# Regarding MLCC ceramic capacitors, this summary is too comprehensive!

### Regarding MLCC ceramic capacitors, this summary is too comprehensive!

There are many types of capacitors in Electronic components, and ceramic capacitors are the most used types, so hardware engineers must master their characteristics.

1 Introduction

There are many types of capacitors in electronic components, and ceramic capacitors are the most used types, so hardware engineers must master their characteristics.

As a hardware engineer who has worked for many years, the author summarizes this document based on his own experience and through consulting various materials, focusing on the key points and difficulties that need to be mastered in hardware design. By writing documents, the purpose is to be able to make one’s own knowledge more systematic, review the past and learn new things, and at the same time hope to be helpful to readers, so that everyone can learn and progress together.

2. Definition of capacitance

2.1 The nature of capacitors

Two conductors that are close to each other with a non-conductive insulating medium sandwiched between them form a capacitor. A capacitor stores charge when a Voltage is applied between its two plates.

2.2 The size of the capacitance

The capacitance of a capacitor is numerically equal to the ratio of the amount of charge on one conducting plate to the voltage between the two plates. The basic unit of capacitance of a capacitor is the farad (F). Capacitive elements are usually represented by the letter C in circuit diagrams.

The formula for the size of the capacitance:

: The dielectric constant of the medium between the two polar plates

S: the facing area between the two polar plates
k: electrostatic constant, equal to k=8.987551×10^9N・m^2/C^2
d: the distance between the two polar plates

The simplified formula is:

There are three ways to make the capacitor capacity large:

①Use a medium with a high dielectric constant
②Increase the area between the plates
③ Reduce the distance between the plates.

3. Physical structure of MLCC ceramic capacitors

MLCC (Multi-layer Ceramic Capacitors) is the English abbreviation of Multi-layer Ceramic Capacitors. The ceramic dielectric diaphragms with printed electrodes (internal electrodes) are 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 The monolithic structure is also called monolithic capacitor.

It can be seen that the internal electrodes are stacked one by one to increase the area of ​​the two electrode plates of the capacitor, thereby increasing the capacitance.

The ceramic dielectric is the internal filling medium. The characteristics of capacitors made of different dielectrics are different, such as large capacity, good temperature characteristics, good frequency characteristics, etc. This is why there are so many types of ceramic capacitors.

4. Basic parameters of ceramic capacitors

4.1 Units of Capacitance

The basic unit of capacitance is: F (method), in addition to μF (microfarad), nF, pF (picofarad), because the capacity of capacitor F is very large, we generally see μF, nF, pF units, not units of F.

The specific conversion between them is as follows:

1F=1000000μF
1μF=1000nF=1000 000pF

4.2 Capacitance capacity

Commonly used ceramic capacitor capacity range: 0.5pF~100uF.

The ceramic capacity value of the actual production capacitor is also discrete, and the commonly used capacitor capacity is as follows:

The capacitance of ceramic capacitors starts from 0.5pF and can reach 100uF, and the capacitance will vary according to the capacitor package (size).

When purchasing capacitors, you cannot blindly choose large capacity. It is correct to choose the right one. For example, 0402 capacitors can achieve 10uF/10V, and 0805 capacitors can achieve 47uF/10V. Select the capacitor for the top grid.

It is generally recommended to choose 4.7uF-6.3V for 0402, 22uF/6.3 for 0603, and 47uF/6.3V for 0805. Others with higher withstand voltage need to reduce the capacity accordingly.

In the case of meeting the requirements, the choice mainly depends on whether it is commonly used and whether the price is low.

4.3 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. Wait.

4.4 Capacitor Type

Different types of media have different response speeds and polarizabilities to electric field changes due to their different main polarization types. The capacity under the same volume is different, and the dielectric loss and capacity stability of the capacitor are also different. Dielectric materials can be divided into two categories according to the temperature stability of capacity, namely, class I ceramic capacitors and class II ceramic capacitors. NPO belongs to class I ceramics, while other X7R, X5R, Y5V, Z5U, etc. belong to class II ceramics.

MLCC ceramic capacitors are mainly divided into two categories: high dielectric constant type and temperature compensation type

Domestic: Fenghua FH, Yuyang Technology EYANG, Xinchang Electric Ceramics PSA, Sanhuan CCTC, etc. Murata muRata, Panasonic PANASONIC, Samsung Samsung, TAIYO YUDEN, TDK, VISHAY, YAGEO, etc. 4.5

5. Characteristics of ceramic capacitors

5.1 Actual circuit model of capacitor

As one of the basic components, the capacitors actually produced are not ideal, and there will be parasitic inductance and equivalent series resistance. At the same time, because the medium between the two electrode plates of the capacitor is not absolutely insulated, there is a large insulation resistance.

Therefore, the actual capacitance model, etc. is as follows:

5.2 Impedance-Frequency Characteristics

According to the above capacitance model, we can get the complex impedance formula of the capacitance:

The insulation resistance of the actual ceramic capacitor is very large, which is in the mega-ohm level, so R is much larger than that, so the simplified formula is:

Among them is capacitive reactance, is inductive reactance, is equivalent series resistance. It is easy to see that when the frequency is relatively low (smaller), the capacitive reactance is much larger than the inductive reactance, and the capacitance is mainly capacitive, and when the frequency is relatively high, the capacitance is mainly inductive.

When, that is, resonance, the impedance is equal to the equivalent series resistance, and the impedance reaches the minimum value at this time. If it is used for filtering, the effect is the best at this time.

The impedance frequency curve of a Murata 10uF capacitor is as follows:

Note that this coordinate system is a logarithmic coordinate system, and the vertical axis is the modulus of the complex impedance.

5.3 Resonant frequency

As can be seen from the previous section, the impedance of the capacitor is the lowest at the resonant frequency, and the filtering effect is the best. So what is the resonant frequency of the capacitors of various specifications?

The following figure is the resonance frequency table of Murata’s commonly used capacitors:

The frequency curve is as follows:

5.4 Equivalent series resistance ESR

As can be seen from the previous section, the equivalent series resistance of ceramics is not constant, it has a great relationship with the frequency. The ESR of the above 10uF capacitor is 3Ω at 100hz, and reaches the minimum at 700Khz, and the ESR is 3mΩ, a difference of 1000 times, which is very large.

We are very concerned about how big the ESR of the ceramic capacitor is, especially when it is used in switching power supplies, it needs to be used to calculate the size of the ripple. So what is the ESR of each capacitor model?

The figure below shows the ESR table of Murata ordinary capacitors.

The ES frequency curve is as follows:

5.5 Precision size

Compared with the accuracy of the resistor, the accuracy of the capacitor is much lower. The following is the accuracy of the general capacitor.

The same type of capacitor precision generally produces 2 to 4 precision grades to choose from.

5.6 Temperature characteristics

The operating temperature ranges of different types of capacitors are different, and their capacity varies with temperature, and the difference is very large, as shown in the following table

When designing the circuit, it is necessary to consider the temperature coefficient of different capacitors, and select the capacitor that meets the requirements according to the usage scenario. In some places where capacitance is required, Y or Z series capacitors cannot be selected.

5.7 DC bias characteristics

Another characteristic of ceramic capacitors is their DC bias characteristics.

Special attention should be paid to capacitors (X5R, X7R characteristics) that are classified as high inductivity series among ceramic capacitors, since the capacitance may differ from the nominal value due to the application of DC voltage.

For example, as shown in the figure below, the higher the DC voltage applied to a high dielectric constant capacitor, the lower its actual capacitance.

The higher the capacitance, the more obvious the DC bias characteristics. For example, the capacitance of 47uF-6.3V-X5R, at 6.3V, the capacitance is only about 15% of its nominal value, while the capacitance of 100nF-6.3V-X5R The capacitance value of the capacitor is its nominal value, as shown in the figure below.

So, what is the principle of the DC bias characteristic?

Among the ceramic capacitors, the high-inductivity series capacitors mainly use BaTiO3 (barium titanate) as the main component of the dielectric.

BaTiO3 has a perovskite-shaped crystal structure as shown in the figure below. Above the Curie temperature, it is a cubic crystal, with Ba2+ ions at the apex, O2- ions at the center of the surface, and Ti4+ ions at the center of the cube. Location.

The above figure shows the crystal structure of a cubic crystal when the Curie temperature (about 125°C) or more is exceeded. In the normal temperature range below this temperature, one axis (C axis) is extended and the other axis is slightly shortened (tetragonal). Crystal structure.

At this time, polarization occurs as a result of the displacement of Ti4+ ions in the extension direction of the crystal unit. However, this polarization occurs even in the absence of an external electric field or voltage. Therefore, it is called spontaneous polarization. (spontaneous polarization). In this way, it has a characteristic of spontaneous polarization and the orientation of the spontaneous polarization can be changed according to an external electric field, which is called ferro electricity.

The same as the phase transition of spontaneous polarization in a unit volume is the permittivity, which can be observed as an electrostatic capacitance.

When no DC voltage is applied, the spontaneous polarization is in a random orientation state, but when a DC voltage is applied from the outside, the free phase transition during spontaneous polarization is not easy to occur because the spontaneous polarization in the dielectric is bound by the direction of the electric field. As a result, the obtained electrostatic capacity is lower than that before the application of the bias voltage.

This is the principle of the decrease in electrostatic capacity when a DC voltage is applied.

In addition, the capacitors for temperature compensation (CH, C0G characteristics, etc.) are mainly made of dielectric ceramics, and the capacitance does not change due to the DC voltage characteristics.

5.8 Leakage current and insulation resistance

Ceramic capacitors have relatively large insulation resistance and small leakage current.

The insulation resistance is mainly related to the capacity. The larger the capacity, the greater the leakage current. The insulation resistance table of several common capacitors from Murata is listed below for reference.

Although the leakage current of ceramic capacitors is not large, the capacitance of large capacitors has also reached the microamp level. If it is a product with ultra-low power consumption, it is also necessary to choose some capacitors with large insulation resistance.

6.1 Capacitor failure due to mechanical stress

The most pitted failure of ceramic capacitors is short-circuit. Once the ceramic capacitor is short-circuited, the product cannot be used normally, and the harm is very large. So what is the cause of the short-circuit failure?

The answer is that mechanical stress and mechanical stress will cause cracks, which will reduce the capacitance or short-circuit.

Why do twist cracks occur? This is due to the fact that the patch is soldered to the circuit board. Twist cracks occur when excessive mechanical force is applied to the board, causing the board to bend or age.

When a twist crack extends from one end of the lower outer electrode to the upper outer electrode, the capacity drops, making the circuit appear open (open). Therefore, even if the crack is not very serious, if it reaches the internal electrode of the chip, the organic acid and moisture in the flux will penetrate through the gap of the crack, resulting in a decrease in the insulation resistance performance. In addition, the voltage load will become high, and when the current flow is too large, the worst case will lead to a short circuit.

Once a twist crack occurs, it is difficult to remove it from the outside, so in order to prevent the crack, it should be controlled not to apply excessive mechanical force.

Generally, the larger the capacitor package is, the easier it is to cause mechanical stress failure.

6.1.2 Mechanical stress behavior

So, what are the common behaviors that cause stress?

①Shipment reason: The placement machine picks up the capacitor with too much force, the force point is not in the center, and the unevenness of the capacitor may damage the capacitor.

②Excessive soldering: When the temperature changes, the excessive soldering produces a high tension on the chip capacitor, so that the capacitor is broken, and when the soldering is insufficient, the capacitor will be peeled off from the PCB.

③PCB bending: After soldering to the PCB board, the PCB is bent, pulling the ceramic capacitor, and it will be damaged after overstressing.

④Drop, collision: The PCB/finished product is dropped causing vibration or deformation, which makes the capacitor subject to mechanical stress.

⑤ Manual welding: sudden heating or cooling leads to relatively large tension (the solution is to preheat first)

6.1.3 PCB Design Considerations

The capacitor placement direction is parallel to the PCB bending direction, and the placement position is far from the position where the PCB is greatly deformed. Avoid stress on the long side of the capacitor, as shown in the figure below, the capacitor on the right should be placed on the left.

The following PCB puzzle, the force size is: A>B, A>B, A>C, A>D

Capacitors also need to be kept away from screw holes to reduce stress.

6.2 Howling

In general, high dielectric constant ceramic capacitors with temperature characteristics of X5R/B and X7R/R use ferroelectric barium titanate-based ceramics as the dielectric material, which has a piezoelectric effect.

When an AC voltage is applied, the monolithic ceramic capacitor chip expands and contracts in the stacking direction. Therefore, the circuit board also expands and contracts in parallel, and noise is generated due to the vibration of the circuit board. The amplitude of the patch and circuit board is only about 1pm to 1nm, but the sound is very loud.

In fact, it is almost impossible to hear the noise emitted by the capacitor itself, but after installing it on the circuit board, the vibration will be enhanced, and the period of the amplitude will reach the frequency band (20Hz ~ 20kHz) that the human ear can hear, so the sound can pass through the human ear. ear recognition. For example, you can hear “ji–“, “ki–“, “pi–” and other sounds.

The “whistling” phenomenon of ceramic capacitors, its vibration change is only about 1pm ~ 1nm, which is 1/10 to several tenths of piezoelectric applications, very small, so we can judge that this phenomenon is very important for monolithic ceramics. There is no reliability problem due to the influence of the capacitor itself and surrounding components.