Counterfeit and re-marked capacitors remain a persistent risk in the electronic component supply chain, particularly for high-turnover parts such as aluminium electrolytic and multilayer ceramic capacitors. For OEM buyers and procurement engineers, the cost of a counterfeit component is not the unit price tag but the downstream consequence: field failures, warranty claims, and production line halts. This article provides a systematic, decision-threshold approach to incoming inspection that separates suspect lots from acceptable stock, with a focus on measurable parameters rather than visual guesswork.
Where Incoming Inspection Applies and Where It Does Not
Incoming inspection is a screening process, not a substitute for supplier qualification. It is most effective for passive components that are widely counterfeited, have short lead times, and are used in high volumes. The procedure described here applies to aluminium electrolytic capacitors, MLCCs, and film capacitors rated between 6.3 V and 450 V DC. It is intended for lot-level verification where the procurement contract already includes a supplier declaration of conformity and a Certificate of Conformance (CoC).
The inspection protocol does not extend to components that require destructive physical analysis (DPA), such as X-ray inspection of internal electrode alignment, or to parts that must meet military or aerospace-grade screening. Those require a separate, documented test flow. Incoming inspection here focuses on electrical and dimensional tests that can be performed with a standard LCR meter, a digital multimeter, a caliper, and a temperature-controlled soldering iron.
Key Parameter Thresholds and Rejection Criteria
For each lot, test a sample size based on an AQL (Acceptable Quality Limit) of 0.65, or a minimum of 20 pieces for lots under 1,000 units. Use the following thresholds as decision gates. Failing any single gate should trigger 100% inspection of the lot; if more than 5% of the full lot fails, reject the entire lot and escalate to the supplier.
Gate 1: Capacitance and Dissipation Factor (DF)
- Capacitance tolerance: Measure at 1 kHz for electrolytic capacitors and 1 MHz for MLCCs below 1 µF. Accept only if the measured value falls inside the marked tolerance band (e.g., ±20%, ±10%). A part marked ±20% that measures at −25% is a reject.
- Dissipation factor (DF) / tan δ: For aluminium electrolytic capacitors, DF should be below 0.15 at 120 Hz for parts rated above 16 V. For MLCCs, DF below 0.025 at 1 kHz is typical for X7R dielectric. A DF reading that drifts upward over a 30-second measurement window indicates moisture ingress or a re-marked, older part.
Gate 2: Leakage Current and DC Resistance
- Leakage current (electrolytic): Apply the rated voltage through a 1 kΩ series resistor for 5 minutes. Acceptable leakage current is I = 0.01 CV (µA) or 3 µA, whichever is greater, where C is rated capacitance in µF and V is rated voltage. For example, a 470 µF / 25 V capacitor should leak no more than 0.01 × 470 × 25 = 117.5 µA.
- Insulation resistance (film and MLCC): For film capacitors, measure IR at 100 V DC for 60 seconds; accept if greater than 10,000 MΩ for values below 1 µF. For MLCCs, IR should exceed 10 GΩ at rated voltage.
Gate 3: ESR and Ripple Current Handling
- Equivalent series resistance (ESR): Measure at 100 kHz for electrolytic capacitors. Use a threshold of ESR ≤ 0.5 × (DF / (2π × 120 Hz × C)) as a sanity check. If a 1000 µF / 35 V part measures ESR above 80 mΩ, it is likely a re-marked low-voltage part or an aged component.
- Ripple current rating: The printed ripple current rating must match the case size. A 10 mm × 16 mm case claiming 3 A ripple at 100 kHz is physically implausible for a standard aluminium electrolytic; typical values for that case size range from 0.8 A to 1.5 A. Reject any part whose printed rating exceeds the case-size typical range by more than 20%.
Gate 4: Physical Dimensions and Marking Integrity
- Case size: Measure diameter and height with a caliper. The measured dimensions must be within the series datasheet tolerance (±0.5 mm for diameter, ±1 mm for height). A 10 mm diameter part that measures 8 mm is either a re-marked smaller capacitor or a different series.
- Marking method: Laser-etched markings are common on authentic parts. If the marking is pad-printed and easily scratched off with a fingernail, this suggests a re-marked part. For MLCCs, verify the case size (e.g., 0805 = 2.0 mm × 1.25 mm) and the voltage code on the body.
| Parameter | Test Frequency / Condition | Typical Accept Range | Reject Indicator |
|---|---|---|---|
| Capacitance (electrolytic) | 120 Hz / 1 kHz | Marked tolerance ±10–20% | Outside tolerance band |
| Capacitance (MLCC X7R) | 1 kHz / 1 MHz | ±10% of rated value | More than −20% deviation |
| Dissipation Factor | 120 Hz (electrolytic) | < 0.15 | > 0.20 |
| Leakage Current | Rated DC voltage, 5 min | I ≤ 0.01CV µA | 10× above calculated value |
| ESR | 100 kHz | ≤ 0.5 × (DF / (2π × 120 × C)) | > 1.5× the calculated value |
| Case Dimensions | Cal, ambient temp 20–25°C | ±0.5 mm (D), ±1 mm (H) | Outside ±1 mm |
Step-by-Step Inspection Procedure
Run the following sequence in order. Each step uses the thresholds above to make a go/no-go decision.
- Prepare the lot: Record manufacturer name, lot date code, capacitance, voltage rating, and series code from the packing slip. Compare this against the physical marking. A mismatch in date code format or series code is sufficient cause for rejection.
- Visual inspection (100% of samples): Check for scratched-off markings, double markings, or marking that sits at an angle. For radial electrolytic capacitors, verify the vent pattern on the top is consistent across the sample. Re-marked parts often show a dimple or indentation where the original marking was ground down.
- Dimensional check: Measure 20 pieces. Calculate the average and standard deviation. If any part is outside the ±1 mm range on height or diameter, mark the lot as suspect.
- Electrical screening (LCR meter): Measure capacitance and DF at the specified frequency. For electrolytic capacitors, also measure capacitance after a 60-second warm-up at room temperature. Capacitance that rises by more than 5% during warm-up indicates a dry or degraded part.
- Leakage current test: Set the DC power supply to rated voltage with a current limit of 1 mA. Apply voltage through a 1 kΩ resistor. Record the leakage current after 5 minutes. If the current does not stabilise and keeps rising, the capacitor is likely a re-marked lower-voltage part.
- ESR verification: Use an ESR meter or LCR meter at 100 kHz. Compare against the calculated threshold. For surface-mount MLCCs, ESR values above 50 mΩ at 1 MHz are unusual for X7R dielectrics below 10 µF.
- Soldering test (for MLCCs): Solder one sample onto a test board using a standard reflow profile or a soldering iron at 350°C for 3 seconds. After cooling, re-measure capacitance and DF. A significant capacitance drop (more than 10%) after soldering indicates internal electrode cracking or a counterfeit part with a wrong dielectric.
- Decision: If all gates pass, release the lot. If any gate fails, move to 100% inspection. If more than 5% of the full lot fails after 100% inspection, reject the lot and file a discrepancy report with the supplier.
Documenting every measurement, including the ambient temperature and the instrument model, is standard practice. This data protects both buyer and supplier when a field failure later questions the lot’s performance. For procurement contracts, specifying that the supplier must accept returns on lots that fail these incoming gates is a common and workable term.