In order to meet the needs of miniaturization, large volume quantification, high reliability and low cost of electronic equipment, the chip capacitor itself is developing rapidly: the variety is increasing, the volume is shrinking, the performance is constantly improving, the technology is progressing, the material is constantly updated, and the light and thin series of products have been tending to be the standard. Standardization and generalization. Its application is gradually being penetrated and developed from consumer equipment to investment equipment.

In addition, chip capacitors are developing towards diversification.

First, in order to meet the needs of portable communication tools, chip capacitors are developing towards low voltage, large capacity, ultra small and ultra-thin.

In order to adapt to the development of some electronic equipment (such as military communications equipment), high voltage, high current, high power, ultra high Q value, low ESR type medium and high voltage chip capacitor are also an important direction of development. (3) in order to meet the requirements of highly integrated circuits, multi-functional composite chip capacitors are becoming the focus of technology research.

The 1 laminated ceramic dielectric capacitors used in chip capacitors are the laminated multilayer ceramic dielectric capacitors.

The sheet laminated ceramic capacitor (MLCC), referred to as the laminated capacitor (or further abbreviated as a chip capacitor), is a ceramic dielectric diaphragm with a printed electrode (internal electrode) stacked in a misplaced way, forming a ceramic chip through a one-time high temperature sintering, and sealing a metal layer (outer electrode) at both ends of the chip, thus forming a metal layer (external electrode). A monolithic structure is also called monolith capacitor, as shown in Figure 1. Figure 1 shows that the chip laminated ceramic capacitor is a multilayer laminated structure, which is essentially composed of many parallel parallel capacitor banks. Therefore, the capacitance calculation formula of the capacitor is C=NKA/t, C is the capacitance; N is the number of electrode layers; K is the dielectric constant (commonly known as K); A is the relative electrode coverage area; t is the electrode spacing (medium thickness).

It can be seen that in order to achieve the requirement of large capacity and small volume of chip laminated ceramic capacitors. As long as the increase of N (increase the number of layers), it will increase the capacitance. Of course, high K value material (reduce stability), increase A (increase volume) and reduce t (reduce voltage tolerance) is also a way to take.

Here, let's talk about the dielectric constant K value. It depends on the ceramic material filled in the capacitor. The environment temperature, working voltage and frequency, and working time (long-term stability) will have different effects on different media. Generally, the greater the dielectric constant (K), the worse the stability, reliability and durability.

The main components of commonly used ceramic mediums are MgTiO3, CaTiO3, SrTiO3 and TiO2, and then add proper amount of rare earth oxides. It is characterized by large medium coefficient, low dielectric loss, low temperature coefficient, wide application range of ambient temperature and high frequency characteristics. It is used in high requirements (I type ceramic capacitor).

The other is that low frequency and high dielectric materials are called strong dielectric ferroelectric ceramics, which are commonly used as medium for the type II ceramic capacitors. The ferroelectric ceramics, which are mainly based on BaTiO3, are characterized by high dielectric coefficient, up to thousands, or even tens of thousands, but the dielectric constant varies with the temperature, and the dielectric constant is also different with the applied external electric field. Linear relation.

At present, the most commonly used multi-layer ceramic capacitor medium has three types: COG or NPO is super stable material, K value is 10~100; X7R is a more stable material, K value is 2000~4000; Y5V or Z5U as a general purpose material, K value is 5000~25000. In China's standard, it is divided into two kinds: I ceramics (C C 4 and CC41) and class II ceramics (CT4 and CT41). In the above materials, COG and NPO are super stable materials, and the capacitance of the capacitors does not exceed -55 30ppm/ at -55 C to +125 C. The rest materials are named according to the range of their working temperature and the change rate of capacitance. For details, see Table 1.

Several commonly used dielectric materials for chip capacitors: X7R represents the temperature range from -50 C to +125 C, and the capacitance variation within this range can reach up to 15%. Z5U represents the temperature range from +10 to +85 C, and the capacitance in this range varies from -56% to +22%.

Y5V represents the temperature range from -30 to +85 C, and the capacitance in this range varies from -82% to +22%.

These relationships show that in actual use, the selection of the chip capacitor can not only consider the volume and price. If there are environmental temperature problems used, we should pay attention to the capacitance change caused by the dielectric of the capacitor. Figure 2 draws the curves of capacitance and dielectric loss of different materials and capacitors.

Performance and application of different material capacitors

(1) NPO capacitor NPO capacitor is the most stable capacitor and dielectric loss capacitor. Its filling medium is composed of rubidium, samarium and some other rare oxides. When the temperature is from -55 to +125, the capacitance changes to 0 + 30ppm/. The capacitance varies with frequency and relative service life. With different packaging forms, the capacitance and dielectric loss vary with the frequency, and the large size NPO capacitor is better than the small package size. The NPO capacitor is suitable for the slot capacitance of oscillator and resonator, as well as the coupling capacitance in high frequency circuit.

X7R capacitor X7R capacitor is known as a temperature stable ceramic capacitor. When the temperature is between -55 and +125, the capacity change is 15%. We should notice that the capacitance change is nonlinear. The capacity of the X7R capacitor is at different voltages and frequencies