Detailed explanation of the production process of SMD Resistors
As one of the most widely used passive components in modern electronic devices, the production of surface mount resistors is a highly precise and automated process. The core process mainly revolves around building precise resistors on tiny ceramic substrates and forming reliable electrode connections. The following is its typical production process:
Raw material preparation:
Ceramic substrate: mainly uses alumina ceramic powder. Mix the powder with organic binders, solvents, etc. to form a uniform slurry.
Resistance paste: It is made by mixing and grinding conductive materials (such as ruthenium dioxide RuO ₂, carbon, metal alloy powder, etc.), glass powder (flux), and organic carriers (solvents, resins, dispersants), and determines the resistance value and TCR (temperature coefficient).
Electrode paste: usually silver palladium alloy paste or pure silver paste, used to form internal electrodes.
Protective paste: Glass glaze paste (for primary protection) and epoxy resin paste (for secondary protection/marking layer).
Terminal electrode material: Metal materials used for electroplating or impregnation (usually copper, nickel, tin).
Substrate Forming:
Casting molding: The ceramic slurry is uniformly coated on a continuous carrier tape through a casting machine, and the thickness is controlled by a scraper (which determines the thickness of the final substrate). After drying, a flexible ceramic green belt is formed.
Stamping/cutting: Accurately stamping or cutting the dried green strip into small squares (individual resistor substrates) of the required size.
Printing internal electrode:
Screen printing: Accurately printing inner electrode paste (usually silver palladium paste) on both ends of a ceramic substrate (with a reserved middle area for printing resistors). After printing, it needs to be dried and cured.
Printed resistor:
Screen printing: precise printing of a specific formula of resistive paste layer in the central area of a ceramic substrate, located between two inner electrodes. This is a crucial step in determining the range of resistance values. Use slurries with different square resistances (unit area resistance) for different resistance values. After printing, it also needs to be dried.
High temperature sintering:
Co firing: Send the substrate with printed internal electrodes and resistors into a high-temperature sintering furnace (tunnel kiln). Under strictly controlled temperature curves (usually peak temperatures above 800 ° C and 1000 ° C) and atmospheres, the ceramic matrix is densified, the organic carriers in the resistance slurry and internal electrode slurry are burned off, the conductive material and glass powder melt and solidify, forming a strong bond with the matrix. This process shapes the resistor and internal electrode and obtains stable electrical characteristics.
Laser resistance adjustment (repair):
Core precision control: This is a key step in achieving high-precision resistance values (such as ± 1%, ± 0.5%, ± 0.1%).
Principle: Using a high-precision laser beam, etching is carried out on the surface of the resistor according to a predetermined cutting path (usually an L-shaped or double L-shaped groove), accurately "trimming" off some of the resistor material, thereby increasing the effective length of the resistor and reducing its effective cross-sectional area, so that the resistance value can accurately rise from the low state after sintering to the target value.
Closed loop control: While laser cutting, the probe measures the real-time change in resistance value and feeds it back to the control system until the target resistance value is reached.
Apply a protective layer once:
Printing/dispensing: Printing or dot coating a layer of glass glaze paste on the surface of the resistor after resistance adjustment is completed.
Baking/curing: Baking and curing at a lower temperature to form a dense glass protective layer, covering and protecting the precision resistor, isolating environmental moisture and pollutants, and improving long-term stability.
Apply secondary protective layer/marking layer:
Printing: On top of a protective layer, print a layer of epoxy resin paste (usually with added pigments such as black, gray, beige).
Curing: Curing at a lower temperature. This layer mainly serves as mechanical protection and insulation. At the same time, its color is often used as a marker to distinguish the type or accuracy of resistance (such as black is often a regular product, and blue is often a precision product).
End electrode formation:
End face metallization:
Immersion/Rolling Plating: Dip or roll the two ends of the resistance strip into contact with a slurry or plating solution containing metal powder (usually copper powder), so that the metal adheres to the end face and some side faces, covering the inner electrode and extending outward.
Burnt end: Heat treatment is carried out in a protective atmosphere furnace to sinter and solidify the attached metal layer, forming a firm bottom electrode.
Electroplating:
Nickel plating: Electroplating a layer of nickel on the copper layer formed at the firing end. The nickel layer serves as a barrier layer to prevent copper from diffusing into the subsequent tin layer, and provides good solderability and mechanical strength.
Tin plating: Electroplating a layer of tin or tin alloy (such as tin lead, but mostly pure tin or tin copper/tin bismuth under lead-free trend) on the outermost layer. The tin layer provides excellent solderability, ensuring good soldering to PCB pads during SMT reflow soldering.
Electrical testing and sorting:
Automated testing: Using high-speed automated testing equipment, conduct 100% resistance measurement (usually using four terminal measurement method to ensure accuracy), withstand voltage testing, insulation resistance testing, etc. for each resistor.
Sorting: Sort resistors into different bins based on the measured resistance accuracy (such as ± 5%, ± 1%, ± 0.5%, etc.), TCR (temperature coefficient) level, and other parameters (such as size, power level).
Appearance inspection:
Automatic optical inspection: Use AOI equipment to check the size and appearance defects of resistors (such as cracks, missing corners, poor electrodes, unclear markings, etc.).
Tape packaging:
Taping: Qualified resistors are automatically loaded into the grooves of the Tape on Reel by a tape machine according to the sorting results.
Cover film: Cover the carrier tape with a layer of heat sealing tape and heat seal it.
Roll up: Wrap the packaged carrier tape onto a roll up.
Identification and packaging: Mark the specifications, quantity, production batch number, and other information on the reel, then proceed with outer packaging and prepare for shipment to the SMT factory.
Core Logic Summary
The core logic of the entire production process is to form internal electrodes and resistors on a tiny insulating ceramic substrate through precision printing and high-temperature sintering → precisely adjust the resistance value using laser → apply multi-layer protection to ensure long-term reliability → construct multi-layer end electrodes to ensure excellent weldability and connection reliability → undergo strict testing, sorting, and packaging to ultimately form standardized components that meet specification requirements. The application of high automation, strict process control, and material science is the key to ensuring stable, consistent, and cost-effective performance of surface mount resistors.
This process description strives for originality, following the logical sequence of material preparation → substrate forming → functional layer construction (electrode+resistor) → key resistance adjustment → protection → end electrode strengthening → testing and sorting → packaging. The sentences are clear and easy to understand, covering the main steps of producing surface mount resistors.