Capacitor is basically a container that can store electricity. The basic principle of the capacitor is to use two parallel conductive metals that are not in contact with each other, and fill with air or other materials as insulation. Connect one of the two metals to the positive terminal of the battery, and the other to the negative terminal. The device which stores charge is called a capacitor.
Capacitors are mainly divided into electrolytic capacitor, tantalum electrolytic capacitor, multi layer ceramic capacitor (MLCC).
The principle of MLCC
The capacity of the capacitor is directly proportional to the area of the metal sheet electrode, so how to make the maximum metal area of charge storage with the minimum volume can make the maximum capacitance value of the capacitor with the minimum volume.
The MLCC work in the same volume as traditional electrolytic capacitors because they can form comb sheets. MLCC can greatly increase the capacity of its capacitors through the structure of the comb sheet, so that electronic products can be thinner and smaller.
MLCC formula expression：C=εK(A/D)n
C: capacitance, in F (farad), while the capacitance of MLCC is mainly PF, nF, and µF.
ε : dielectric constant of the insulation between electrodes, expressed in farads/meter.
K: dielectric constant (depending on the type of ceramic)
A: conductive area (different from product size and printing area)
D: the thickness of dielectric layer
n: number of layers
Different types of MLCC
MLCC classified by temperature characteristics :Capacitance value varies with temperature, can be divided into C0G(NP0), X7R, Z5U, Y5V, etc.
★ Classified by the size of MLCC products: 0402 ; 0603 ; 0805 ; 1206 etc.
★ MLCC classified by the capacitance: 10 PF, 100P, 1nF, 1µF, 10µF.
★ MLCC classified by operating voltage： 10V, 16V, 25V, 50V, 100V, 200V, 500V, 1KV, 2KV, 3KV。For the same series of products, the higher the operating voltage, the thicker the dielectric layer thickness must be, and the relative capacitance value is lower.
★ MLCC classified by tolerance：±0.1pF(B)、±0.25pF(C)、±0.5pF(D)、±1%(F)、±2%(G)、±5%(J)、±10%(K)、±20%(M)-20%～+80%(Z)
Therefore, a complete MLCC product specification must include at least all of the above features.
The manufacturing process of MLCC
The dielectric material of MLCC include barium titanate, titanium oxide, magnesium titanate, strontium titanate… According to the type of product (NP0, X7R, Y5V) will determine different sintering temperature and sintering atmosphere.
Thick film deposition technology
Raw embryo shape: strip embryo, thickness: 5 (including m – including 25 m.
Electrode printing: conductive electrode printing, depending on the size.
Laminated technology: 4, 250.
Cutting technology: knife cutting and laser cutting, sawing.
Ceramic co-firing technology
Ceramic and metal electrode materials: use the matching materials.
Sintering technology of ontology: temperature (950 ~ 1300 ° C) and atmosphere control (air, nitrogen/hydrogen mixture).
Side electrode technology: high temperature burning (750 ~ 900 ° C) and atmosphere control (copper electrode).
Electroplating technology (nickel plating, tin/lead), pure tin plating.
According to the material properties, MLCC is divided into two kinds of process technologies, including NME(noble metal electrode) and BME(base metal electrode), which have slightly different generation and application characteristics. NME is relatively stable, often used as a high pressure resistant product, the price is relatively expensive; BME is a low-cost product with a relatively large margin, which is generally used for products without high requirements.
Disadvantages of MLCC
Reasons cause the crack of MLCC
When micro crack occurs in PCB soldering process, common capacitor will be open circuit, and insulation resistance will be increased. However, when MLCC is microcracked,the insulation impedance of MLCC is decreased and current leakage between the layers when it is broken.
Roughly speaking, the causes of MLCC rupture can be divided into the following three parts:
MLCC rupture caused by thermal shock
MLCC rupture caused by extrinsic defect and overstress failure
MLCC rupture caused by intrinsic defect
Thermal shock occurs when the temperature around a part rises or falls too rapidly. For example, in wave soldering, reflow, touch-up, or repair, high temperatures are applied to the material quickly. When MLCC are manufactured, a variety of different compatible materials will be used.
These materials will have different coefficient of thermal expansion (CTE) and thermal conductivity due to their different material properties. When these different materials exist in the interior of a capacitor at the same time and the temperature changes rapidly, Volume changes of different ratios will be formed and push and pull each other, finally resulting in the phenomenon of crack.
This kind of rupture tends to start from the most vulnerable part of the structure, or the place where the structural stress is most concentrated. It usually occurs near the central ceramic interface where the exposed end joins, or where the greatest mechanical tension can be generated . generally in the four hardest corners of the crystal.
The distortion and fracture is usually caused by extrinsic, which usually happens during SMT or the assembly process of the whole machine product. The possible reasons are as follows:
1. Pick and place machine form PCB assembly factory grabs parts improperly, causing breakage.
2.During the installation of the capacitor, if the nozzle of the nozzle of the nozzle is under too much pressure when taking the parts or putting the parts, or if the spring of the nozzle snaps, resulting in the failure of the buffer or the error of the nozzle, a crack may occur if the parts are bent and deformed.
3. The size of the corresponding land-pattern layout is not uniform (including a welding pad connected with a large area of copper foil, the other pad is not), or the amount of solder paste is not symmetrical when printing. It is also easy to receive different thermal expansion forces when going through the Reflow oven. So that one side is lifted by a larger tension or thrust, resulting in cracks.
4. The thermal shock in the welding process and the bending deformation of the welded substrate are also easy to lead to cracks.
MLCC material intrinsic defect
MLCC material intrinsic defect is generally divided into three categories of, this kind of failure usually comes from the failure of the capacitor, and is enough to damage product reliability, this kind of problem is usually caused by the MLCC process or improper selection of materials.
3. Firing crack
MLCC cracks caused by thermal shock spread from the surface to the interior of the assembly. MLCC cracks caused by excessive mechanical tension can be formed on the surface or inside the component, and these MLCC cracks will spread at an angle of nearly 45 degrees. As for the failure of the raw material, it will lead to rupture in the direction perpendicular to or parallel to the internal electrode.
In addition, the thermal shock rupture generally spreads from one end to zero and one end. In the rupture caused by the taking and placing machine, there will be multiple rupture points below the end connection. Damage caused by a twisted circuit board usually has only one point of rupture.