PCB
Polychlorinated biphenyls (PCBs) are a group of 209 synthetic organochlorine compounds formed by adding chlorine atoms to biphenyl, resulting in highly stable, lipophilic molecules resistant to biological and chemical degradation.[1] These man-made chemicals, lacking natural sources, were commercially produced starting in 1929 primarily for use as dielectric fluids in transformers and capacitors, hydraulic fluids, and plasticizers due to their non-flammability and electrical insulating properties.[2] Production ceased in the United States in 1979 following regulatory bans prompted by their persistence in the environment, bioaccumulation in fatty tissues, and documented toxicity in wildlife and laboratory animals, including liver damage, reproductive impairments, and carcinogenic effects.[1][3] Despite the bans, PCBs remain a global environmental contaminant through legacy sources like improperly disposed equipment and atmospheric transport, with human exposure occurring mainly via contaminated food such as fish and dairy products.[4] Empirical studies link high-level exposures to immunotoxicity, endocrine disruption, and increased cancer risk in exposed populations, though causal mechanisms at low environmental doses involve complex congener-specific interactions rather than uniform effects across the mixture.[5] Designated as persistent organic pollutants under the Stockholm Convention, remediation efforts focus on decontamination of soils, sediments, and electrical infrastructure, underscoring ongoing challenges in eliminating their ecological footprint.[6] Debates persist over the proportionality of regulatory responses, given discrepancies between acute high-dose animal data and subtler human epidemiological outcomes, highlighting the need for first-principles assessment of dose-response relationships over precautionary assumptions.[7]Technology and Engineering
Printed Circuit Board
A printed circuit board (PCB) consists of an insulating substrate, typically laminated with thin sheets of copper foil, from which conductive pathways, pads, and vias are etched to provide electrical connections between mounted components. These boards mechanically support components while enabling reliable signal routing and power distribution in electronic circuits, replacing earlier methods like point-to-point wiring that were prone to failures from vibration and loose connections. PCBs form the foundational platform for assembling complex electronics, with modern variants including single-layer, double-layer, and multilayer configurations up to 100 layers or more for high-density applications.[8] The concept of PCBs traces to 1903, when German inventor Albert Hanson patented a method of embedding flat metal conductors between layers of insulating material to create a rudimentary circuit on a wooden base, aimed at telephone equipment. Practical development advanced in 1936 when Austrian engineer Paul Eisler created the first etched foil circuit for a radio receiver while working in the UK, patenting it in 1943; this design used copper foil on bakelite substrate and gained traction during World War II for military radios due to its compactness and reliability. Post-war commercialization accelerated in the 1950s, driven by U.S. firms like the National Cash Register Company, which automated production; by 1960, PCBs were integral to computers like the IBM 7090, enabling miniaturization and mass production in consumer electronics.[9][10] Design of PCBs begins with schematic capture and component placement, followed by routing traces using electronic design automation (EDA) software, ensuring compliance with standards like IPC-2221, which specifies guidelines for trace width (minimum 0.1 mm for signal lines), spacing, via sizing, and thermal management to prevent overheating or signal integrity issues. Material selection favors FR-4, a composite of woven fiberglass cloth impregnated with epoxy resin (approximately 60% glass fiber and 40% resin by volume), offering a dielectric constant of 3.8 to 4.8 and standard board thickness of 1.57 mm; copper cladding is typically 1 oz per square foot (35 μm thick), equivalent to 1.37 mils, sufficient for currents up to 1 A per 10-mil trace at acceptable temperature rises. Multilayer boards incorporate prepregs for interlayer bonding, with via plating adding 25-50 μm of additional copper for interconnections.[11][12][13][14] Fabrication starts with plotting the design onto film, followed by applying photoresist to copper-clad laminate, exposing to ultraviolet light through the mask, developing to reveal the pattern, and etching away unwanted copper in ferric chloride solution. For multilayers, inner layers are aligned and laminated under heat and pressure (up to 200°C and 500 psi), then drilled (mechanically or laser for microvias down to 0.1 mm diameter), followed by electroless copper deposition (0.5-1 μm seed layer) and electroplating to fill vias. Outer layers receive solder mask (green epoxy for insulation and protection) and silkscreen legends, with surface finishes like HASL (hot air solder leveling, tin-lead or lead-free) or ENIG (electroless nickel immersion gold, 0.05-0.1 μm gold over 3-6 μm nickel) applied for solderability and corrosion resistance. Final steps include electrical testing via flying probe or bed-of-nails fixtures to verify continuity and shorts, achieving defect rates below 100 ppm in high-volume production. Assembly involves surface-mount technology (SMT) for dense placement using pick-and-place machines and reflow soldering (peak 220-260°C for lead-free SAC alloys), or through-hole for robust connections via wave soldering.[15][16] PCBs underpin applications across electronics, including consumer devices like smartphones (high-density interconnects with <0.1 mm lines), computers (motherboards with 8-16 layers handling gigahertz signals), automotive systems (ECUs enduring -40°C to 125°C), medical equipment (implantable devices requiring biocompatibility), and aerospace (rigid-flex boards for vibration resistance). Standards like IPC-6012 ensure qualification for performance classes: Class 1 for general consumer (basic reliability), Class 2 for dedicated service (e.g., servers, automotive), and Class 3 for high-reliability (e.g., military, where failures cost millions). Advances include high-frequency materials like Rogers laminates for RF/microwave (dielectric constant ~3.0) and embedded components to shrink form factors further.[17][18]Process Control Block
The Process Control Block (PCB) is a kernel-level data structure in operating systems that encapsulates all metadata required to represent and manage an active process, enabling efficient multitasking through scheduling, context switching, and resource allocation.[19][20] It serves as the operating system's internal abstraction of a process, distinct from the process's user-space code and data, and is created upon process initiation and maintained until termination.[21] The PCB's design supports multiprogramming by allowing the kernel to suspend and resume processes without losing their execution context, a necessity formalized in early systems like the THE multiprogramming system developed in 1968 at Technische Hogeschool Eindhoven.[22] Key purposes of the PCB include tracking process states (such as new, ready, running, waiting, or terminated), facilitating CPU time slicing via scheduler queues, and handling interrupts or signals by saving and restoring hardware contexts.[20][19] During context switches, the kernel updates the current process's PCB and loads the next one's, minimizing overhead in systems supporting thousands of concurrent processes; for instance, modern Linux kernels use the PCB-equivalent task_struct to manage up to millions of tasks in high-load environments.[19] This structure ensures causal isolation between processes, preventing interference while enabling shared kernel services, grounded in the principle that process execution depends on verifiable state preservation rather than ad-hoc memory probes. The PCB typically comprises several fixed fields, with variations across operating systems but adhering to core requirements for portability and efficiency:- Process Identification: A unique process ID (PID), often a 32- or 64-bit integer, and pointers to parent/child processes for hierarchy tracking.[20][21]
- Process State: An enumerated value indicating current status (e.g., ready, blocked on I/O), updated during state transitions to guide scheduling decisions.[19]
- Program Counter and Registers: The address of the next instruction and saved CPU register values (e.g., stack pointer, accumulator), essential for resuming execution post-interruption.[20]
- Scheduling Information: Priority levels, quantum timers, and queue pointers, used by algorithms like round-robin or multilevel feedback queues to allocate CPU time.[21]
- Memory Management Details: Base and limit registers for virtual memory mapping, page tables, or segment descriptors to enforce address space isolation.[19]
- I/O and Resource Status: Open file descriptors, allocated devices, and signal handlers, tracking pending operations to avoid resource contention.[20]
- Accounting Data: Cumulative CPU usage, elapsed time, and limits, aiding in billing or termination decisions in resource-constrained systems.[21]