Applied 370degC at the input of an ADC in a Microcontroller

Question

I am using this Microcontroller - 100 pin

I have configured pin 82 as an ADC input. ADC Reference voltage is 5V (provided from an LDO whose input voltage is 12V)

To test my ADC accuracy, I took a thin 45cm copper wire from that pin and started to measure.

But while the board was in functioning, by mistake I used the soldering iron at the other end of the wire taken from the ADC pin. The soldering iron was around 370degC for 3sec contact at the wire.

Noticing this immediately, I turned off the module power and restarted it again. Upon restarting, the power module started consuming an excess current of 80mA.

1. What would have happened at the ADC input pin due to this mistake?

2. And what would the same scenario if those pins were output (open drain or push pull)?

Answer 1

What would have happened at the ADC input pin due to this mistake?

It's likely that an ESD event occurred - maybe the circuit was unearthed and had attracted some charge (not a problem by itself but you have to know how to handle these events). The worst thing you can do is earth one single pin and, by the sounds of it, that is what happened when the soldering iron tip touched the wire connected to the pin.

And what would the same scenario if those pins were output (open drain or push pull)?

More than likely it would still damage the chip.

Even an unearthed soldering iron tip can be a huge problem - capacitive coupling to the tip from the AC supply can push a few mA into the chip's pin and cause damage. See this from the data sheet for the part you used: -

Answer 2

lets talk about thermal time constants.

A cubic meter of copper has 9,600 seconds thermal time constant, with 4 of the 6 faces insulated, and heat applied and removed from 2 opposite faces.

We can chop the cubic meter into 0.1 meter columns, and then into 0.1 meter cubes, and we learn the time constant has sped up by 100X, to 96 seconds.

Chop more, into 1cm cubes, and you now have 0.96 second time constant.

Your 45 cm wire (even assuming NO HEAT FLOW from the wire along that 45cm distance, will have approx. 0.96 * (45 * 45) = 2,000 seconds time constant.

Thus the misbehaving ADC will not be caused by soldering iron heating.

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4 of 6 faces ----- start with any face, exit any edge and continue in that direction until you return to the original face; now insulate the 4 faces you traveled thru.

time constant: compute the thermal resistance of one meter cube; compute the thermal storage of that 1 meter cube; multiply those 2 numbers.

The inverse of what you computed is also called "thermal diffusivity".