The structure and main processing links of ceramic capacitors:
Ceramic capacitors are stacked with ceramic dielectric diaphragms with printed electrodes (internal electrodes) in a dislocation manner, and are sintered at a high temperature to form a ceramic chip, and then a metal layer (external electrode) is sealed on both ends of the chip to form a A monolithic structure, so it is also called a monolithic capacitor.
The internal electrodes are stacked layer by layer to increase the area of the two plates of the capacitor, thereby increasing the capacitance. The ceramic medium is the inner filling medium.
The inner electrode conductor is generally Ag or AgPd, the ceramic medium is generally BaiTiO3, and the multilayer ceramic structure is sintered at high temperature. The external electrode is generally sintered Ag/AgPd, and then a Ni barrier layer is prepared (to block the internal Ag/AgPd material and prevent it from reacting with the external Sn), and then a Sn or SnPb layer is prepared on the Ni layer for welding.
Capacitors made of different media have different characteristics, such as large capacity, good temperature characteristics, good frequency characteristics, etc., which is why there are so many types of ceramic capacitors.
The main processing links of ceramic capacitors:
1. Material preparation and molding: After the raw materials are calcined, crushed and mixed, they reach a certain particle fineness. In principle, the finer the particles, the better. Then, according to the structure and shape of the capacitor, ceramic dielectric blanks are formed.
2. Firing: High-temperature treatment is performed on the porcelain body to make it into a porcelain body with high mechanical strength and excellent electrical properties. The firing temperature is generally above 1300°C. If the high temperature holding time is too short, the solid phase reaction will not be complete and complete, which will affect the structure of the whole green body and cause the deterioration of electrical properties, which is the so-called "burning"; Deterioration of electrical performance, causing "overburning"
3. Finally, electrode manufacturing, wire welding, coating, encapsulation
The origin of ceramic capacitors:
In 1990, Italian L. Lombardi invented ceramic dielectric capacitors. In the late 1930s, it was discovered that adding titanate to ceramics could double the dielectric constant, thus manufacturing cheaper ceramic dielectric capacitors.
Around 1940, people discovered that BaiTiO3 (barium titanate), the main raw material of ceramic capacitors, was insulating, and ceramic capacitors began to be used in military electronic equipment that required both small size and high precision.
On the other hand, ceramic laminate capacitors were developed as commercial products around 1960. By 1970, with the advancement of hybrid ICs, computers, and portable electronic equipment, it developed rapidly and became an indispensable part of electronic equipment. The total number of ceramic dielectric capacitors now accounts for about 70% of the capacitor market.
The insulating material of ceramic dielectric capacitors mainly uses ceramics, and its basic structure is to overlap ceramics and internal electrodes.
There are several types of ceramic materials. Since considering the harmlessness of electronic products, especially lead-free, PB (lead) with high dielectric coefficient has withdrawn from the field of ceramic capacitors. Now TiO2 (titanium dioxide), BaTiO3, CaZrO3 (calcium zirconate) are mainly used )wait.
Compared with other capacitors, it has the advantages of small size, large capacity, good heat resistance, suitable for mass production, and low price.
Due to abundant raw materials, simple structure, low price, and wide range of capacitance (generally several PF to hundreds of μF), the loss is small, and the temperature coefficient of capacitance can be adjusted in a wide range according to requirements.
Classification of ceramic capacitors:
There are many kinds of ceramic capacitors, and the dimensions vary greatly, from 0402 (about 1×0.5mm) chip capacitors to large power ceramic capacitors. According to the characteristics of the dielectric material used, it can be divided into type I, type II and semiconductor ceramic capacitors; according to the size of reactive power, it can be divided into low-power and high-power ceramic capacitors; according to the working voltage, it can be divided into low-voltage and high-voltage ceramic capacitors; according to the structural shape It can be divided into disc shape, tube shape, drum shape, bottle shape, cylinder shape, plate shape, lamination, monolith, block shape, pillar shape, etc.
Class I ceramic capacitors, formerly known as high-frequency ceramic capacitors (High-frequency ceramic capacitors), refer to ceramic dielectrics with small dielectric loss, high insulation resistance, and dielectric constants that vary linearly with temperature. the capacitor. It is suitable for resonant circuits, and other circuits that require low loss and stable capacitance, or for temperature compensation.
Class II ceramic capacitors (Class II ceramic capacitors) used to be called low frequency ceramic capacitors (Low frequency ceramic capacitors), which refer to capacitors using ferroelectric ceramics as a medium, so they are also called electric ceramic capacitors. This type of capacitor has a large specific capacitance, the capacitance changes nonlinearly with temperature, and the loss is large. It is often used in bypass, coupling or other circuits that do not require high loss and capacitance stability in electronic equipment.
Basic parameters of ceramic capacitors:
1. Capacitance unit: The basic unit of capacitance is F, and there are also μF, nF, and pF. Since the capacity of capacitor F is very large, we generally see the units of μF, nF, and pF instead of F. .
The specific conversion between them is as follows:
1F＝1000000μF
1μF=1000nF=1000 000pF
2. Capacitor capacity: Commonly used ceramic capacitor capacity range: 0.5pF~100uF
The ceramic capacity value of the actually produced capacitor is also discrete, and the commonly used capacitor capacity is as follows:
pF级 | 0.5pF、1pF、2pF、3pF、4pF、5pF、6pF、7pF、8pF、9pF、10pF、11pF、12pF、13pF、15pF、16pF、17pF、18pF、19pF、20pF、21pF、22pF、23pF、24pF、27pF、30pF、33pF、36pF、39pF、43pF、47pF、51pF、56pF、62pF、68pF、75pF、82pF、91pF、100pF、120pF、150pF、180pF、220pF、270pF、330pF、390pF、470pF、560pF、680pF、820pF、910pF |
nF级 | 1nF、1.2nF、1.5nF、1.8nF、2.2nF、2.7nF、3.3nF、3.9nF、4.7nF、5.6nF、6.8nF、8.2nF、10nF、12nF、15nF、18nF、22nF、27nF、33nF、39nF、47nF、56nF、68nF、82nF、100nF、120nF、220nF、330nF、470nF、680nF |
uF级 | 1uF、2.2uF、4.7uF、10uF、22uF、47uF、100uF |
The capacity of ceramic capacitors starts from 0.5pF and can reach 100uF, and the capacity will vary depending on the capacitor package (size).
When purchasing capacitors, you can’t blindly choose a large capacity. It is correct to choose the right one. For example, the 0402 capacitor can achieve 10uF/10V, and the 0805 capacitor can achieve 4.7uF/10V. However, in order to facilitate procurement and reduce costs, generally no Capacitors will be selected at the top.
It is generally recommended to choose 4.7uF/10V for 0402, 22uF/6.3 for 0603, and 47uF/6.3V for 0805. Others with higher withstand voltage need to reduce the capacity accordingly
When the requirements are met, the choice mainly depends on whether it is commonly used and whether the price is suitable
3. Rated voltage: Common rated voltages of ceramic capacitors are: 2.5V, 4V, 6.3V, 10V, 16V, 25V, 50V, 63V, 100V, 200V, 250V, 450V, 500V, 630V, 1KV, 1.5KV, 2KV, 2.5KV, 3KV, etc.
The rated voltage value is related to the distance between the two poles of the capacitor. The larger the rated voltage is, the larger the distance is generally, otherwise the medium will be broken down. This results in a capacitor with the same capacity, with a high withstand voltage value, generally having a larger size.
The applied voltage of the capacitor must not exceed the rated voltage specified in the specification. Actually, in the circuit design, when selecting a capacitor, the rated voltage will be left with a margin of about 70%.
Common Causes of Capacitor Failure
If the ceramic capacitor fails in a short circuit, the product cannot be used normally. The main cause of its failure is mechanical stress, which will cause cracks, which will reduce the capacitance or short circuit
The twist cracks occur because the patch is soldered to the circuit board. Twist cracks are caused by excessive mechanical force applied to the board, bending or aging of the board
If a twist crack extends from one end of the lower external electrode to the upper external electrode, the capacity will drop and the circuit will appear in an open state (open). Therefore, even if the crack is not particularly serious, if it reaches the internal electrode of the patch, the organic acid and moisture in the flux will invade through the gap of the crack, resulting in a decrease in insulation resistance performance. In addition, the voltage load will become high, and the flow of current will be too large, and the worst case will cause a short circuit.
Once a distortion crack appears, it is difficult to remove it from the outside. In order to prevent this from happening, it should be controlled not to apply excessive mechanical force. Generally, the larger the capacitor package, the easier it is to fail due to mechanical stress.
1. Mechanical stress behavior
Mechanical stress behavior can occur for the following reasons
(1) Reason for placement: The placement machine picks up the capacitor with too much force, the force point is not in the center, and the capacitor may be damaged if the capacitor is uneven
(2) Excessive soldering tin: When the temperature changes, excessive soldering tin will generate high tension on the SMD electrical appliances, which will cause the capacitor to break and the soldering tin will not
If enough, the capacitor will be stripped from the PCB
(3) PCB bending: After soldering to the PCB board, the PCB bends, pulls the ceramic capacitor, and is damaged after overstress
(4) Drop and collision: The drop of the PCB or finished product causes vibration or deformation, which causes the capacitor to be subjected to mechanical stress
(5) Manual welding: sudden heating or cooling leads to relatively large tension (the solution is to preheat first)
2. PCB design considerations
The capacitor placement direction is parallel to the PCB bending direction, and the placement position should be away from the large deformation position of the PCB to avoid the stress on the long side of the capacitor; the capacitor also needs to be away from the original screw hole on the board to reduce stress
3. Capacitor whistling
Generally, in high dielectric constant ceramic capacitors with X5R/B and X7R/R temperature characteristics, the dielectric material is ferroelectric barium titanate-based ceramics, which have piezoelectric effect.
Monolithic ceramic capacitor chips expand and contract in the lamination direction when AC voltage is applied. Therefore, the circuit board also expands and contracts in a parallel direction, and noise is generated due to the vibration of the circuit board. The amplitude of the patch and circuit board is only about 1pm-1nm, but the sound is very loud
In fact, it is almost impossible to hear the noise from the capacitor itself, but after installing it on the circuit board, the vibration will increase accordingly, and the period of the amplitude will reach the frequency band (20Hz ~ 20kHz) that can be heard by the human ear, so the sound can pass through the human body. ear for identification
The whistling phenomenon of ceramic capacitors, the vibration change is only about 1pm-1nm, which is one tenth to a few tenths of piezoelectric application products, which is very small, so we can judge that this phenomenon has a great influence on the monolithic ceramic capacitor itself. And the impact of surrounding components, there is no reliability problem.
