The core material composition of metal foil resistors
The core secret of why metal foil resistors can achieve ultra-high precision, ultra-low temperature drift, and excellent stability lies in their unique structural design and key materials selection. The material composition mainly revolves around the following core parts:
Core resistor: precision alloy foil
Mainstream material: Nickel chromium alloy (NiCr)
This is the most core and representative material of metal foil resistors. It is usually a nickel chromium alloy foil that has undergone special formulation and precision processing, with an extremely thin thickness (usually in the micrometer range).
Key features:
Extremely low temperature coefficient of resistance: By precisely controlling the alloy composition (such as chromium content usually adjusted within the range of 20% -80%) and adding trace amounts of other elements (such as aluminum, iron, etc.), a TCR close to zero or extremely low (such as ± 0.2 ppm/° C, ± 1 ppm/° C, ± 2 ppm/° C) can be achieved. This is the cornerstone of high-precision metal foil resistors.
Excellent long-term stability: The material itself has a stable structure and minimal aging effects.
Low noise: The characteristics of the alloy itself determine that its current noise is very low.
Good mechanical performance: It has a certain degree of ductility and is easy to form by precision etching.
Other alloys (less commonly used but present):
Copper nickel alloy (Constantan): Sometimes used for specific requirements or cost considerations, but its TCR is usually lower than that of precision NiCr alloy (typical value around ± 20 ppm/° C), limiting its application in high-precision fields.
Special alloys: Some specific applications or manufacturers may develop alloy foils with other components (such as adding other precious metals or rare earth elements) to pursue specific performance combinations (such as ultra-high stability, specific TCR curves, high temperature resistance, etc.), but this belongs to the more specialized or customized category.
Supporting substrate: ceramic substrate
Material: High purity and high thermal conductivity alumina ceramic (Al ₂ O ∝) is usually used. Other high-performance ceramics, such as aluminum nitride AlN, with better thermal conductivity but higher cost, are also used as substrates.
Function:
Provide strong, flat, and insulating physical support for ultra-thin alloy foils.
Efficiently conduct and dissipate the heat generated during the operation of the resistor, ensuring its performance and stability.
Its extremely low coefficient of thermal expansion helps maintain the stability of the overall structure.
Adhesive layer: high-performance adhesive
Material: A special adhesive is required that can withstand high temperatures during subsequent processes such as sintering and welding, and has excellent bonding strength, insulation, and long-term stability.
Common types:
High temperature cured epoxy resin: designed with a special formula, it has high adhesive strength, low shrinkage rate, high temperature resistance, and good electrical performance.
Glass glaze: an inorganic adhesive material used at higher process temperatures, providing excellent heat resistance, chemical stability, and long-term reliability.
Function: To firmly, stably, and stress free bond precision alloy foil onto ceramic substrates. The quality of adhesion directly affects the long-term stability of resistance and TCR performance.
Protective layer: encapsulation coating
Material:
Silicone Gel: Provides excellent moisture, dust, and mechanical protection, with minimal stress and minimal impact on resistance values. High transparency, easy to adjust laser resistance (some processes).
Modified epoxy resin: provides stronger physical protection and sealing.
Function: To protect the precision resistor foil and internal structure from environmental factors such as moisture, pollutants, and mechanical damage, ensuring long-term reliability.
Termination materials: electrodes and pins
Material:
Electrode: It is usually a silver palladium alloy (AgPd) or other precious metal thick film paste printed/sintered at both ends of a ceramic substrate, forming a reliable electrical connection with the internal alloy foil. Sometimes nickel/gold plating is used to improve weldability and corrosion resistance.
Pin (lead type): Tin plated copper wire or copper alloy wire, providing external electrical connections and mechanical support.
Solder end (patch type): Lead free solder (such as SnAgCu) or other solderable coatings cover the end electrode.
Function: To achieve reliable electrical connection and mechanical fixation between the internal and external circuits of the resistor.
The outstanding performance of metal foil resistors comes from the composite structure composed of its precision alloy foil (core, providing ultra-low TCR and stability) and high-performance ceramic substrate (support and heat dissipation) through a special adhesive (stress free stable bonding). This structure, combined with a protective coating (isolating the environment) and reliable termination materials (electrical connections), achieves its characteristics as a top-level resistive element. Among them, nickel chromium precision alloy foil is the most critical material for achieving its iconic ultra-low temperature coefficient and ultra-high precision. The selection of other materials (ceramics, adhesives, protective layers, terminations) serves to support, protect this core, and ensure long-term stability of overall performance.