Arduino Overheating?

The ATMEL datasheet gives the details:

If you are using the ATMEGA328PB (mind the "B"!) the total current is only 100 mA:

When I do the math I get the result that I may connect six LEDs each with a resistor of 47 ohms. And that is what I did - one after the other. And continuously I checked the internal temperature which can be read using channel 8 of the ADC, and displayed it on the Serial Plotter of the old IDE, showing 500 samples, and a TFT display showing 160 samples.

You can see that the temperature rises each time I add one LED, and drops when I switch all of them off. The Serial Plotter performs autoscaling, the figures shown on the y-axis probably do not match the exact temperature as calibrating of the internal temperatur sensor takes long time and has to be repeated for each and every single chip.

This time, at the beginning of peek #3 I covered the Atmel chip using my finger-tip, so it could no longer radiate its heat to the surrounding air. Obviously, the internal temperature was rising even more.

The same can be seen in the TFT display. The tickmarks indicate the times when another LED was added.

So driving six LEDs with their maximum current can be regarded as safe.