Capacitors are among the components most often subjected to counterfeiting, and the problem tends to intensify when a production line needs an obsolete part on short notice. Re-marking, that is, grinding off the original marking and printing a higher voltage, a larger capacitance value, or a different brand, is the most common counterfeiting method. A re-marked 400 V capacitor relabeled as 450 V, or a general-purpose electrolytic relabeled as a low-ESR type, will pass a basic capacitance check and fail only after it is installed. This article outlines the inspection gates that matter at incoming receiving, and which parameters can be relaxed when a genuine substitute is sourced instead.
Why Original Parts Disappear
Original electrolytic and film capacitors become unavailable for several reasons. Manufacturers retire older series without a direct replacement, particularly for niche formats such as snap-in terminals with a specific can height. Lead times for current series often stretch to 12–26 weeks, which pushes buyers toward distributors or brokers offering fast delivery. In some cases, the original part was a customer-specific variant that was never publicly catalogued.
Whatever the cause, the response should not be an unauthorized broker without a test report. A legitimate distributor will provide a traceable origin, a copy of the original datasheet, and a lot code. If that documentation cannot be produced, treat the parts as suspect.
Parameter Matching: What Must Match, What May Deviate
When the original part number is truly discontinued, evaluate a substitute against the circuit’s requirements rather than the old datasheet. Three parameters must match exactly or be exceeded:
- Rated voltage. The substitute must be rated at or above the original voltage, with the same voltage type (DC versus AC). For DC-link applications, derate by at least 20% from the nominal bus voltage.
- Capacitance within tolerance. The nominal value must match, and the tolerance must be equal or tighter. For electrolytic types, ±20% is standard; a substitute claiming ±5% on a large electrolytic is suspect.
- Ripple current. The allowable ripple current at 105 °C, or at the application’s ambient temperature, must be equal or greater. A lower ripple current rating causes overheating and premature failure.
Parameters that may deviate, provided the application allows it:
- Case size. A slightly larger diameter or height is acceptable if the PCB layout has clearance. A smaller case is a red flag: a smaller can with the same capacitance and voltage rating is unlikely to be genuine.
- ESR and dissipation factor. For bulk energy storage, a modestly higher ESR may be tolerable. For switch-mode input and output filtering, ESR must match closely; higher ESR increases output ripple and heating.
- Lifetime rating. A 2,000 h part instead of a 5,000 h part at rated ripple will still function but requires scheduled replacement in applications with a known service life.
Be careful: any substitute with the same values but a significantly different physical weight, terminal style, or internal construction should be quarantined for testing.
Cross-Reference: Parameter Gates for Incoming Verification
| Parameter | Measurement Condition | Acceptance Gate | Counterfeit Red Flag |
|---|---|---|---|
| Capacitance | 1 kHz, 0.5 Vrms for electrolytic; 1 kHz for film | Within ±20% of marked value (electrolytic); ±10% for film | Marked 470 µF measures 330 µF — likely re-marked upward |
| ESR / DF | ESR at 100 kHz for electrolytic; DF at 1 kHz for film | ESR within 1.5× datasheet maximum | Low-ESR marking but DF above 0.2 at 1 kHz |
| Leakage current | Rated voltage applied for 5 minutes | ≤ 0.01 CV or datasheet limit (electrolytic) | Leakage 3× higher than spec — degraded dielectric |
| Dielectric withstand | 1.5× rated DC for 60 s (film); 1.2× rated for electrolytic | No breakdown or audible discharge | Fails at 1.1× rated — relabeled lower-voltage part |
The withstand test is the fastest discriminator between a genuine 450 V part and a re-marked 400 V one. It is destructive if the part is defective, so sample testing rather than 100% inspection is the practical route. A common sampling plan for a lot of 10,000 is 125 pieces with a zero-acceptance number, following general inspection level II.
Verification Steps at Incoming Inspection
Sequence the checks from least to most destructive:
- Visual inspection. Examine marking quality under magnification. Laser-etched markings on a ground surface, uneven print, or a visible step where the original marking was removed are telltale signs. Compare the print font and logo with a known-good sample or the manufacturer’s published example.
- Weight and dimensions. Measure mass and compare with an authentic sample of the same series. A can that is 10% lighter than expected suggests a shorter element or a different dielectric inside.
- Capacitance and ESR. Use an LCR meter at the frequencies stated in the datasheet. Record temperature and correct readings to 20 °C.
- Leakage current. Apply the rated voltage through a current limit and measure after 5 minutes. High leakage indicates aged or degraded dielectric material.
- Dielectric withstand sample test. Apply 1.2× to 1.5× rated voltage for 60 seconds on a sample. Any failure warrants quarantine of the full lot.
- Soldering test. Solder a sample onto a test board and re-measure capacitance after reflow. A capacitance shift greater than 2% after soldering may indicate a hidden internal defect.
If a lot fails any gate, hold it for disposition. Do not return suspect parts to stock. For critical applications, third-party verification through a qualified laboratory is justified by the cost of field rework.
The practical point is that incoming inspection does not require expensive equipment. An LCR meter with 0.1% accuracy, a current-limited DC power supply, and a dielectric withstand tester cover the essential gates. For overseas procurement, note that re-marked capacitors often arrive with ambiguous origin documentation; require packaging photographs and full lot-code traceability in the purchase order from the outset, and verify RoHS/REACH declarations against the actual product rather than the invoice.
