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44 DESIGN007 MAGAZINE I DECEMBER 2024 traces or to provide for power and ground distribution. e fabricators emphasize the importance of the layer stackup order and how it affects both end-product performance and functionality. Key concerns include maximiz- ing signal integrity, impedance control, power and ground distribution, and avoiding exces- sive thermal concentration (hot spots). e primary materials used for the multiple layer circuit board are the copper foil to form the interconnect, the glass-reinforced epoxy-based laminate material for the core structure, and pre- preg material for bonding the layers together. • ere is a wide range of copper foil thick- nesses available, but the most common PCB copper specified for multilayer PCB applications is 17.5-micron. However, when very higher circuit density is war- ranted, the circuit board fabricator may implement a semi-additive copper forming process that uses a much thinner (5-miron, 9-micron, or 12-micron) copper foil. • A glass-reinforced epoxy material devel- oped expressly for printed circuit core has been (and will continue to be) the work- horse of interconnection technology. e materials are widely available, very ver- satile, and can be tailored to accommodate differ- ent product functions or applications. Although all epoxy/glass laminates have similar physical attributes, the products that adapt these materials oen have very different manufactur- ing focus and performance requirements. • e prepreg materials are a dielectric engineered to bond the copper foils to each other and to the copper-clad core struc- ture of the multilayer cir- cuit board. e thickness of the prepreg material selected for the multilayer board will vary, depending on the number of circuit layers and the maxi- mum finished board thickness specified. In the past, the circuit board designer did not get an opportunity to be involved with the planning of the layer stackup sequence strategy used to build the circuit board. e design tools simply didn't have the sophistica- tion for board layer stackup and configuration capabilities that they do today. at responsi- bility for stackup planning was transferred to the experts, the actual fabricator of the circuit board. Once established, the designer would then document the circuit board's formation with the specific material set and construction. e fabrication process for the multiple core stack examples illustrated in Figure 2a begins with hole forming and plating, imaging, and chemically etching the circuit pattern on both sides of each core section with the exception of the two outer layers of the stack. Although the holes will be formed and plated, only the inside- facing circuit pattern will be processed. Next, the core layers are assembled using the prepreg material between the opposing copper surfaces for lamination. Holes that are furnished to inter- 1 2 3 4 5 6 1 2 3 4 5 6 A B Figure 2: Comparing two 6-Cu layer circuit board stackup construction variations, one having three cores to one with cap layers requiring only two cores.