1.Structural Reliability Design of Reliability of Feedthrough Capacitors

The C-type and L-type filters produced by XUANSN are designed for different applications.  The general C-type filter is suitable for connecting to high-impedance sources and high-impedance loads, and is mainly used to suppress high-frequency signals; while the L-type filter is suitable for high-impedance loads and low-impedance sources. When designing the reliability of feedthrough capacitors, the influence of the structure on high-frequency performance should be fully considered.

The C-type filter (solder-mounted type) structure (see Figure 1 for a schematic diagram) has one end encapsulated with glass powder and the other end encapsulated with epoxy resin;

Figure 1 Schematic diagram of the C-type filter structure (left) and circuit diagram (right)

In this product structure, the two ends of the metal lead are the signal input and output terminals. Due to the symmetry of the circuit principle, the input and output terminals can be interchanged during use; the chip indicated in the schematic diagram is a multilayer ceramic capacitor, which is connected to the nut lead (connected to “ground” through welding, which can be earth ground or circuit ground depending on the overall design) and the metal lead through welding material to form a capacitor; the glass powder part is an improved packaging method for this series of products according to design requirements, which can greatly reduce the product structure size, reduce distributed parameters, and improve the reliability of feedthrough capacitors.

The chip indicated in the schematic diagram is a multilayer ceramic capacitor. In the traditional design of C-type structures, the material of the shell and leads often uses Kovar alloy. When comparing the samples initially produced according to the design parameters with foreign samples, it was found that the high-frequency performance was consistently lower than that of foreign products. To determine the reason and seek solutions, the foreign samples were subjected to material analysis, and the analysis results are shown in Figure 2.

 Figure 2. Analysis of Foreign Sample Casing Materials

Through the analysis of similar foreign samples, it was found that their casings and leads were made of ferrous materials, with an iron content of almost 100%. We tested the inductance of these ferrous leads and found that their inductance reached 2-6 nH at 10 kHz, while the inductance of copper and Kovar alloy leads of the same specifications was almost zero. Further analysis revealed that iron is more magnetically active than Kovar alloy, and the ferrous lead acts as a tiny inductor. The higher the frequency, the greater its contribution to the attenuation characteristics of the product. Therefore, by using ferrous materials for the casing and leads, even a C-type filter structure will have inductive characteristics in its leads, which is equivalent to adding a small magnetic bead to the lead. Using the simulation design software described earlier, the inductance value of the lead was input to fit and compare the insertion loss curves (see Figure 3), verifying the positive effect of ferrous leads on the filtering effect and the reliability of feedthrough capacitors under high-frequency conditions.

Figure 3. Comparison of Fitted Insertion Loss Curves

From the comparison of the fitted insertion loss curves, it can be seen that the insertion loss value of the C-type filter with ferrous leads is approximately 35 dB at 6 GHz, while the insertion loss value of the C-type filter with copper leads is approximately 24 dB at 6 GHz. Therefore, the high-frequency filtering effect of the filter using ferrous materials for the leads is far superior to that of the filter using copper materials.

L-type filter (screw-mounted type) (structural diagram shown in Figure 4), both ends are encapsulated with epoxy resin. Figure 4 Schematic diagram of the L-type filter structure (left) and circuit diagram (right)

In this product structure, the two ends of the metal lead are the signal input and output terminals, respectively. Due to its asymmetrical circuit principle, swapping the input and output will affect the performance. The chip indicated in the schematic diagram is a multilayer ceramic capacitor, which is connected to the nut lead (the user connects the “ground” lead through the nut fastening method; this “ground” can be earth ground or circuit ground depending on the user’s overall system design) and the metal lead through welding material to form a capacitor; the magnetic core is made of magnetic material and is sleeved on the metal lead to form an inductor.

The filter housing and the mounting metal plate should be reliably connected, minimizing contact resistance to ensure the bypass effect of the filter capacitor.  It is best to add an internal and external toothed washer and a flat washer between the filter and the isolation plate; the internal wiring of the filter should be as short as possible. Generally, the input and output leads should not be on the same side. If necessary, a shielding isolation layer should be added between the two sets of leads to avoid coupling interference, especially to avoid excessively long and exposed unshielded leads in strong electromagnetic fields, thus ensuring the reliability of feedthrough capacitors.

2.Thermal Matching Design

To address the phenomenon of reduced reliability or even complete failure of the product caused by the welding process, the analysis concludes that the thermal expansion properties of the metal and ceramic materials are inconsistent (see Table 1 for relevant material thermal expansion coefficients (CTE)). The thermal expansion coefficient of iron is closest to that of ceramic materials, resulting in better thermal matching between the two.

Table 1 Thermal Expansion Coefficients of Metal and Ceramic Materials

3.To improve product reliability, the following reliability optimization designs are required for its core component—the multilayer ceramic capacitor:

3.1 Reliability Design of Inner and Outer Electrode Margins

To prevent breakdown at the edge of the capacitor chip, the inner and outer electrode margins must be considered in the design. The inner and outer electrode margins of the chip are 0.3mm to ensure the reliability of feedthrough capacitors.

(Where Ut—test voltage kV, K′—edge coefficient, i.e., 1/E0 mm/kV)

Considering both process factors and capacitance requirements, a certain margin is given for the inner and outer electrodes. The margin for the inner and outer electrodes of the chip is 0.3mm.

3.2 Reliability Design of Interlayer Dielectric Thickness

Based on the dielectric strength E0 of the dielectric material and the required operating voltage of the product, the theoretical dielectric thickness is calculated as Ut/E0.  Compared with the theoretically calculated dielectric thickness, to prevent interlayer breakdown, the number of dielectric film layers is increased to improve voltage withstand reliability. The number of dielectric film layers in the chip is 5 layers.

3.3 Design of Attachment Layer Thickness

Based on the above voltage reliability design, if the required operating voltage is 200VDC, the minimum thickness of the attachment layer is 200μm. While meeting the operating voltage and capacitance requirements, a larger margin can be given for the number of dielectric film attachment layers. However, for better high-frequency filtering performance, the attachment layer thickness should not be designed to be too thick to avoid increasing the thickness of the chip. The attachment layer thickness of the chip is designed to be approximately 200μm, further enhancing the reliability of feedthrough capacitors.