1 Welding and Assembly Process of Feedthrough Capacitor EMI Filter

The Feedthrough Capacitor EMI filter manufactured by XUANSN consists of a feedthrough capacitor chip, magnetic core, metal casing, and lead terminals. During assembly, the feedthrough capacitor EMI filter chip is inserted into the metal casing. During welding and assembly, the lead wire must not be eccentric. The outer electrode of the chip is welded to the inner wall of the metal casing to ensure good grounding contact with the casing, while the inner electrode of the chip is welded to the lead wire to ensure good contact with the transmission line. The specific process is as follows.

For products of different models, sizes, and structures, dedicated assembly tooling is designed and precisely machined (see Figure 1). The accuracy of the assembly fixture design ensures correct assembly and relatively avoids arbitrary operation by operators; the precision of the assembly fixture ensures product consistency and relatively reduces operator-related influencing factors.

1.2 Welding Heating Control Process

Due to the special structure of this type of feedthrough capacitor EMI filter, it is necessary to consider the heat absorption effect of the metal casing and the welding metal fixture. The welding heating method shall adopt preheating, slow heating, uniform heating, and limitation of maximum temperature to reduce cracks, high-temperature delamination, or other physical damage caused by sudden temperature rise. The same control methods are also applied during the cooling process.

Therefore, a hot air reflow soldering machine is used. Considering the heat capacity of the product and fixture, the optimal welding temperature profile is set through process experiments and temperature curve testing (see Figure 2 and Figure 3) to ensure welding quality.

Figure 2. Single-layer capacitor structure diagram

Figure 3. Schematic diagram (left) and circuit schematic (right) of an L-type filter structure

The assembly and welding procedure is as follows:

• Apply solder paste uniformly along the outer circumference of the chip at 360°

• Apply solder paste uniformly at the junction between the lead wire and the inner hole of the chip

• Place the assembly into the dedicated fixture (see Figure 3)

• Perform welding using a hot air reflow soldering machine

After welding, the solder between the chip and the casing must form a seamless connection. Inspection is performed under a 10× magnifying glass, and products with visible solder seams are rejected. Gaps between the chip and the casing may cause magnetic leakage at the input and output terminals, preventing effective grounding and shielding of input and output signals, thereby affecting high-frequency filtering performance.

Figure 4. Optimal soldering temperature curve

1.3 Post-Weld Inspection Requirements

After welding, the ceramic capacitor and sealing glass beads in the feedthrough capacitor EMI filter must not have cracks, fractures, or fragments, and other physical damage shall be minimized.

In addition to requiring seamless soldering between the chip and the casing (inspected under 10× magnification), the welding quality for reliability shall be inspected in accordance with MIL-28861, with the following requirements:

1.1.2 Chip Capacitor

(1) Cracks or deformation in the ceramic dielectric.
(2) Fragments, voids, or pinholes exceeding 0.25 mm in the main area of the ceramic dielectric that penetrate an embedded electrode layer.
(3) Terminal metallization leakage or bulging.
(4) Excessive metal markings on the surface reducing the spacing between inner and outer terminals to less than 75% of their radial distance.
(5) Foreign materials or contaminants.

1.1.3 Soldering Defects

• Solder balling

• Icicle formation

• Solder splashing

• Desoldering

1.1.4 Solder Overflow

1.1.5 Magnetic Beads

Broken or cracked ferrite is not allowed. Nicks exceeding 10% of the cross-sectional area are not permitted.

1.1.6 Casing

Burrs, lead damage, coating flaking, or indentations.

2 Testing Process

The electrical parameters of the EMI filter include:

• Rated voltage

• Rated current

• Dielectric withstand voltage

• Capacitance to ground

• Insulation resistance

• Temperature rise

• Voltage drop

• Insertion loss

2.1 Rated Voltage

Rated voltage refers to the voltage that the feedthrough capacitor EMI filter can withstand for a long time during normal operation. It is directly related to circuit safety; therefore, special attention must be paid to the voltage rating.

DC types must never be used in AC applications, otherwise breakdown may occur. In addition, almost all complete system electromagnetic compatibility tests include pulse interference testing. Therefore, when selecting a filter, its resistance to high-voltage pulse interference must be considered, and sufficient voltage margin shall be reserved.

2.2 Rated Current

Rated current refers to the current value that can pass through the filter for a long time during normal operation.

The rated current is determined by the lead diameter; the larger the wire diameter, the greater the allowable rated current. For filter assemblies, the rated current is also related to the saturation characteristics of the inductor coil. When the current exceeds the rated value, filter performance will degrade.

2.3 Withstand Voltage

The maximum instantaneous voltage that the filter can withstand (at least 60 seconds).

2.4 Insertion Loss

Insertion loss refers to the frequency attenuation value (dB) of the filter in the stopband region. Each filter type has a parameter table showing insertion loss versus frequency. When selecting a filter, the required insertion loss can be determined according to the frequency of the interference signal and the desired attenuation level.

It should be noted that the insertion loss provided in product catalogs is measured in a 50Ω system. If the actual operating condition is not a 50Ω system, the insertion loss will differ from the catalog value. The load in the actual circuit will also significantly affect the measured value.

2.5 Temperature Rise

Refers to the change in surface temperature when rated current is applied to the filter leads.

2.6 Voltage Drop

Refers to the voltage difference measured across the filter terminals when current is applied to reach specified requirements.

LCR comprehensive testers, insulation resistance testers, and withstand voltage testers are used to accurately test capacitance, dielectric withstand voltage, insulation resistance, and other fundamental electrical parameters.

① Withstand voltage test: 2.5 times rated voltage, 5s ±1s, maximum charging current not exceeding 50mA, no arcing or breakdown.
② Insulation resistance test: Test voltage 100V, Rj ≥1500MΩ is qualified.
③ Capacitance and loss measurement: Capacitance (test condition: 1 MHz) 10pF–100pF.
Accuracy: K: ±10%.
Loss factor tgδ ≤0.0015 is qualified.

Temperature Rise Test Method

Using a DC regulated power supply, apply rated current to the filter leads. After the surface temperature stabilizes, measure the surface temperature using a point thermometer. Temperature rise shall not exceed 15°C.

3 Voltage Drop Test Method

Place the product into a dedicated test fixture. Apply current to reach the specified voltage requirement. Adjust the variable resistor to achieve the specified current. Use a four-wire voltage tester to measure the voltage across the filter terminals. The difference in measured data is the voltage drop. Generally, the voltage drop shall be less than 0.1V.

4 Reliability Screening and Power-On Aging

To ensure the service life reliability of the feedthrough capacitor EMI filter products manufactured by XUANSN, after completing routine electrical parameter tests (rated voltage, dielectric withstand voltage, capacitance to ground, insulation resistance), electrical performance screening is required.

This process further eliminates early component failures and process-related early failures. Products undergo 100% screening under the following conditions:

• Temperature shock

• Moisture exposure

• High-temperature load with applied current and voltage

A power-on test fixture was designed and manufactured. Through multiple trials, the fixture precision was improved, increasing mounting hole accuracy (specified as φ3.3 +0.1/0). After improvement, the mounting hole gap is less than 0.06mm.

The fixture pressure plate was changed to 2mm thickness (originally 0.5mm) to tightly press the edge of the product casing. The metal mounting plate was changed from aluminum to copper, and its thickness was increased.

Comparative testing of contact resistance before and after improvement showed a reduction of approximately 43%–91%. Since heating power P = I²R, the heating power during energization decreased accordingly, reducing power consumption due to heating by approximately 60%.

Under identical conditions, a temperature rise comparison test showed:

• Original fixture temperature rise: 14°C

• New fixture temperature rise: 9°C

This ensures reliable power-on contact.

Explanation:
24h — Process-related early failure period
24h–96h — Component early failure period
168h — Component aging stabilization period

5 Design Verification of C-Type and L-Type Filters

Through the above electrical performance tests, unqualified products are eliminated, ensuring the reliability of XUANSN feedthrough capacitor EMI filters in practical applications.

This chapter completes the design and fabrication of C-type and L-type EMI filters.

5.1 C-Type Filter

Measured insertion loss at 470MHz: −3.5 to −3.9dB, meeting the cutoff frequency requirement of 470MHz.

Insertion loss at 6GHz: 32.1–34dB, meeting the maximum −28dB specification.

Specification Requirements

Frequency range: (1–6000) MHz
Cutoff frequency: 470MHz
Insertion loss: 28dB

Simulation result at 6GHz: 30.136dB.

Actual measured insertion loss at 6GHz: 32.5–34dB.

The deviation between actual test data and simulation results is approximately 2–4dB.

5.2 L-Type Filter

Measured insertion loss at 500MHz: −2.1 to −2.5dB, meeting the cutoff frequency requirement of 500MHz.

Insertion loss at 6GHz: −28 to −29dB, meeting the maximum −28dB specification.

Specification Requirements

Frequency range: (1–8000) MHz
Cutoff frequency: 500MHz
Insertion loss: 30dB

6 Conclusion

This article systematically presents the complete manufacturing, inspection, testing, and reliability verification process of XUANSN Feedthrough Capacitor EMI filter components. From precision fixture design and controlled reflow soldering temperature profiles to strict post-weld inspection requirements, every stage is engineered to ensure structural integrity, stable grounding performance, and consistent electromagnetic shielding effectiveness. Welding quality is verified in accordance with MIL-28861 standards, ensuring that ceramic dielectrics, metallization layers, ferrite components, and casing structures meet high-reliability requirements.

Comprehensive electrical testing—including rated voltage, rated current, dielectric withstand voltage, insulation resistance, capacitance, temperature rise, voltage drop, and insertion loss—provides multi-dimensional validation of both safety and performance. The implementation of 100% screening under temperature shock, moisture exposure, and high-temperature load conditions effectively eliminates early failures and enhances long-term operational stability. Optimized power-on fixtures significantly reduce contact resistance and temperature rise, further improving reliability during energization.

Finally, design verification of C-type and L-type filters demonstrates strong agreement between simulation and measured insertion loss data, confirming the accuracy of structural and electromagnetic design. Overall, the integration of controlled manufacturing processes, rigorous inspection standards, full-parameter electrical testing, and reliability screening ensures that XUANSN feedthrough capacitor EMI filters deliver dependable EMC protection, stable signal integrity, and extended service life in demanding electronic applications.