Incoming inspection is the last point where a bad capacitor can be stopped before it reaches production. Re-marked and counterfeit electrolytic capacitors and MLCCs are not a niche problem; they appear in bulk lots sold through third-party brokers and occasionally in otherwise clean distribution channels. For an OEM buyer, the cost of a counterfeit part is not the unit price but the field failure rate, warranty claims, and reputational damage. This guide defines what incoming inspection can and cannot catch, gives numeric thresholds for common tests, and presents a stepwise procedure you can implement in a typical receiving inspection area.
Incoming Inspection Scope and Limits
Incoming inspection cannot replicate a full accelerated lifetime test, and it cannot verify the internal chemistry of an aluminum electrolytic capacitor without destroying the part. What it can do is identify parts whose construction, electrical behavior, or marking does not match the genuine article. Any lot that fails these checks should be quarantined and returned, because a single suspect unit indicates a systemic issue.
Focus on the three case styles most often counterfeited: aluminum electrolytic through-hole (radial and snap-in) and MLCCs in sizes 0603 to 1210. The thresholds below assume you have the original manufacturer datasheet for the parts you ordered. If the supplier cannot provide one, that alone is reason to hold the lot.
Decision Thresholds for Rejecting Suspect Capacitors
| Parameter | Screening method | Reject threshold | Counterfeit signature |
|---|---|---|---|
| Dimensions | Caliper, 10 units per reel or box | Outside datasheet min/max | Re-marked smaller case presented as a larger one |
| Weight | Analytical balance, 10 units | Deviation above ±5% from lot median | Smaller die inside a larger package |
| Capacitance | LCR meter at 120 Hz or 1 kHz | Below 80% or above 120% of rated value | Die value not matched to stated rating |
| ESR (electrolytic) | LCR meter at 100 kHz | Above 120% of datasheet maximum | Used or undersized element |
| DF (MLCC) | LCR meter at 1 kHz | Above datasheet limit (2.5% for X7R, 5% for X5R) | Wrong dielectric material |
| Leakage current | DC supply and microammeter, 2 min | Electrolytic: above 2× the datasheet limit (0.01·C·V or 3 µA, whichever is greater) | Internal contamination, wrong chemistry |
| Marking integrity | Solvent swab with isopropyl alcohol or acetone | Ink smudges, lifts, or reprint marks | Re-marked or re-printed body |
| Internal structure | X-ray, 5 units per batch | Missing MLCC electrode layers; electrolytic element visibly smaller than the can | Empty or partially filled package |
The electrical limits above are deliberately conservative. A genuine part will normally sit inside its tolerance band, while a re-marked part built from a lower-value die often falls outside. ESR is the most useful single test for aluminum electrolytics: genuine parts show ESR within the datasheet maximum, while re-marked parts frequently exceed it by a wide margin. For MLCCs, measure insulation resistance at rated voltage; typical limits are in the range of 10 GΩ or 1,000 MΩ·µF, but always apply the datasheet value.
Step-by-Step Incoming Inspection Procedure
- Verify documentation first. Confirm that the lot code format, date code, and manufacturer logo match examples from a known-good source. A part carrying a recent date code but arriving from a discontinued series is a red flag.
- Document the lot visually. Photograph the vendor labels and a sample of 10–20 units before handling. Good lighting and a macro lens are enough to record most marking discrepancies.
- Perform the weight check. Measure 5–10 identical units per package size and capacitance value, then compare each reading to the lot median. Flag any unit deviating more than ±5%. Mixed weights within one visually identical lot suggest re-marked leftover stock from different manufacturers.
- Check case dimensions. Radial and snap-in electrolytics must match the datasheet case size. A 25 mm can carrying a label intended for a 30 mm part is often a re-marked lower-voltage unit.
- Run electrical sampling. Use the AQL level already applied to your passive components, typically 0.65 to 1.0. Measure capacitance, dissipation factor or ESR, and leakage current against the thresholds above. A first failure should trigger a doubled sample; a second failure rejects the lot.
- Apply the solvent test. Rub a cotton swab wetted with isopropyl alcohol or acetone across the marking. Original laser or baked ink markings survive; re-printed markings tend to smear or lift.
- Use X-ray when available. Five units per batch is enough to identify missing MLCC electrode layers or an electrolytic element visibly smaller than the can.
- Quarantine and escalate. Any lot failing one or more checks should be quarantined pending supplier response. Do not re-test failed units and pass them, because the defect is statistical rather than isolated.
Sourcing-Level Countermeasures
Inspection catches defects, but sourcing choices reduce their likelihood. Buy through franchised or authorized distribution channels with a clear chain of custody. If a broker offers a hard-to-find capacitor at a price well below the current market level, treat the lot as suspect from the outset. Confirm that parts are covered by the manufacturer’s warranty and that the supplier can provide RoHS and REACH documentation on request. This matters most for end-of-life components: when a series is discontinued, re-marked and recycled units re-enter the market and original traceability is lost. For critical designs, a two-year supply locked in through your authorized channel is a more reliable strategy than a last-minute broker purchase.
