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46 DESIGN007 MAGAZINE I DECEMBER 2024 face with the conductor pattern within the now laminated structure are formed, the circuit pat- tern imaged, followed by copper plating to build up the surface of the conductor pattern and plate the holes. Aer chemical etching to define the circuit pattern on the two outer layers, the boards are made ready for solder mask application and final electrical test. e process sequence for Fig- ure 2b will be the same for the two inner core sec- tions, but the outer circuit pattern is referred to as a cap layer that will interface with the conductor patterns on the core sections with laser-ablated and copper-plated microvias. Today, the printed circuit board layout pro- cess is ver y different, as most major CAD so- ware developers are furnishing design tools equipped with advanced PCB layer configu- ration features. Although these tools establish the general order of the conductive layers, the actual construction can have significant varia- tions. e 12-layer stackup shown in Figure 3 incorporates all of the process steps noted above but it includes a center-located core section that is referred to as a floating core. 1 2 3 4 5 6 7 8 9 10 11 12 Figure 3: Twelve-Cu layer circuit board includes four core layers plus one floating core and two cap layers. Assembly sequence: 1. e two double core sections (circuit lay- ers 2, 3, 4, and 5 and circuit layers 8, 9, 10, and 11) are first assembled and processed separately, typical of the example shown in Figure 2b, but without the copper foil cap layers 1 and 12 on both. 2. The two multi-core sections are then assembled with the addition of single copper-clad (floating) core for layers 6 and 7 (reser ved for power and ground distribution) between the two multiple layer sections. 3. e two sections and floating core are then topped off with the two copper foil cap layers (1 and 12) and made ready for lamination. 4. Following lamination, through-holes for inner layer interconnect are drilled and cap layer microvia holes for joining layers 1 and 2 and layers 11 and 12 are formed. 5. e final stages provide copper hole plating operation, final imaging on both outer sur- faces, chemical etching, and solder mask application previously described.