Solutions to the failure of capacitor piezoelectric effect
Time：2021.12.08
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Capacitors with high withstand voltage and high capacitance are generally realized by electrolytic capacitors or film capacitors, and their volume is generally large. Despite years of development, the miniaturization of capacitors with high withstand voltage and high capacity is still very limited.
The current progress is mainly in high withstand voltage, but it is difficult to take into account high capacity at the same time; or to achieve high capacity but the voltage is generally less than 50V. In the power supply industry, some applications require high withstand voltage and high capacity capacitors, such as in switching power supplies. For input and output filtering, energy storage, peak absorption, DC-DC conversion, DC blocking, voltage multiplication, etc., in addition, in some applications, size and weight are very important, requiring small-volume electronic components.
In order to obtain high withstand voltage and high capacity at the same time, a common practice in the industry is to stack multiple ceramic capacitors together according to the specifications of DSCC 87106/88011 and MIL-PRF-49470. This approach occupies a large space, is heavier, and is expensive. expensive. Therefore, there has always been a demand for lighter, smaller, high-voltage, high-capacity capacitors in the industry.
Past technical limitations
The failure mode determines the design limitation, and the existence of multiple failure modes also limits the increase in capacitance of medium and high withstand voltage capacitors. Some failure modes are external, such as fracture caused by mechanical stress or thermal stress, but at the same time we also need to explore the internal failure mode in depth, which is within the manufacturer's control.
The limiting factors in the design of multilayer ceramic capacitors change with the times. The main limiting factors faced by early multilayer ceramic capacitors are the point defects and impurities of the dielectric material itself. These factors affect the quality and purity of the material, as shown in Figure 1, thereby limiting the upper limit of the number of layers inside the capacitor and the minimum thickness of each layer. value.
Figure 1: Pollution defects
With the improvement of the quality of the dielectric material itself and the improvement of the operation process, the limiting factor has turned into the strength of the dielectric material itself. Once this factor has been resolved, we could have expected the manufacture of larger and thicker capacitors without worrying about the production of dielectric materials. Breakdown or point failure, as shown in Figure 2.
Figure 2 Dielectric breakdown
However, a new failure mode has emerged, which we call piezoelectric stress fracture, which usually refers to the piezoelectric effect or electrostriction phenomenon, as shown in Figure 3. This failure mode has so far been the limiting factor facing the manufacture of multilayer ceramic capacitors. It affects most of the second type of barium titanate (Class II dielectric, and limits the capacitance range of ceramic capacitors with a size above 1210 and a withstand voltage above 200V).
As shown in Figure 3, the fracture usually runs through the middle of the entire capacitor along one or two dielectric layers. Most of the solutions are to stack multiple capacitors by adding pins to increase the capacitance value under a given size, but this requires a lot of manpower, costs more, and will cause reliability problems. Other solutions use special dielectric formulations, but at the same time at the expense of dielectric constant, and affect the final obtainable capacitance value.
Figure 3 Piezoelectric effect stress fracture failure
Figure 4 Deformation of X7R multilayer ceramic capacitor under DC bias
solution
StackiCapTM is a monolithic capacitor solution that addresses the limitation of piezoelectric failure. The patented technology GB Pat./EP2013/061918 applied innovatively adds a pressure buffer layer inside the capacitor, so that the capacitor can not only exhibit the performance of multiple superimposed capacitors, but also has a single The advantages of capacitors.
Figure 5 StackiCap
The pressure buffer layer uses a ready-made material system combination and goes through a standard manufacturing process. The pressure buffer layer is added to one or more parts with the largest mechanical stress, thereby alleviating the mechanical stress caused by piezoelectric deformation. According to the experiments so far, the pressure buffer layer can divide the multilayer capacitor into 2, 3, or 4 internally, thereby greatly alleviating the mechanical stress caused by internal deformation. At the same time, the flexible terminal technology of FlexiCap releases the mechanical stress on the terminal. In this way, we do not need to stack multiple capacitors, and we do not need to assemble pins for capacitors, which facilitates standardized tape and reel packaging and automated placement.
Cross section of "sponge"-like pressure buffer layer (SEM micrograph)
miniaturization
While greatly increasing the capacitance value, StackiCapTM can achieve a significant reduction in component size. The following pictures visually demonstrate the superiority of StackiCapTM.
Figure 7 shows the product sizes of StackiCapTM that have been developed: 1812, 2220, 2225 and 3640. Figure 8 shows a pin capacitor assembly with a maximum of 5 capacitors superimposed, and the size of a single capacitor is 2225, 3640, 5550 and 8060. Figure 9 and Figure 10 show the capacitive components that a single StackiCapTM capacitor can replace. An extreme example is that 8060, 1kV, 470nF capacitors can now be replaced by a single 2220, 1kV, 470nF StackiCapTM; 3640, 1kV, 180nF capacitors can now be replaced by a single 1812, 1kV, 180nF StackiCapTM, and the size is reduced to the original size. 1/10 and 1/7.
Figure 7 StackiCap sizes from 1812 to 3640
Figure 8 The maximum size of a five-capacitor stacked capacitor component is 8060
Figure 9 Comparison of a 2220 500v StackiCap and three 2225 500v stacked capacitors
Picture 10
Reliability test certification
StackiCap has passed the following reliability tests:
(1) Life test. StackiCap series capacitors can work continuously for 1000 hours at 125°C, 1 times or 1.5 times the rated voltage.
(2) 85/85 test. StackiCap series capacitors work continuously for 168 hours at 85℃/85%RH.
(3) Bending plate test. StackiCap series capacitors are installed on Syfer/Knowles' test PCB for bending test to evaluate the mechanical properties of the components.
