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)?

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  • \$\begingroup\$ What kind of 12V power supply you used to power the device (earthed, unearthed, floating)? Is the device connected to any other equipment, or just to the power supply and the soldering iron? Not to PC or any other equipment? \$\endgroup\$ – Justme May 22 at 11:19
  • \$\begingroup\$ 12V is earthed power supply. Not connected to anything else apart from the circuit board. The soldering station is also earthed \$\endgroup\$ – Newbie May 22 at 11:46

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: -

enter image description here

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  • \$\begingroup\$ So, the Microcontroller is to be replaced for proper operation? The other of the ADC pin is connected to this IC - st.com/resource/en/datasheet/vn7140aj.pdf - pin 4 ? Will this IC also get damaged due to this? \$\endgroup\$ – Newbie May 22 at 10:51
  • \$\begingroup\$ Replace the microcontroller is my advice. \$\endgroup\$ – Andy aka May 22 at 10:52
  • \$\begingroup\$ ok. But actually, I am still not able to understand how you found that it is an ESD event? I actually thought that the heat may have caused the internal capacitor and diodes circuit to fail. Could you please explain a little more on how it is an ESD event \$\endgroup\$ – Newbie May 22 at 10:55
  • \$\begingroup\$ Yes, the circuit is unearthed. But the soldering station is earthed. How did I earth the single pin when I touched the soldering iron? Not able to get clarity on this. Please help \$\endgroup\$ – Newbie May 22 at 10:57
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    \$\begingroup\$ You can't assume that the damage hasn't caused neighboring parts of the silicon to also become damaged. Excess current could be due to the old CMOS ESD-latch-up problem (worth googling) or just a general degradation of components around the area where the event hit. You probably won't be able to use the pin's alternate functionality but there's nothing wrong in trying it. However, I'd just take the chip and throw it in the trash and fit a new one. \$\endgroup\$ – Andy aka May 22 at 16:57

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.


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".

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  • \$\begingroup\$ Could you please explain what you mean by "4 of the 6 faces insulated" and how you arrived at 9600 seconds initially \$\endgroup\$ – Newbie May 22 at 14:14
  • \$\begingroup\$ Thank you for the explanation \$\endgroup\$ – Newbie May 23 at 4:49

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