Operating limit vs destruct limit: what's the difference?
An operating limit is the stress level beyond which a product stops working correctly but recovers when the stress is reduced: a soft failure. A destruct limit is the level at which permanent damage occurs and the product does not recover: a hard failure. HALT finds both, for cold, heat and vibration.
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The limits HALT measures
| Term | Meaning |
|---|---|
| LOL / UOL | Lower / upper operating limit: the stress level beyond which the product stops functioning correctly but recovers when the stress is reduced (a soft failure). |
| LDL / UDL | Lower / upper destruct limit: the stress level at which permanent (hard) damage occurs; the product does not recover. |
| VOL / VDL | Vibration operating limit / vibration destruct limit: the same two boundaries for repetitive-shock vibration. |
| Operating margin | The difference between the specification limit and the operating limit. |
| Destruct margin | The difference between the operating limit and the destruct limit. |
| FLT | Fundamental limit of technology: a limit set by an intrinsic material property, such as a solder melting point, that design changes cannot improve. |
What each limit is used for
Operating limits become the upper bound of the HASS screen: both the precipitation and the detection segments run below them and above specification. They are also the basis for judging margin against the specification. Destruct limits are a measure of design robustness, and show how much margin lies beyond the screen.
There is no universal required margin. Common practice looks for operating limits comfortably beyond specification, and treats any weakness inside or near specification as mandatory to fix. Where a limit is set by the fundamental limit of the technology, document it and stop; where it is set by a fixable weakness, fix it.
How they are found
HALT steps each stress, cold, heat and then vibration, past the soft failure to the hard failure, checking recovery at the operating limit before continuing. The procedure, with step sizes and dwells, is in the methodology guide.
In the standards
IEST-RP-PR003, “HALT and HASS”, from the Institute of Environmental Sciences and Technology, is the principal industry recommended practice dedicated to HALT and HASS methodology. GM’s GMW8287 defines the same two limits: the operational limit, where the product recovers when the stress is reduced (a soft failure), and the destruct limit, where it is damaged and does not recover (a hard failure). The standards guide summarizes both.
The short answer
What is the difference between an operating limit and a destruct limit?
At an operating limit the product malfunctions but recovers when the stress is reduced (a soft failure). At a destruct limit it is permanently damaged and does not recover (a hard failure). HASS screens run below the operating limits; destruct limits show how much margin lies beyond the screen.