Classification of ceramic capacitor design and failure reasons


Ceramic capacitors are capacitors made from ceramic. Its structure consists of two or more alternating ceramic layers and metal layers, with the metal layers connected to the capacitor electrodes.

The composition of ceramic materials determines the electrical characteristics and applications of ceramic capacitors, which can be divided into the following three categories based on their stability:

Class 1 ceramic capacitors: high stability and low losses, suitable for resonant circuit applications.

Class 2 ceramic capacitor: has high volumetric efficiency but poor stability and accuracy and is suitable for buffer, decoupling and bypass circuits.

Class 3 ceramic capacitors: These are more economical, but less stable and less accurate.

Ceramic capacitors are the most commonly used capacitors in electronic equipment, with an annual production of about one trillion. The most commonly used of these are multilayer ceramic capacitors (MLCC), and there are also components using surface mount technology.

Ceramic capacitors are non-polar components with two terminals. In the early days, the most commonly used ceramic capacitors were disk capacitors, which predated transistors. They were used in many vacuum tube devices (such as broadcast receivers) from the 1930s to the 1950s. Later, ceramic capacitors were also widely used in transistor devices... As of 2007, ceramic capacitors are still widely used in various electronic devices due to their high capacitance and low cost compared to other low cost capacitors.

Ceramic capacitors can be divided into the following shapes and styles:

Disc, resin coated, plug-in capacitor

Surface mount capacitors made up of multilayer capacitors

Contactless disk capacitors are usually placed in PCB slots and soldered directly to the PCB. They are often used in ultra high frequency (UHF) devices.

Cylindrical, not currently used.

Commonly sold ceramic capacitors can be divided into the following three categories:

Class 1 ceramic capacitors:

Temperature compensated capacitors with precise capacitance values. The stability under different voltage and temperature is the best, and the loss is the smallest. But it also has the lowest volumetric efficiency. The temperature coefficient of a typical class 1 capacitor is 30 ppm/°C and the linearity to temperature is very high. The dissipation factor of class 1 ceramic capacitor is about 0.15%, so it is suitable for high quality filter. factor. As a rule, the capacitance error of class 1 capacitors is between 5% and 10%, it is also possible to find high-precision capacitors with an error of only 1%. Class 1 capacitors of the highest accuracy are marked C0G or NP0.

Class 2 ceramic capacitor

Its volumetric efficiency is better thanclass 1 capacitors, but the accuracy and stability of the capacitance is not high. In the temperature range from -55°C to 85°C for ordinary class 2 capacitors, the capacitance measurement error will be within 15%. The dissipation factor of a class 2 capacitor is about 2.5%.

Class 3 ceramic capacitor

Its volumetric efficiency is better, but its accuracy and capacitance stability are also worse. General class 3 capacitors will have a capacitance change of -22% to +56% over a temperature range of 10°C to 55°C. The heat dissipation factor of class 3 capacitors is about 4%. Typically, class 3 capacitors are used in decoupling capacitors and other power supplies.

In the past, class 4 ceramic capacitors were sold, their electrical performance was worse, but their volumetric efficiency was higher. However, advanced multilayer ceramic capacitors can have better electrical performance in a small package, thus replacing class 4 ceramic capacitors.

The above three types of capacitors roughly correspond to low K value (dielectric coefficient), medium K value, and high K value. None of the three types of capacitors is the best, and it is necessary to choose the appropriate capacitor according to the requirements of the application. Class I capacitors are larger than Class 3 capacitors. If only used for shunt and non-filtering applications, the capacitor only needs to consider cost and volumetric efficiency, and its accuracy, stability, and loss factor are not the main considerations. this time using class 1 capacitors is not practical, so the filters mostly use class 1 capacitors. In addition to using class 1 ceramic capacitors in this area, low frequency applications can also use film capacitors, while RF applications require more complex capacitors. Class 3 capacitors are commonly used in power supplies. Due to size limitations, it is difficult to find other suitable capacitors other than class 3 capacitors. With the development of ceramic technology, the capacitance range of ceramic capacitors is gradually expanding. be up to 100 uF. Many applications have begun to replace electrolytic capacitors with ceramic capacitors. The performance of ceramic capacitors is better than that of electrolytic capacitors with the same capacity. Although their cost is higher than that of electrolytic capacitors, as technology improves, its price is also getting lower and lower.

The causes of failure of multilayer ceramic capacitors are divided into external and internal factors

Internal factors

01 Voids in ceramic media

The main factors leading to the formation of voids are organic or inorganic contamination of the ceramic powder, improper control of the sintering process, etc.The formation of voids can easily lead to leakage, and the leakage will cause local heating inside the device, which will further degrade the insulating performance of the ceramic dielectric and increase the leakage. This process is cyclical and continues to deteriorate, in severe cases leading to serious consequences such as cracking, explosion and even burnout of multilayer ceramic capacitors.

02 Shooting Crack

Caking cracks often occur on a single end electrode and propagate in the vertical direction. The main reason is related to the rate of cooling during the sintering process, and cracks and hazards are similar to voids.

03 Layering

Multilayer Ceramic Capacitor (MLCC) sintering is the co-firing of stacks of multilayer materials. The sintering temperature can reach 1000°C or more. Inadequate interlayer bonding, volatilization of internal contaminants during sintering, and improper control of the sintering process can lead to delamination. Delamination is similar to the risk of voids and cracks and is an important birth defect of multilayer ceramic capacitors.

External factors

01 Thermal crack

Mainly due to the temperature shock of the device during soldering, especially wave soldering, improper finishing is also an important cause of thermal cracks.

02 Flex Crack

Multilayer ceramic capacitors are characterized by their ability to withstand large compressive stresses, but their ability to resist bending is relatively low. Any operation that may cause bending deformation when assembling the device may result in cracking of the device. Common voltage sources include: patch alignment, PCB operation during the process; factors such as people, equipment and gravity in the circulation process; inserting components through a hole; circuit testing, segmentation of a separate board; installation of a printed circuit board; PCB positioning, rivet ;Screw installation, etc. This type of crack typically originates from the top and bottom ends of the device plating and propagates into the device at an angle of 45°C. This type of defect is also the type of defect that is actually the most common.