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PCBD-Apr2014

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April 2014 • The PCB Design Magazine 23 The semi-additive process is essentially the opposite of the subtractive PCB fabrica- tion process. It starts with a laminate that is clad with much thinner copper than the thin- nest conventional foils. For example, quarter- ounce copper is approximately 8.5 µm thick, but the ultra-thin foil is typically 2 µm or 3 µm thick. Such ultra-thin foils have a backing that is removed after the foil is laminated to the substrate. The holes are drilled and then a bath of electroless copper coats the bores and the foil surface. A "reverse" resist is applied, developed, and selectively removed to expose only the areas that will become the traces, vias, and other conductive features. Those exposed areas are plated up, the resist is stripped, and the ultra-thin copper remaining in the well be - tween the conductors is etched away. The plated-up traces and features are far more accurately defined than those produced by even the most-well-controlled subtractive processes (Figures 2 and 3). Trace width and spacing down to 1.25 mils can be maintained over distance within extremely tight toler - ance. There is no penalty in manufacturing time. The semi-additive process frees designers who would otherwise be challenged to break- out dense fine-pitch BGAs and also enables very strict impedance control to ensure signal integrity. And finally, while I have the chance to dis- pel myths harbored by some PCB designers, let me clear up another misconception. If any manufacturer tells you there's a setup charge for board fabrication or assembly, tell them no thanks, you'll shop elsewhere. Remember, no special jigs or one-off fixtures are necessary to build these boards. Tell them I said so. PCBDESIGN References 1. Abdelghani Renbi, Arash Risseh, Rikard Qvarnstrӧm, and Jerker Delsing, "Impact of etch factor on characteristic impedance, cross- talk and board density," IMAPS 2012. Amit Bahl directs sales and marketing at sierra Circuits, a PCB manufacturer in sunnyvale, California. he can be reached by clicking here. design for manufacturing MyTHBuSTING: SEMI-ADDITIVE PROCESS MAkES SuB-3/3 A REALITy continues Figure 3: This pair of traces designed to be 3 mils wide on 3 mil spacing was produced by the best possible subtractive process.

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