Supplier eBooks

| 28 lines, filters, waveguides, and other passive components. Typically, the external components and devices integrated into a PCB are either surface mount technology (SMT) parts placed on the top and bottom (Figure 1) or through-hole components connected to vias that penetrate the entire PCB stack and protrude from the top or bottom of the PCB. Sheet capacitive and resistive films can be integrated into the PCB stack and used to develop resistors and capacitors within internal PCB layers. Moreover, there are methods of spiraling or patterning metallic layers to make inductors, transformers, magnetic field sensors, and even motors. A flex circuit (Figure 2), or flex PCB, is a PCB technology that uses extremely thin laminate layers or intrinsically flexible materials to develop a circuit that is lighter, conforms to non-planar geometries for mounting, and can be used as a sticker, integrated into clothing, or for other purposes. Due to the need to etch or machine the metallic foils, there are practical limits to the dimensions and proximity of PCB conductive elements. Depending on the machine or etching method, these tolerance limitations constrain how narrow the conductive traces and elements can be and how tightly they can be spaced. These constraints impact the size of planar transmission lines, coupled structures, filters, waveguides, and other high-frequency RF elements, ultimately limiting PCB technology's upper-frequency capability. HTCC & LTCC HTCC and LTCC planar technology differs from PCB technology in a variety of ways. These ceramic circuit technologies are advantageous in high-frequency, high-power, and harsh-environment applications due to their excellent thermal stability, low signal loss, and ability to withstand extreme temperatures and mechanical stress. Unlike PCBs, which rely on organic materials that can degrade over time, ceramic-based circuits offer superior reliability and durability in aerospace, defense, and industrial applications. These technologies are based on using planned layers of ceramic— often alumina (aluminum oxide, Al 2 O 3 ) or beryllium oxide (BeO)— with conductive, resistive, or dielectric elements screen-printed or photo-imaged as pastes on the ceramic substrate sheet or tape. These layers of ceramic substrates or tapes, pastes, and any embedded components are then fired (known as sintering) in a stack to create a final circuit. LTCC is generally sintered at less than 1000°C, while HTCC may be sintered as high as 1600°C. Due to the sintering temperatures and metallic compatibility, LTCC technologies can use low-resistivity conductors, like silver and gold, while HTCC processes (Figure 3) tend to rely on molybdenum and tungsten due to the refractory temperatures. A key advantage of LTCC technology is its ability to integrate passive components such as resistors, capacitors, and inductors directly into the ceramic layers, reducing circuit size and improving performance by minimizing parasitic elements and signal interference. Figure 2: Johnson Space Center's Passive Smart Container, an RFID technology that uses flex PCB technology to quantify and track liquids and bulk goods. (Source: NASA) 2 Figure 3: A test assembly of a silicon carbide (SiC) package made with HTCC alumina. (Source: NASA) 3

• Cover