[L1] 1 Gold's Engineering Characteristics and Applications in Ultra-Precision Systems [L2] 1) Engineering Definition & Mechanical Characteristics [L4] - Gold (Au) is a transition metal with atomic number 79, possessing a face-centered cubic (FCC) structure where atoms are most densely packed, resulting in superior malleability and ductility. [L4] - Its density is very high at $\rho = 19-32 \text{ g/cm}^3$, and among natural metals, it exhibits the most perfect chemical resistance to oxidation and sulfidation reactions, possessing thermodynamic stability that prevents the formation of any insulating film on its surface. [L2] 2) Industrial Examples [L4] - Semiconductor Wire Bonding: Used as an ultrafine wire material, tens of micrometers in diameter, to electrically connect the silicon die of an integrated circuit (IC) to its lead frame. During ultrasonic thermocompression bonding, the absence of an oxide layer ensures the formation of perfect intermetallic compounds (IMCs). [L4] - Electroless Nickel Immersion Gold (ENIG): Applied as a thin immersion plating, approximately $0-05 \mu\text{m}$ thick, to completely prevent the oxidation of copper pads on printed circuit boards (PCBs), thereby ensuring component solderability for several years or more. [L2] 3) Pros, Cons & Limitations [L4] - Pros: Contact resistance remains permanently unchanged even in harsh environments with high temperatures, high humidity, and corrosive gases, making it essential for fail-safe systems such as aerospace and autonomous driving sensors. [L4] - Cons: Pure gold has an extremely low yield strength of approximately $30 \text{ MPa}$. Therefore, in applications involving mechanical friction, such as connector terminals or slip rings, hard gold plating processes, where trace amounts of cobalt (Co) or nickel (Ni) are alloyed to forcibly increase hardness, must be applied. [L1] 2 Silver's Engineering Characteristics and Applications in High-Power Transmission [L2] 1) Engineering Definition & Mechanical Characteristics [L4] - Silver (Ag) is a transition metal with atomic number 47, serving as the benchmark medium for physical limits due to possessing the highest electrical conductivity ($\sigma \approx 6-3 \times 10^7 \text{ S/m}$) and thermal conductivity ($k \approx 429 \text{ W/mK}$) among all elements at room temperature. [L4] - Like gold, it has an FCC structure with excellent plastic deformability. However, it possesses a critical chemical property: reacting with trace hydrogen sulfide ($H_2S$) gas in the atmosphere to form a black silver sulfide ($Ag_2S$) film on its surface. [L2] 2) Industrial Examples [L4] - Solar Cell Front Electrode: To collect electrons generated by light in crystalline silicon photovoltaic panels without series resistance loss, a conductive paste based on silver powder is screen-printed and then sintered. [L4] - High-Voltage Circuit Breaker and Relay Contacts: Rapidly dissipates the high heat generated by arc discharge when thousands of amperes (A) of current are interrupted. Even if a silver oxide ($Ag_2O$) film forms, the oxide itself is conductive, optimizing it as a contact material for high-power switching systems. [L2] 1) Pros, Cons & Limitations [L4] - Pros: Minimizes Joule heating ($P = I^2R$), drastically reducing power loss. Its reflectivity exceeds 95% in the visible light spectrum, achieving maximum efficiency as a coating material for ultra-precision optical mirrors. [L4] - Cons: In environments where DC voltage and moisture are simultaneously present, silver ions ($Ag^+$) migrate from the anode to the cathode, growing in a dendrite form (electromigration). This causes critical short circuits in microcircuits, thus necessitating strict moisture-proof packaging. [L1] 3 Copper's Engineering Characteristics and Applications in Structural-Functional Integration [L2] 1) Engineering Definition & Mechanical Characteristics [L4] - Copper (Cu) is a transition metal with atomic number 29. It possesses excellent electrical conductivity ($\sigma \approx 5-96 \times 10^7 \text{ S/m}$), second only to silver, yet its procurement cost is overwhelmingly lower, making it the absolute foundational material for modern power grids and mechanical infrastructure. [L4] - Its tensile strength can increase to over $400 \text{ MPa}$ through work hardening via cold working, allowing it to serve not only as a simple conductor but also as a structural material that supports mechanical stress. [L2] 2) Industrial Examples [L4] - EV Motor Windings: High-density windings of enamel-coated copper wire are formed inside the stator slots of Permanent Magnet Synchronous Motors (PMSMs) to create powerful rotating magnetic fields. Recently, hairpin copper windings with rectangular cross-sections have been introduced instead of round wires to maximize the fill factor. [L4] - Mechanical Heat Exchangers and Piping Networks: Due to its excellent thermal conductivity ($k \approx 401 \text{ W/mK}$) and internal fluid corrosion resistance, it is widely used for refrigerant tubes in HVAC systems and as heat transfer fins in oil coolers for construction heavy equipment. [L2] 3) Pros, Cons & Limitations [L4] - Pros: Possesses excellent metallurgical versatility, essential for general-purpose mechanical component design, such as alloying with zinc (Zn) to create brass for valves or with tin (Sn) to create bronze for wear-resistant bearings. [L4] - Cons: Easily oxidizes in the atmosphere to form a non-conductive insulating film of copper oxide ($CuO$). Therefore, for use in precise electrical contacts or PCB patterns, a protective plating process with dissimilar metals such as tin (Sn), nickel (Ni), or gold (Au) must necessarily be applied to the surface, presenting a limitation in manufacturing cost.