries from the semi-additive process are lithographi- cally defined. Photolithography creates a well- defined trace. Trace width and spaces produced by the semi-additive process are extremely close or identical to the as-designed dimensions. design for manufacturing Figure 2: A typical cross-section along a pair of 2 mil wide traces on 1.8 mil spacing formed by the semi-additive process. note the near-vertical sides. MyTHBuSTING: SEMI-ADDITIVE PROCESS MAkES SuB-3/3 A REALITy continues