From Wikipedia the common temperature range for electrical components is:
Commercial: 0 to 70 °C
Industrial: -40 to 85 °C
Military: -55 to 125 °C
I can understand the lower part (-40°C and -55°C) as these temperatures do exist in cold countries like Canada or Russia, or at high altitudes, but the higher part (85°C or 125°C) is a bit confusing for some parts.
Transistors, capacitors, and resistors heating is very understandable, but some ICs have approximately constant low heat generation (like logic gates)
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If I am considering a microcontroller or operated in a Sahara deserts at 50 °C ambient ( I don’t know if there is higher temperature on earth) why would I need 125 °C or 85 °C? The heat built up from power loss inside shouldn’t be 50 °C or 70 °C otherwise the Commercial part would fail immediately in for example, 25 °C environment?
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If I live in a moderate climate where the temperatures can only fluctuate in the 0–35 °C range all year around, and designing industrial products for the same country only (no export) could I use commercial grade components (assuming no certification, legislation, and accountability exist and only engineering ethics govern your actions)?
Best Answer
The maximum temperature the silicon experiences can be much more than ambient. 50 °C ambient certainly happens. That's only 122 °F. I've personally experienced that in the Kofa Wildlife refuge north of Yuma Arizona. You need to design to worst case, not wishful case. So let's say ambient can be 60 °C (140 °F).
That by itself isn't much of a problem, but you don't get that by itself. Take the same thermometer that reads 60 °C in open air and put it in a metal box sitting on the ground in the sun. It's going to get much hotter.
I've seen someone fry a egg on the hood of a car in the sun in Phoenix AZ. Granted, this was a stunt deliberately set up for this purpose. The car was parked at the right angle, the piece of hood was tilted at the right angle, and painted flat black. However, it still shows that just a piece of metal sitting in the sun can get really hot.
I once left a car parked at the Las Vegas airport for a few days. I had left one of those cheap "stick" ballpoint pens on the dashboard, partly sticking out over the side. When I got back the pen was bent at 90° over the lip of the dashboard. I don't know what temperature such pens melt at, but clearly it gets a lot hotter than ambient under common enough conditions in a enclosed box.
If you left some cheap piece of consumer electronics on the dashboard in the sun and it didn't work, you'd probably be a little irritated, then toss it and replace it. If the controller for your oil pump stopped working in the summer because it got too hot, you'd lose a lot of money, be pretty upset, and probably buy the replacement from a different company that takes quality more seriously. If your missile defense system stopped working because you deployed it in the desert of Iraq instead of some nice comfy test range in Massachusetts where it was developed, you'd be dead. The procurement officers that don't get fired will be extra careful to require all electronics to work at high temperature, and insist it get tested under those conditions.