Solid polymer and liquid (wet) electrolytic capacitors both store charge in an anodized aluminum oxide layer, but their electrolytes differ fundamentally. A wet capacitor uses a liquid or gel electrolyte that can evaporate and partially re-form the oxide layer; a solid polymer capacitor uses a conductive polymer with no liquid to lose. The practical consequence is that the two types age through different mechanisms, and the lifetime models used for procurement and obsolescence replacement are not interchangeable.
Lifetime models in plain terms
For wet electrolytics, the dominant ageing mechanism is electrolyte loss. As core temperature rises, the electrolyte evaporates faster and the oxide layer degrades; capacitance falls and ESR climbs. The conventional engineering estimate is that lifetime doubles for every 10°C drop in core (hot-spot) temperature. A wet capacitor rated at 105°C for 5,000 hours, run at 95°C, is typically expected to last roughly 10,000 hours. This approximation holds well within the rated temperature range and is useful for planning replacement intervals in industrial supplies.
Polymer capacitors behave differently. There is no liquid to evaporate, so the 10°C rule does not predict dry-out failure. The limiting factors are thermal-mechanical stress at the polymer-to-oxide interface and gradual ESR increase under high ripple. Many polymer types are rated at 105°C or 125°C with endurance ratings from 2,000 to 10,000 hours, but the temperature acceleration factor is typically 1.5 to 2 per 10°C rather than the classic 2 per 10°C of wet parts. Voltage derating, which does little for wet electrolytic life beyond reducing oxide stress, has a similar limited effect on polymer service life; the dominant variables are core temperature and ripple current.
Temperature and ripple derating with numbers
Both families share one practical rule: the core temperature, not the case temperature, drives ageing. Core temperature is approximated by ambient temperature plus the rise caused by ripple current. For a wet part with ESR in the range of 30 to 50 mΩ at 100 kHz, a 1 A ripple contributes 30 to 50 mW of self-heating, which typically raises the core a few degrees. A comparable polymer part with ESR of 5 to 15 mΩ contributes roughly a third to a quarter of that heat, which is why polymer parts are often selected for high-frequency, high-ripple positions such as switcher outputs and DC-DC input rails.
The trade-off appears at high ambient temperatures. Because a polymer capacitor cannot re-form its oxide layer electrochemically, its capacitance tends to degrade cumulatively above roughly 105°C hot-spot. A wet capacitor, by contrast, can partly repair the oxide film at moderate temperatures as long as electrolyte remains. So in a 105°C ambient with low ripple, a wet part may match or exceed the life of a polymer part, despite the polymer’s higher ripple capability.
Operating conditions versus expected life
The table below gives typical endurance numbers for a generic 470 µF / 25 V part in each family. Treat the values as planning estimates, not datasheet guarantees; always verify the supplier’s endurance test conditions and recommended load profile.
| Condition | Hot-spot temp | Ripple as % of rated | Wet electrolytic life | Polymer life |
|---|---|---|---|---|
| Low ambient, moderate ripple | 85°C | 50% | ~20,000 h | ~15,000 h |
| Nominal rating | 105°C | 100% | 5,000 h | 5,000 h |
| High ambient, low ripple | 110°C | 30% | ~3,500 h | ~2,500 h |
| High ripple, forced cooling | 95°C | 120% | ~7,000 h | ~10,000 h |
These numbers illustrate the crossover point: the polymer withstands higher ripple at the same core temperature, but at sustained high core temperature with modest ripple, the wet part holds capacitance longer because its oxide layer can partially re-form.
Design rules for cross-replacement
- Define the actual hot-spot temperature from the board thermal profile, not from ambient alone. Use ripple current and ESR to calculate self-heating; a polymer part with half the ESR may run 5 to 10°C cooler at the same ripple.
- Apply ripple derating conservatively. If the switching frequency is above 100 kHz, verify that both the wet and polymer candidates have impedance data at that frequency; polymer parts typically provide lower ESR and flatter impedance, while some wet types are optimized for low-frequency bulk storage.
- Do not assume voltage derating extends polymer life proportionally. Most polymer datasheets list an endurance rating at rated voltage; a 20% derating may add only marginal hours compared with a core temperature reduction of 5°C.
- For obsolescence replacement, check the failure mode of the original part. If the legacy wet part failed by ESR rise, a polymer part is usually a safe cross; if it failed by capacitance loss at high ambient, verify the polymer candidate’s capacitance change over the target life, not just its initial rating.
- Request sample support for the specific ripple waveform and thermal cycling profile in your board before committing to a bulk order. A polymer part that looks attractive on paper can show faster capacitance drift under repetitive thermal cycling in confined enclosures.
When sourcing replacements, consider carrying both families in inventory for different positions: polymer parts for output and point-of-load rails where ripple dominates, and wet parts for bulk storage in hot, low-ripple sections. Matching the lifetime model to the actual stress profile is more reliable than comparing datasheet endurance hours alone.