Incoming inspection is the last controlled checkpoint before suspect capacitors enter production. For distributors and design engineers sourcing electrolytic and film capacitors, the cost of a counterfeit part is not the unit price — it is the field failure, the recall, and the qualification rework. Re-marked components, where a lower-grade part is cleaned and printed with a higher-grade brand or tolerance, are difficult to catch by visual inspection alone. The following test sequence builds a practical, parameter-based defense.
Target Equipment and Electrical Stress Profile
The inspection procedure below targets capacitors intended for switched-mode power supply (SMPS) output stages and industrial motor drive DC links. These applications impose a defined stress set: high RMS ripple current (typically 1.5 A to 6 A per capacitor), DC bus voltages from 350 V to 800 V, and enclosure ambient temperatures of 45 °C to 65 °C. Under these conditions, the capacitor’s core temperature can exceed the ambient by 10 °C to 20 °C due to ESR losses. A re-marked part, which may have a higher ESR or a lower voltage rating than its label suggests, will run hotter and age faster. The goal of incoming inspection is to confirm that the measured parameters match the declared datasheet values within acceptable production tolerances.
Derived Component Requirements and Test Limits
From the electrical stress profile, the following acceptance criteria should be applied when the capacitor arrives at the dock. These limits are intentionally tighter than the manufacturer’s absolute maximum ratings, because a genuine part will comfortably meet them, while a re-marked or counterfeit part will show measurable deviation.
| Parameter | Test Condition | Acceptance Range (Relative to Label) |
|---|---|---|
| Capacitance | 120 Hz, 20 °C, 0.5 Vrms | ±10 % of stated value; reject if outside ±15 % |
| Dissipation Factor (DF) | 120 Hz, 20 °C | ≤ 100 % of datasheet maximum; reject if > 120 % |
| ESR | 100 kHz, 20 °C (for electrolytic); 1 kHz (for film) | ≤ 110 % of datasheet typical; reject if > 130 % |
| Leakage Current | Rated DC voltage, 5 min soak, 20 °C | ≤ 100 % of datasheet limit; reject if > 150 % |
| Vent / Case Integrity | Visual and mechanical torque check | No deformation, no lifted vent, no seal wobble |
For DC-link film capacitors, add a dV/dt test at 50 % of the rated voltage slew rate. A genuine part will pass without measurable temperature rise; a re-marked part with thinner metallization may show localized heating or audible discharge.
Installation and Test Notes for Reliable Verification
Do not rely on a single measurement. The most effective sequence is capacitance and DF first, as these require only an LCR meter and reveal gross mismatches. Then perform ESR measurement at the specified frequency. A common counterfeit practice is to re-mark a 105 °C, 2000-hour part as a 105 °C, 10000-hour part; ESR at 100 kHz will typically be 20 % to 40 % higher on the shorter-life part, which is a strong screening indicator.
- Sample size: For lots of 1000 pieces or fewer, test 20 units or 5 %, whichever is greater. For larger lots, use an AQL of 0.65 for critical parameters (voltage rating, capacitance) and 1.0 for cosmetic checks.
- Leakage current soak: Use a regulated DC supply with current limit set to 10 mA. A genuine electrolytic capacitor’s leakage current will decay logarithmically over the 5-minute soak. A re-marked part often shows an unstable, slowly rising leakage current after the initial decay, indicating an incorrect oxide layer thickness.
- Temperature rise check: For a quick field check, run the capacitor at its rated ripple current for 30 minutes in a 25 °C still-air chamber. Measure case temperature with a thermocouple. Genuine parts will stabilize within 15 minutes; suspect parts may continue to climb, indicating excessive ESR.
- Date code and batch consistency: In a genuine lot, the date code and lot number print will be consistent in font, alignment, and color density. Re-marked lots often show slight ink mismatches or a laser-etched surface that differs from the surrounding sleeve texture.
Documenting Results and Disposition
Record every measurement on a lot-level form that includes the supplier, the declared part number, the measured average, and the number of rejects per parameter. This documentation serves two purposes: it creates a comparison baseline for future shipments from the same supplier, and it provides evidence if the lot must be returned or quarantined. When a lot fails any single critical parameter — capacitance outside ±15 %, or leakage current above 150 % of limit — quarantine the entire lot. Do not re-screen only the rejected units, because the underlying issue is often a process variation in the original manufacturing, meaning the entire batch is suspect.
A practical note: counterfeiters are aware of basic capacitance and ESR checks. Adding a ripple current life test on a small sample (3 to 5 units, 1000 hours at rated ripple and 105 °C case temperature) is the most reliable discriminator, although it delays incoming release. For high-volume production, run this life test on a quarterly basis per supplier rather than on every lot. The combination of fast electrical screening on every lot, and a periodic life test, catches both the obvious re-marks and the more sophisticated counterfeits that pass single-point measurements.
The inspection sequence described here is not a replacement for a full qualification audit of the supplier. It is a practical, parameter-driven screen that catches the majority of re-marked components before they reach your assembly line. When in doubt, treat the lot as suspect, hold it, and request a certificate of conformance or a sample from the original manufacturer for comparison. The extra days of quarantine are cheaper than a field return.