Created by: Glen Zhu | Updated Date: January 9th, 2025
The core material of MOV is metal oxide powder, especially zinc oxide (ZnO), usually mixed with other metal oxides such as manganese (Mn), iron (Fe), nickel (Ni) in certain proportions to form composite materials with specific electrical characteristics. In addition to metal oxides, other auxiliary materials include:
Binder: Used to bond the powder together before sintering.
Conductive materials: such as silver (Ag) or copper (Cu), used for the production of subsequent electrodes.
Powder mixing: Zinc oxide and other metal oxides are uniformly mixed in specific proportions through methods such as ball milling and mechanical mixing. This process requires ensuring uniform powder particle size and consistent distribution.
Add binder: In order to ensure the moldability of the material, a certain amount of binder or adhesive (such as polyvinyl alcohol) is usually added, and then continue mixing until a paste with moderate viscosity is obtained.
Measurement and testing:
Mold pressing: Put the mixed powder material into the mold, and press it into the desired shape through high pressure (usually 10-50 MPa). Common shapes include disc-shaped, cylindrical, sheet-like, etc.
Molding density control: In order to ensure the performance of the varistor, it is necessary to accurately control the density during the molding process to avoid loose or tight particle accumulation.
Measurement and testing:
Mainly used to remove the binder added in the raw material preparation stage. The role of the binder is to help the raw materials maintain their shape during pressing and forming, but it needs to be removed during sintering process to ensure the electrical and mechanical properties of the final product.
Measurement and testing:
After the pressure-sensitive resistor is degassed, it is neatly placed in the box.
The role of the box: by absorbing a certain amount of heat, help the pressure-sensitive resistor to be evenly heated, avoiding cracks caused by too fast or too slow temperature changes.
Measurement and testing:
The sintering process of MOV usually occurs under the composite reaction of zinc oxide and other additives. Zinc oxide reacts with other metal oxides at high temperatures, forming specific crystal phase structures and creating a stable ceramic structure. This process enhances the bonding strength between particles, thereby improving the mechanical strength and stability of varistor resistors.
Sintering temperature: Usually between 900°C and 1100°C, the specific temperature depends on the type of metal oxide used.
Sintering time: Typically takes 1 to 4 hours, during which powder particles will diffuse and recrystallize, enhancing material stability and resistivity characteristics.
Measurement and testing:
After sintering, the MOV needs to be slowly cooled to avoid cracks or deformation caused by rapid cooling. After cooling, the surface of the MOV is usually treated in some way:
Removal of surface impurities: If there are impurities such as dust or oil stains during the sintering process, they need to be removed mechanically (such as grinding or wiping).
Surface smoothing treatment: In order to increase the adhesion of subsequent electrodes, the surface may undergo fine polishing or chemical treatment.
Measurement and testing:
Coat the sintered MOV surface with a layer of silver paste (Ag) to form an electrical connection.
Use a screen to brush the silver paste on one side of the MOV, commonly in circular or rectangular electrode shapes, ensuring reliable connection between the MOV and external circuit. After heating and drying, cool down and repeat the process by brushing silver paste on the other side before another round of drying treatment.
Measurement and testing:
After the MOV surface is coated with silver, in the reduction furnace, during the reduction process, silver ions will gradually be reduced to metallic silver and deposited on the coating surface, filling the gaps between silver powder particles, reducing silver ions to metallic silver, thereby enhancing the density, adhesion and conductivity of the silver layer.
Measurement and testing:
After restoring all the silver, visually inspect and test with TTK MOV-168EP tester, then randomly select 6 pieces for lightning surge generator testing.
Measurement and testing:
After the electrical sorting, the MOV is painted and cured in a reflow oven.
This process can only be completed manually, painting is to ensure that the MOV has good waterproof, moisture-proof, and insulating performance.
Measurement and testing:
Place the MOV on the dispenser, apply solder paste for better welding of metal electrodes.
Measurement and testing:
Use a long tail clip to clamp the MOV and electrode together, fixing the electrode, soldering, and MOV.
Measurement and testing:
Solder the clipped MOV through a reflow soldering oven.
Measurement and testing:
Clean MOV debris through ultrasonic cleaning machine and oven drying to ensure no solder flux, tin ash, or other debris on the MOV.
Measurement and testing:
Wrap the electrode feet of MOV with decorative paper tape, apply adhesive to the protrusions, and arrange them neatly on the coating rack.
Preheat the MOV in an oven, pass it through a coating machine to spray powder onto the MOV, then bake in an oven to smooth out the surface of the MOV.
Measurement and testing:
Packaging
Gumming
Prehea
After the sprayed MOV is sent into the curing furnace, the surface encapsulation of the MOV is cured by high temperature.
Measurement and testing:
Perform performance testing on the cured MOV using a TTK MOV-168EP tester.
Measurement and testing:
Print logo, model number, and date on the MOV.
Remove excess packaging and electrodes.
Measurement and testing:
Each batch of MOV samples undergo testing.
Testing equipment:
4. Programmable constant temperature and humidity test chamber
Measurement and testing:
OQC inspection with Impulse generator
OQC inspection with High temperature electric aging equipment
OQC inspection with TTK MOV-168EP tester
OQC inspection with temperature and humidity test chamber
OQC inspection with Rapid temperature change test chamber
OQC inspection with ICTS 2ms-3 TEST System
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