Issue link: https://resources.mouser.com/i/1541351
27 | co-fired ceramics (LTCCs), and integrated circuits (ICs). More recently, some RF circuits and components are being made with 3D technologies and other specialized fabrication approaches that can be considered 2.5D. 3D circuits allow complete vertical integration, whereas 2.5D circuits refer to structures that stack multiple layers but don't fully integrate components in all three dimensions. For most RF circuits, PCB, HTCC/ LTCC, and IC technologies are dominant. These circuit technologies require developing an RF circuit using conductors, dielectrics, and semiconductor structures over a single 2D layer that is then bonded to or used as a platform to create additional layers. Interlayer interconnects, sometimes called vias, are often used to connect circuits between layers. Vias are small conductive pathways that electrically connect different PCB layers, allowing for compact, multilayer designs with improved signal routing. This layering technique allows for increased circuit density and improved RF performance, making it a critical factor in modern high- frequency designs. Interconnecting layers in RF and high-speed digital circuits is a complex task, using advanced methods and spurring ongoing research that explores new solutions. PCBs and Flexible Circuits PCB technology is based on the use of relatively thin dielectric substrate layers, called laminates, which are commonly made from glass fabrics and other composites bound with resins and flattened into sheets. PCB laminates may also be made of composite materials that include polymers or polymer foams for enhanced RF performance. Thin metallic foils—typically electroplated or rolled annealed copper—are laminated to the top and bottom surfaces and patterned to provide interconnection between components placed on or through the PCB materials. These metallic layers can also be used to form some passive RF circuits, such as power dividers and combiners, couplers, filter elements, planar transmission lines, and planar waveguides. The most common method of patterning the metallic foils for PCBs is etching. While alternative methods to etching, such as laser ablation and milling, exist, they are typically limited to small custom batches or prototypes and are not commonly used in the mass production of PCBs. In large-scale manufacturing, PCBs are often plated with metals like nickel, tin, silver, palladium, and gold. These metal platings enhance conductivity, improve corrosion resistance, and aid in the solderability of surface metal contacts, pads, and vias. Multiple layers of laminates and metallic foils can be used to fabricate extremely complex circuits with metallized vias that allow for interconnection between the layers. These multilayer designs are essential for reducing signal loss and improving impedance matching in high-frequency RF applications. Designers can make 2.5D structures using carefully constructed layers of laminates, foils, and metalized vias, such as strip line transmission Figure 1: A PCB chip, crafted from functionalized single-walled carbon nanotubes (SWCNTs) and gold nanoparticles (AuNPs). (Source: NASA) 1