The core material composition of disposable patch fuses
Although the structure of disposable surface mount fuses (also known as surface mount fuses) is compact, the material selection is crucial for performance and reliability. The material composition mainly revolves around the following core parts:
Fuse Element Core Conductive and Fusing Materials:
Main material: This is the core functional component of fuses, usually composed of special alloy films. The common substrates are:
Copper (Cu): relatively low cost, good conductivity, but high melting point (about 1083 ° C), often needs to be alloyed with other elements or used in combination with low melting point materials.
Silver (Ag): With excellent conductivity (the best among all metals), good corrosion resistance, and a moderate melting point (about 961 ° C), it is an ideal choice for high-performance fuse melts, especially when precise melting characteristics are required.
Tin (Sn): with a very low melting point (about 232 ° C), it is often used as a coating or alloy component (such as tin copper alloy), utilizing its "metallurgical effect" to reduce the temperature required for melting.
Key features:
Low melting point alloys/structures: The design of the melt (such as geometric shapes: straight, wavy, slotted, etc.) and material composition (such as silver tin, copper tin alloys) are designed to quickly reach the melting point and melt in specific areas when overcurrent occurs, forming an open circuit.
Accurate resistance value: The size (length, width, thickness) and material resistivity of the melt jointly determine the nominal resistance and I ² t value (melting heat value) of the fuse, which is the basis of its protective characteristics.
Stability: Under normal operating current and temperature, material properties must be stable without creep or oxidation causing characteristic drift.
End caps/terminals conductivity and welding interface:
Main material: usually composed of multiple layers of metal plating to meet the comprehensive requirements of electrical connection, weldability, and environmental resistance. Common structures include:
Bottom layer/substrate layer: often made of copper, providing good conductivity and a connection foundation with the internal melt.
Intermediate layer (barrier layer): commonly used nickel (Ni). The nickel layer is crucial as it can effectively prevent internal metals (such as copper and silver) from diffusing outward, prevent tin in external solder from diffusing inward (avoiding the formation of brittle intermetallic compounds that affect reliability), and improve heat resistance and mechanical strength.
Outer layer (solderable layer): usually tin (Sn) or tin alloy (such as SnAgCu, SAC). This layer directly contacts the PCB pads, providing excellent solderability and ensuring reliable solder joints during reflow soldering. Lead free solder (SAC) meets environmental requirements (RoHS).
Key features:
Low contact resistance: ensures smooth current flow, reduces self voltage drop and heat generation.
Excellent solderability: ensuring a strong and low resistance connection with the PCB during SMT mounting process.
Welding heat resistance: able to withstand high temperatures (usually>260 ° C) of reflow soldering without deterioration.
Anti environmental corrosion: protects the internal structure from moisture, sulfides, and other corrosion.
Body/Housing Insulation and Mechanical Support:
Main materials:
Ceramic: usually aluminum oxide (Al ₂ O ∝). This is the preferred packaging material for high-performance patch fuses.
Advantages: extremely high electrical insulation, excellent high temperature resistance (far higher than the melting temperature), outstanding mechanical strength, high thermal conductivity (conducive to heat dissipation), extremely low coefficient of thermal expansion (size stability), high arc resistance, excellent flame retardancy (non flammable itself). The common colors are beige/light yellow.
Epoxy resin: commonly used in cost sensitive or smaller size applications.
Advantages: Low cost, easy to process and shape, good electrical insulation.
Disadvantages: High temperature resistance, mechanical strength, thermal conductivity, and arc resistance are usually not as good as ceramics. Flame retardants (commonly compounds containing bromine or phosphorus) need to be added to achieve flame retardant ratings (such as UL94 V0). Common colors are black or other dark colors.
Key features:
High insulation strength: Ensure effective isolation of the circuit after melting to prevent arcing.
High heat resistance: It must be able to withstand the high temperature and even arc impact generated during melting without cracking or carbonization (especially for ceramics).
Good mechanical strength: protects the internal melt from external damage and withstands SMT process stress.
Flame retardancy: An absolute requirement that must meet the flame retardant rating (such as UL94 V0) of relevant safety standards (such as UL 24814, IEC 60127) to prevent flame spread under fault conditions. Ceramics themselves are non flammable, and flame retardants need to be added to epoxy resin.
Dimensional stability: The thermal expansion coefficient should be matched with the PCB and end electrodes as much as possible to reduce thermal stress.
Internal Connection/Filler (optional):
In some structures, especially ceramic tubular fuses, welding points (using low melting point solder such as tin based alloys) may be required to connect the melt and end electrodes.
Some designs may fill the gaps inside the package with high-purity quartz sand (SiO ₂) or other insulating arc extinguishing materials. Its function is to:
Absorbing melting energy: helps cool the arc and molten metal.
Suppressing arc: Accelerating the extinguishing of the arc and improving the breaking ability.
Prevent molten metal splashing: maintain the integrity and insulation of the package.
Summarize key points:
Melts (special alloy films: Ag/Sn, Cu/Sn, etc.) are responsible for precise melting and are the functional core.
The end electrode (multi-layer electroplating: Cu/Ni/Sn (SAC)) provides low resistance connection, excellent weldability, and environmental protection, and the nickel layer is the key to preventing diffusion.
The encapsulation body (made of ceramic Al ₂ O ∝ or flame-retardant epoxy resin) provides insulation, mechanical support, heat resistance, and flame retardant protection, while the ceramic performance is superior.
Internal fillers (such as quartz sand) are mainly used to enhance high current breaking ability and arc extinguishing.
The selection and combination of these materials collectively determine the rated current/voltage, melting characteristics (fast break/slow break), I ² t value, breaking ability, pulse resistance, operating temperature range, mechanical robustness, and long-term reliability of disposable patch fuses to meet the protection needs of different electronic circuits.