SMD aluminum electrolytic capacitor

SMD aluminum electrolyte capacitors are one of the most common capacitors on today's boards.

1. Manufacturing process:

In fact, other types of chip electrolytic capacitors, such as aluminum solid polymer capacitors, are manufactured in a similar manner, except that the material used for the cathode is not an electrolyte but a solid polymer or the like. SMD aluminum electrolyte capacitor is the most common capacitor on the graphics card. The manufacturing process of the SMD aluminum electrolyte capacitor consists of nine steps. We will explain each one by one in order:

1): Corrosion of aluminum foil. If you open the outer casing of an aluminum electrolyte capacitor, you will see that there are several layers of aluminum foil and several layers of electrolytic paper. The aluminum foil and the electrolytic paper are attached together and wound into a cylindrical structure, so that every two layers of aluminum foil is An electrolytic paper with an electrolyte adsorbed on it. So first let's talk about the manufacturing method of aluminum foil. In order to increase the contact area between the aluminum foil and the electrolyte, the surface of the aluminum foil in the capacitor is not smooth, but is subjected to an electrochemical etching method to form an uneven surface on the surface, which can increase the surface area by 7 to 8 times. The price of ordinary aluminum foil is about 10RMB per square meter, and after this process, its price will rise to 40~50RMB/m2. The process of galvanic corrosion is relatively complicated, involving the type and concentration of the etching solution, the surface state of the aluminum foil, the speed of corrosion, the dynamic balance of the voltage, and the like. The manufacturing process in our country is not mature enough, so the galvanized aluminum foil used to make the capacitor is still mainly imported.

2): An oxide film forming process. After the galvanic corrosion of the aluminum foil, it is necessary to use a chemical method to oxidize the surface to aluminum oxide, which is the medium of the aluminum electrolytic capacitor. After oxidation, the surface of the aluminum oxide should be carefully inspected to see if there are spots or cracks, and the unqualified ones are excluded.

3): Cutting of aluminum foil. This step is easy to understand. It is to cut a whole piece of aluminum foil into small pieces, making it suitable for capacitor manufacturing.

4): Riveting of the leads. The external pin of the capacitor is not directly connected to the inside of the capacitor, but is internally connected to the capacitor through the inner lead. Therefore, in this step we need to connect the inner leads of the anode and cathode to the outer leads of the capacitor by ultrasonic bonding. The outer leads are usually copper-plated iron or copper oxide wires to reduce the resistance, while the inner leads are directly connected to the aluminum foil by aluminum wires. Everyone notices that these small steps are all very demanding for precision machining.

5): Winding of electrolytic paper. The electrolyte in the capacitor is not directly filled into the capacitor, and the aluminum foil is immersed in a liquid state, but is adhered to the aluminum foil layer by the electrolytic paper to which the electrolyte is adsorbed. Among them, the selected electrolytic paper is somewhat different from the ordinary paper, and is microporous. The surface of the paper should not have impurities, otherwise it will affect the composition and performance of the electrolyte. In this step, the electrolytic paper without adsorbing the electrolyte is attached to the aluminum foil, and then wound into the capacitor casing, so that the aluminum foil and the electrolytic paper form a "101010" interval.

6): Impregnation of the electrolyte. After the electrolytic paper is wound up, the electrolyte is poured in, and the electrolyte is impregnated onto the electrolytic paper. With the improvement of electrolyte formulation and the improvement of electrolytic paper manufacturing technology, the ESR value of aluminum electrolytic capacitors has gradually increased, becoming a fraction of the previous one.

7): Assembly. This step is to assemble the aluminum shell on the outside of the capacitor and connect the outer leads. The capacitor is basically formed at this time.

8): curling. If it is a kind of "foreskin" capacitor, you need to go through this step and put the PVC film coated on the outside of the capacitor outside the capacitor aluminum case. However, the capacitance of PVC films is now decreasing, mainly because the materials are not environmentally friendly and have little to do with performance.

9): Combined assembly. If it is a straight-in package, you don't need to go through this step. This is the last step in the manufacture of SMD aluminum electrolytic capacitors. This step is to install the black plastic backplane components required for the SMT chip packaging process on the bottom of the capacitor. The requirements for the components are firstly the sealing effect is good; the second is the heat resistance; the third is to have chemical resistance, and can not react chemically with the electrolyte inside the capacitor. This small plastic plate is called a "terminal plate" and its manufacturing accuracy requirements are very high, because once the size is not suitable, it will affect the sealing of the capacitor (too small).

2. Performance parameters:

After knowing the whole process of manufacturing capacitors and understanding the basic structure and principle of capacitors, we will face a new problem - how to judge the quality of capacitors from the parameters? Only by mastering this method can we change our performance constantly. Even if we don't understand the type of capacitor and the brand itself, we can quickly judge its performance level through several parameters. Regarding the parameters of the capacitor, we divide it into "seeing" and "invisible". The so-called "seeing" is some basic parameters printed on the surface of the capacitor. These parameters are often directly known after we see a capacitor. For example, the capacity of the capacitor (such as "470μF", etc.), the range of capacity deviation, the temperature range, and the voltage value (such as "16V"). The so-called "invisible" parameter is the parameter we need to query based on the model of the capacitor. For example, the ESR value we often say has become an important parameter to distinguish the performance of the capacitor, and we can't see this parameter in the capacitor. We have to go to the relevant website to query by the model of the capacitor. There are quite a few similar parameters, including the following: 1. ESR value; 2. Chopper current value that can withstand;

3. Temperature characteristics;

4. The tangent of the loss angle (TAN) is equivalent to the ratio of reactive power to active power. This value is related to the quality of the capacitor and the amount of heat generated. The smaller the value, the better the capacitor performance.

5. Leakage current value: No matter how large the insulator is, there will always be a slight current leakage through the capacitor. This value represents the specific leakage. In addition, the ESL feature is also one of the performance specifications of the capacitor. However, with the development of capacitor technology, the current high-grade electrolytic capacitors generally have good ESL characteristics. When they are 10MHz or more, they often show the difference, so they lose the meaning of comparison. What is the significance of capacitor ESR? Why is ESR important? First of all, ESR. ESR is the most important performance parameter in high-frequency electrolytic capacitors. Many electronic components emphasize the performance characteristics of “LOW ESR”, which means that the ESR value is small. So, how do we correctly understand the practical significance of LOW ESR? Due to the development of electronic technology, the voltage supplied to the hardware is showing a trend of lower and lower. For example, the latest CPUs of INTEL and AMD have voltages less than 2V, which is much lower than the previous 3 and 4V voltages. However, on the other hand, due to the explosion of transistors and frequencies, the power consumption of these chips is increasing. Therefore, according to the formula of P=UI, the current requirements of these devices are getting higher and higher. For example, two CPUs with the same power consumption are 70W. The former voltage is 3.3V, and the latter voltage is 1.8V. Then, the current of the former is I=P/U=70W/3.3V, which is about 21.2A. The latter's current is I=P/U=70W/1.8V=38.9A, which is nearly double that of the former. In the case where the current through the capacitor is getting higher and higher, if the ESR value of the capacitor cannot be kept in a small range, then a higher chopping voltage than before (the ideal output DC voltage should be a horizontal line, The chopping voltage is the peak and trough on the horizontal line). In addition, even the same chopping voltage has a greater impact on the low voltage circuit than in the high voltage case. For example, for a 3.3V CPU, the 0.2V chopping voltage is a small percentage, which is not enough to form a fatal effect, but for a 1.8V CPU, it is also a 0.2V chopping voltage, which accounts for The ratio is enough to cause the digital circuit to make a mistake. So what is the relationship between the ESR value and the chopping voltage? We can use the following formula: V = R (ESR) × I In this formula, V represents the chopping voltage, and R represents the ESR of the capacitor, and I represents the current. It can be seen that when the current is increased, the chopping voltage is multiplied even when the ESR remains unchanged, and it is imperative to use a capacitor with a lower ESR value. This is why the capacitors used in hardware devices such as today's boards are increasingly emphasizing LOW ESR.

The close relationship between temperature and capacitance performance: the performance of the capacitor is not static, but it is affected by the environment, and the most important factor affecting the capacitor is the temperature. Among the different types of capacitors, capacitors using electrolyte as the cathode material, such as aluminum electrolyte capacitors, are most affected by temperature. Because among different kinds of cathodes, such as electrolyte, manganese dioxide, and solid polymer conductors, only the electrolyte is ion-conducting, and the others are electronically conductive. For ion conduction, the higher the temperature, the stronger the ionic activity and the stronger the degree of ionization. Therefore, under the premise that the temperature does not exceed the rated limit, the performance of the electrolyte capacitor at a high temperature is better than that at a low temperature.

An aluminum solid polymer conductor capacitor that reduces the chopping voltage at 25 degrees Celsius, roughly equivalent to two tantalum capacitors and three aluminum electrolyte capacitors. The performance of aluminum solid polymer conductor capacitors and tantalum capacitors is not changed much, and remains at around 24~25mV, but the chopping voltage of three aluminum electrolyte capacitors in parallel is reduced to 16.4mV. At this time, only two parallels are needed. A kind of capacitor can reach a level of about 25mV at 25 degrees Celsius, and its performance is greatly improved. Below we will look at the performance of these three capacitors in a low temperature environment.

In low temperature environments, the performance of aluminum electrolyte capacitors is greatly reduced. The chopping voltage in three parallel states jumped from 23.8mV at 25 degrees Celsius to 57.6mV. To reduce the chopping voltage to the same value as 25 degrees Celsius, seven such capacitors need to be connected in parallel. In contrast, we can see that the performance of aluminum solid polymer conductor capacitors and tantalum capacitors, whether in 25 degrees, 70 degrees or -20 degrees, is not very volatile. From the above analysis, it is not difficult to see that the ESR value of the aluminum electrolyte capacitor is extremely affected by the temperature.