Lifespan of a hot microchip I'm currently building my own Home Automation System with a bunch of Arduinos and Relais, all of them wired to Ethernet "Shields". The casing I am printing on a 3D printer.

The Arduinos work great with those Ethernet "Shields", but the ENC28J60 chip on those Ethernet boards get quite hot. I am measuring 79.5°C approx.
In the plastic casing, I expect that they reach higher temperatures (because it's closed and has no ventilation whatsoever).



Since I am building this as a permanent solution (House wiring is now all newly adapted and I have no way back), which lifespan can I expect from those Ethernet boards?

The datasheet of that chip is saying that it's rated up to 85°C, but what does this upper rating mean? Will it stop working at this point, or only begin to degrade it's lifetime at this point?

EDIT: I have no space to put a dissipation block on those chips, because the casing is so tight.
The reliability data is typically not in the product datasheet but in pen whitepapers. Generally speaking life span goes down exponentially as temperature rises. For some high temperature semi conductors ( designed specifically for high temperature applications) you are talking about hubdreds or thousands of hours at rated temperatures.

That should give you some sense as to what to expect.