For critical EMI filter applications, high reliability is paramount. In applications requiring long-lifespan operation, such as medical and military systems, or satellite and submarine cable applications, replacement costs for failed components are prohibitively high. Therefore, during the design phase, it is crucial not only to prioritize overall system reliability but also to refer to relevant feedthrough capacitor standards to ensure that the selected filter components possess sufficient stability and safety margins during long-term operation.

Reliability must be designed into EMI filters. From material selection and manufacturing processes to performance testing and quality assurance systems, every step of the manufacturing process must be considered and rigorously controlled. To ensure consistency, product and performance parameters are clearly defined. Electrical performance, mechanical appearance, testing methods, screening, and certification procedures have evolved into standard specifications. Only by adopting high-reliability EMI filter standards, selecting filters that meet the GIB1518 RF interference filter general specification, and referring to US military standards MIL-F-15733 and MIL-F-28861, can highly reliable EMI products be designed and manufactured.

Table 1 Comparison of GIB1518 General Specification and US Military Standards MIL-F-15733 and MIL-F-28861

Taking MIL-PRF-28861 as an example:

1.The specification defines two quality and reliability assurance product levels: Class B and Class S (Aerospace Grade). It stipulates that ceramic capacitors used for bypass capacitors in Class B filters should comply with MIL-C-58861 (GJB924), and ceramic capacitors used in Class S filters should comply with MIL-C-123 (GJB4157-2002); (plastic film dielectric capacitors should comply with GJB972A-2002, and series inductors should comply with GJB675A-2002). Furthermore, in addition to meeting relevant feedthrough capacitor standards, the design, structure, and dimensions of Class S filters should be glass-sealed, the metal casing should be gold-plated, X-ray inspection should be performed, and DPA analysis requirements should be met.

2.The manufacturer shall establish and maintain a QPL system in accordance with the MIL-STD-790 (GJB546A-1996) reliability assurance outline, and establish an SPC system for batch control and online control, including ① capacitors and inductors ② incoming material inspection ③ welding ④ assembly ⑤ statistical process control of finished products;

2.1 Material reliability selection:

2.1.1 Selection of capacitor chip materials in the filter: Three types of dielectric materials are basically used: COG (ultra-stable), X7R (stable), and Z5U or Y5V (general purpose). However, due to the low dielectric constant of COG material, the product capacity is generally below 4000pF, limiting its use. However, when the filter has bias characteristics requirements, S-grade BX characteristic ceramic material (dielectric constant ≤2000, bias characteristics +15%, -25%) and B-grade BR characteristic ceramic material (dielectric constant ≤2000, bias characteristics +15%, -40%) can be selected. According to MIL-F-15733 standard, EMI feedthrough capacitors are permitted to use COG, X7R, and Z5U ceramic materials with a dielectric constant of 10–10K. According to MIL-F-28861 standard and related feedthrough capacitor standards, Class S requires BX characteristic ceramic materials with a dielectric constant ≤2K and bias characteristics ΔC/C ≤ +15% and -25%. Considering that the capacitance of the capacitor chips designed for the product is less than 100pF, Class 1 ceramic dielectrics can be selected. The characteristics and comparison of various Class 1 ceramic dielectrics are as follows (see Table 2 and Table 3).

Table 2: Characteristics of Various Class 1 Ceramic Dielectrics

Table 3: Comparison of Characteristics of Various Class 1 Ceramic Dielectrics

As can be seen from Table 2, for the reliability of the product, the ceramic dielectric material with COG characteristics should be selected. This ceramic material has superior insulation, withstand voltage, and temperature stability compared to other groups of ceramic materials.

2.1.2 Selection Principles for Internal Electrode Paste:

a.The internal and external electrode pastes should be compatible with the dielectric paste (strong adhesion, similar coefficients of linear expansion, no delamination or blistering, low contact resistance) to improve the high-frequency performance and reliability of the product (temperature shock, etc.). Generally, high-palladium electrodes can reduce silver ion migration; nickel electrodes, being anti-reduction ceramics, exhibit oxygen vacancy migration, resulting in a sharp decrease in insulation resistance at high voltages above 200V and high temperatures of 125℃.

b.The internal and external electrode pastes and dielectric pastes should not react miscibly to avoid reducing withstand voltage and insulation strength. Simultaneously, strong adhesion is essential to prevent delamination, blistering, and increased contact resistance, which would lead to a decrease in product reliability.

The metallized silver paste for through-holes and grounding terminals should be compatible with the internal and external electrode pastes, exhibiting high adhesion with the dielectric paste and good solderability to ensure the reliability of filter assembly and meet the material consistency and reliability requirements of relevant feedthrough capacitor standards. 2.1.3 Selection of Inductor Material in the Filter:

c. First, the inductor material used in the EMI filter is analyzed. EMI soft magnetic materials include MX, NX, Ni-Cu-Zn, and ferric hydroxide; soft magnetic alloys include Fe-Ni-Mo, amorphous alloys, nanocrystalline materials, silicon steel sheets (Fe-Si), and permalloy (Fe-Ni). MX has low insulation resistance and is not suitable for high voltage, while NX has high insulation resistance and is suitable for high voltage. Soft magnetic alloys require an insulating jacket for high voltage applications. The μ of MX, NX, and Ni-Cu-Zn drops sharply to near 1 under high current, therefore they cannot be used for high current. Ferric hydroxide and soft magnetic alloys can be used for high current; increasing the Curie temperature, increasing the saturation flux, or increasing the cross-section of the magnetic material can increase the operating current.

Based on the above analysis and considering the product’s structural dimensions and electrical parameters (current 5A, voltage 200V), NX material is selected.

Based on the product’s attenuation and frequency characteristics and using the simulation results from ADS software, the inductance value was designed through multiple experiments and data collection. Three companies’ magnetic cores were selected for testing, and the test data comparison is shown in Table 4.

Table 4: Test Data of Purchased Magnetic Cores

Table 5: Comparison of Test Data of Finished Products After Assembling Different Magnetic Cores

The comparison of the above two sets of data shows that, although the filter assembled using the magnetic core from Company B meets the insertion loss requirements, its dimensions cannot meet the requirements because the manufacturer does not have the mold for the required magnetic core.

The filter assembled using the magnetic core purchased from Company C partially meets the insertion loss requirements, but the core consistency is poor, and it can only be used after screening tests.

The filter assembled using the magnetic core purchased from Company B fully meets the insertion loss requirements.

After comparing and analyzing the above test results, the final selected magnetic cores are φ1.4×φ0.9×4.5 or two φ1.2×φ0.9×2.5, with an inductance of 3~50nH (1MHz), and NX10 material. Meanwhile, in the selection of the magnetic core and the design of the filter, feedthrough capacitor standards were referenced, and XUANSN was responsible for component manufacturing and assembly to ensure that the overall filter performance and reliability meet the requirements of high-reliability applications.