Key takeaways
Storage is unpowered and asks whether materials and interfaces degrade when held hot, judged by parameter drift after recovery. Operation is powered and loaded, and asks whether function survives electrical stress plus self-heating. They interrogate different failure mechanisms, so neither substitutes for the other.
High-temperature storage and high-temperature operation are often treated as the same test at two different temperatures. The real difference is not the temperature but whether the sample is powered. In a storage test the sample is unpowered and unloaded, and the question is whether materials and interfaces age under pure thermal stress. In an operating test the sample runs under load, junction temperature equals chamber temperature plus self-heating, and the question is whether function survives that combined stress. Passing one does not imply passing the other, and substituting one for the other is a frequent source of rework at sample submission.
1. The root of the difference is power
A storage sample is inert: no supply, no load, no in-situ monitoring. The chamber holds a constant temperature for the specified duration, and the sample is measured after recovery. The test interrogates materials - whether encapsulants and potting compounds oxidise, whether plasticisers migrate, how far diffusion has progressed at plating and solder interfaces, how much electrolyte has been lost.
An operating sample is live: it runs in a specified mode, the chamber temperature is only the ambient, and the junction temperature of chips and power devices adds self-heating on top. This test interrogates function - at that temperature, under power, does the product still hold its specified functional status.
| State | What is assessed and by which mechanism | Where the criterion sits |
|---|---|---|
| --- | --- | --- |
| High-temperature storage (unpowered) | Material oxidation, plasticiser migration, interfacial diffusion, electrolyte loss | Function, parameter drift, appearance and sealing after recovery |
| High-temperature operation (powered, loaded) | Electrical stress plus self-heating; junction above chamber temperature | In-situ monitoring during exposure, judged by functional status class |
| Low-temperature storage (unpowered) | Embrittlement, seal shrinkage, grease viscosity rise | Function, appearance and sealing after recovery |
All three share heat. What differs is what is stacked on top of heat: heat alone, heat plus electrical stress, heat plus cold. Different stacks mean different mechanisms and therefore different criteria.
2. Storage cannot be judged by "it still powers on"
The step most often dropped in a storage test is the parameter measurement. The sample comes out of the chamber, powers up, communicates, and the report says function normal. That report usually does not survive an OEM audit.
The reason is simple: early thermal ageing shows up as drift, not failure. Leakage current rises, capacitance falls, contact resistance climbs, insulation resistance drops - all still inside the acceptance window, so the product behaves normally. Push those same parameters once more with vibration, damp heat or vehicle-level ageing, and the problem surfaces after installation.
A storage criterion therefore needs three layers: function, parameters, and appearance together with sealing. Drop one layer and the report holds a single sentence, leaving nothing to work with when a failure analysis is needed later. Ordering between climate items is covered in how climate loads are sequenced; this article deals with the criterion of a single item.
The second item easily missed is recovery. Measuring straight out of the chamber lets residual heat and moisture bias the readings, and one batch can yield two different conclusions. Recovery environment, recovery duration and measurement time point all belong in the outline, referenced to the applicable standard revision and the customer technical agreement.
3. Self-heating rewrites the stress in an operating test
In an operating test the chamber temperature is ambient, not device temperature. At the same chamber setting, full load versus standby, continuous versus duty-cycled operation, forced air versus still air, and different sample density can separate junction temperatures by a wide margin.
The outline must therefore state four things: the operating mode (standby, full load or cycled), how the load is connected, sample spacing and placement inside the chamber, and whether the monitoring point sits inside or outside the housing. Omit one and the applied stress becomes uncontrolled; an uncontrolled stress means the temperature in the report is only a chamber setpoint.
Two details recur. First, sample density: with several units in one chamber, neighbouring samples act as heat sources and the actual temperature varies by position. Second, the monitoring point: measuring inside the housing needs feed-through wiring, which brings sealing and routing problems of its own; measuring only outside says nothing about junction temperature.
In one sentence: operating stress equals chamber temperature plus self-heating, and if self-heating is uncontrolled the results are not comparable.
4. Three pairs that get confused
| Confused pair | Where they differ | Consequence of mixing them up |
|---|---|---|
| --- | --- | --- |
| Storage / operation | Powered or not; presence of self-heating | Storage used in place of operation misses electro-thermal failures |
| Storage / damp heat (85/85, cyclic) | Dry-heat oxidation versus moisture absorption, hydrolysis and electrochemical migration | Dry heat used in place of damp heat misses absorption and bias-driven migration |
| High-temperature storage / low-temperature storage | Thermal ageing versus embrittlement and shrinkage | Only one end tested; the other surfaces in cold-climate use |
One more concept is frequently muddled: there are two temperature ceilings, not one. The operating ceiling comes from the temperature class assigned to the mounting location, defined by class codes in GB/T 28046.4 or ISO 16750-4. Storage temperature is a separate parameter - the operating ceiling cannot be quoted as a storage temperature, and a storage temperature cannot be reported to a customer as the product's operating capability. The 2023 revision of the class codes is covered in ISO 16750-4:2023 temperature classes.
On the humidity side, the difference in condensation location between steady-state and cyclic profiles is set out in the two damp-heat profiles, and the steady-state case in the 85/85 test. The two fast temperature profiles are compared in temperature cycling versus thermal shock. No condition values are repeated here.
5. Four columns the outline must carry
- Sample state: powered or not, operating mode, load connection, monitoring items.
- Conditions: temperature value, hold duration, applicable standard revision year - all three, or it is not a condition.
- Criteria: functional status class, parameter items and measurement points, method for checking appearance and sealing.
- Recovery and measurement timing: recovery environment, recovery duration, the moment of formal measurement.
In public practice, +85 °C / 1000 h is often quoted for high-temperature storage and -40 °C / 1000 h for low-temperature storage. Actual values follow the applicable standard revision, the OEM specification tied to the part number and the customer technical agreement; no values are preset at quotation or outline stage.
A few plain words
Storage and operation are not a strict and a loose version of the same test, and they are not alternatives. They interrogate different mechanisms: what happens to materials held hot for a long time, and whether a powered product still works while hot.
When TMC LAB takes on this kind of commission, the power state, operating mode and criteria go into the test outline first, the condition levels are then confirmed against the temperature class for the mounting location, and testing is scheduled only after that confirmation. Capability scope for material and climate work is listed under material and environmental testing and damp heat capability; the relationship between the standard families is in GB/T 28046 and ISO 16750.
Sending the specification revision tied to the part number, the power requirements and the monitoring items along with the samples saves a round of rework. Scheduling is arranged per project; you can state your expected delivery date when submitting samples, and we will confirm a feasible plan against the current schedule and report progress at each milestone.
