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20 DESIGN007 MAGAZINE I APRIL 2024 ally for less complex and/or cost-sensitive applications. 2. Double-sided circuits will rely on copper- plated holes for interconnecting conductor patterns between opposing circuit layers. For surface mount applications, compo- nent land patterns are incorporated into the design to enable solder attachment. Land patterns for surface-mounted com- ponents may be furnished on one or both sides. 3. Multilayer circuits will rely on copper- plated holes for interconnecting the outer layer conductor patterns and the inner layer conductor patterns. Typical of the less complex two-sided PCB, surface mount component land patterns may be incorporated into the design to enable solder attachment on one or both sides of the circuit board. Providing a Copper Balanced PCB Structure e most desirable PCB construction is one where copper layers are built up in pairs and arranged symmetrically about the core of the board: two, four, six, eight, and so on. e vari- ous pre-etched circuit layers will be assembled and aligned with partially cured prepreg mate- rial between layers, as compared in Figure 1. e sequence in which the circuit board lay- ers are assembled (signal, power, ground, etc.) is a key factor that will affect signal transmis- sion performance. In addition to performance concerns, controlling fabrication cost should be a priority as well. Layer count and the method selected for interconnecting between circuit layers, for example, will have significant influence in controlling process complexity. Implementing blind microvia technology for layer-to-layer interconnect will enable signifi- cantly greater circuit routing density, but the build-up process has substantial impact on the manufacturing complexity since it will affect the number of lamination cycles. Any layer on which a microvia begins or ends requires a sub-construction, and each sub-construction will require an extra lamination cycle. Some of the more complex sequential buildup (SBU) multilayer designs will require several lamina- tion cycles, and with each lamination cycle, the core or base materials will also be subjected to repeated exposure to elevated temperatures and high pressure. e concern is that exces- sive lamination cycles can contribute to base material decomposition. Stacked and Staggered Microvias: Pros and Cons Two variations of microvia stacking are commonly employed: offset via stacking and vertical via-on-via stacking (Figure 2). Mul- tilayer SBU circuit boards can use the verti- cally stacked microvia format when routing channels are restricted, or when the subsur- face circuit layers are less restricted, the stag- gered microvia that offsets the blind via hole's position from one layer to the nex, is recom- mended. Figure 1: Layer balanced 3-4-3 circuit board, cross-section view.