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Many integrated circuits I have seen have the specification of maximum junction temperature (150°C) but storage temperature is invariably higher (165°C) and often lower, e.g. -55°C. Why the discrepancy? Surely the junction will be damaged whether or not it has current flowing through it...?

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    \$\begingroup\$ With regards to the lower bound, some chips (mostly analog) will not start properly unless they are above a certain temperature. I read some article (Bob Pease?) a while ago about how some NI part wouldn't start at some cryogenic temp, but if it was started then cooled it would be fine. \$\endgroup\$
    – Nick T
    Oct 26, 2010 at 18:33

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Temperature extremes will damage the physical package itself and interconnecting wires. It is the different expansion rates or silicon, metal wires (gold??), and plastic housing that will effectively destroy the part. The silicon itself will normally survive to a limit. That is why earlier on in electronic development there was a difference between plastic dip and ceramic dip packages which had different temperature ranges. The silicon was the same, just the packaging was different. It was this difference in packaging that allowed the 'military' ceramic packages to have a greater usable temperature range than commercial plastic package.

With SMD packages, this has changed slightly and they all tend to have greater temperature range, but you can still get commercial, industrial and military temperature ranges of 0 to 85, -40 to 125, and -55 to 125 as a general rule.

That is why devices have lower limits and medium range upper limits, due to packages constraints.

There are also phsyics limits for the upper end where the silicon itself may become damaged and the dopants might defuse? I am not sure of the exact function here or temperature limit.

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Max junction temp guarantees it will still work within spec, storage temp is damage threshold. Semiconductor characteristics vary with temp, so there will be areas where it may not work within spec but won't suffer damage.

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