One: the definition of capacitance
The so-called capacitor is actually two conductors that are close to each other, with a non-conductive insulating medium sandwiched between them, forming a capacitor. A capacitor stores charge when a voltage is applied between its two plates.
Two: MLCC ceramic capacitor physical structure
MLCC (Multi-layer Ceramic Capacitors) is the English abbreviation of Multi-layer Ceramic Capacitors. It is composed of ceramic dielectric diaphragms with printed electrodes (internal electrodes) stacked in a dislocation manner, and a ceramic chip is formed by one-time high-temperature sintering, and then metal layers (external electrodes) are sealed at both ends of the chip to form a Similar to the monolithic structure, it is also called monolithic capacitor.
It can be seen that the internal electrodes are stacked in layers to increase the area of the capacitor's two electrode plates, thereby increasing the capacitance. The ceramic dielectric is the internal filling medium. The characteristics of capacitors made of different media are different, such as large capacity, good temperature characteristics, good frequency characteristics, etc. This is why there are so many types of ceramic capacitors.
Three: Features
Since monolithic capacitors are ceramic capacitors, they have the basic characteristics of ceramic capacitors. Monolithic capacitors have large capacitance (capacitance value can be 1uF), small size, relatively stable capacitance, small temperature drift coefficient, long life, small equivalent DC resistance, large allowable pulsating current, high reliability, and low high frequency impedance High temperature resistance, good insulation, low cost, etc., but the manufacture of monolithic capacitors is complicated and stricter than the manufacturing requirements of ordinary capacitors.
Advantages: non-polar, cheap, small size, can be packaged in 01005. Large volume ratio, suitable for use in miniaturized electronic products. Class1 capacitors have good temperature characteristics. The insulation resistance is above several GΩ, and the leakage current is very small.
Disadvantages: small volume ratio, no tantalum capacitors in capacity, large aluminum electrolytic capacitors and non-Class1 capacitors, and the capacitance value varies greatly with temperature. Poor toughness, easy to break when squeezed.
4. Capacitor manufacturing process
Since monolithic capacitors are multilayer chip capacitors, they have strict requirements for dust-free environments during the manufacturing process. The main steps are:
①Porcelain film forming: This part includes ball milling of ingredients and flow edge, and the paste slurry is evenly coated on the film.
②Printing: The material of the electrode is printed on the paste slurry after the flow edge according to the established rules, the purpose is to dislocate the electrode.
③Stacking: The flow of the printed electrodes is superimposed along the slurry blocks according to the different capacitance values to form the prototype of the monolithic capacitor.
④ Voltage equalization: Tightly combine the stacked monolithic capacitor prototypes.
⑤Cut: Cut the capacitor prototype into monomers.
⑥Removal of glue: Use high temperature method to remove the binder contained in the raw material.
⑦Sintering: The ceramic powder is sintered into ceramic material at high temperature and forms ceramic particles.
Five, MLCC FAQ
MLCC—Multilayer Chip Ceramic Capacitors, referred to as SMD capacitors, can cause noise whistling problems...
Ceramic dielectric is the main component of MLCC, and electrostriction is inevitable under the action of voltage. If electrostriction is strongly expressed as piezoelectric effect, vibration will occur.
Countermeasures against Howling There are many ways to reduce the audible noise generated by MLCC capacitors, but all solutions add cost.
①Changing the type of capacitor is the most direct method, and replace it with capacitors that do not have piezoelectric effect, such as paraelectric ceramic capacitors, tantalum capacitors and film capacitors. However, issues such as volume space, reliability and cost need to be considered.
②Adjust the circuit to eliminate the large alternating voltage applied to the MLCC or move its frequency out of the human ear hearing frequency band (the most sensitive audio frequency of the human ear is 1KHz--3KHz).
③ Pay attention to the PCB layout and PCB board specifications to help reduce the shouting level.
④Select MLCC with no noise or low noise.
The design of noiseless/low noise MLCC is currently aimed at the whistling phenomenon of ML CC, and design solutions
There are three kinds:
(1) Thickening the bottom protective layer Since the thickness of the protective layer has no internal electrodes, the BaTiO3 ceramics in this part will not be deformed. When the height of the solder at both ends does not exceed the thickness of the bottom protective layer, the resulting deformation will affect the PCB. small and effectively reduce noise.
(2) The structure diagram of the additional metal bracket is as follows, which uses a metal bracket to overhead the MLCC chip.
The MLCC is separated from the PCB board, and the deformation generated by the inverse piezoelectric effect is elastically buffered by the metal bracket, which reduces the effect on the PCB board and effectively reduces the noise.
(3) Design and manufacture of dielectric materials with weak piezoelectric effect By further doping barium titanate (BaTiO3) with sacrificing a certain dielectric constant and temperature characteristics, a dielectric material with greatly weakened piezoelectric effect is obtained. MLCC can effectively reduce noise. All major MLCC manufacturers have MLCC product series with corresponding low-noise materials.