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AUGUST 2026 I I-CONNECT007 MAGAZINE 65 The spacing between circuit conductors must ac- count for the established minimum electrical clear- ance for fabrication process variables, solder mask surface adhesion, land pattern features, via-hole lands, and other fixed elements on the board. PCB Circuit Density Analysis When defining the board's complexity level, the de- signer will first establish a criterion for fabrication, including the board outline and thickness limita- tions. Regarding the specified circuit board thick- ness limit, clear objectives must be established to determine how many circuit layers are dedicated to signal routing and how many are reserved for power and ground distribution. This helps determine the component density and interconnect complexity. While a significant number of semiconductor packages will have a moderate level of complex- ity (I/O and terminal pitch), others may have an ex- cessively high I/O density. One solution to conduc- tor routing roadblocks is to adopt blind via-in-land techniques to assign a majority of the interconnect to the sub-surface layers. When component density restricts circuit routing on the outer surface, subsur- face circuit routing can be justified. Adapting blind and buried microvia holes as well as predefined conductor routing channels will help the designer facilitate efficient interconnection of these often very fine-pitch and high I/O array terminal config- ured semiconductor packages. The complexity of fabricating the circuit board must always be a consideration in cost control so a limit may be preestablished to restrict the number of circuit layers. Figure 2 represents a 3-4-3 multilayer circuit board with a four-layer core and three additional circuit layers sequentially laminated onto both outer surfaces. Sequential Build-up (SBU) PCB Processing SBU offers a variety of design options. Designers can choose sequential build-up using stacked mi- crovias for layer-to-layer interconnect or the stag- gered microvia, where via-holes are offset from one layer to another. The SBU board's complexity is ul- timately determined by component density, circuit interconnect density, and the number of signal con- ductor layers plus the number of layers dedicated to power and ground. The sequence in which the circuit board layers are assembled (signal, power, ground, etc.) is a key fac- tor affecting signal transmission performance. Layer count and the method selected for interconnect between circuit layers, for example, will have a sig- nificant influence on controlling process complexity. Figure 2: Sequential 3-4-3 build-up circuit board cross-section view: a) Vertically stacked, laser-formed, Cu-filled microvias joining build-up layers; b) Buried, mechanically drilled, and plated via in four-layer core section; c) Stagger-stacked, laser- formed microvias to join build-up layers; d) Blind and plated laser-formed and Cu-filled layer-to-layer microvia; e) Mechanically drilled and plated through-hole. (Source: KSG Europe) D ES I G N E R 'S N OT E B O O K