Recently one of my mirocontrollers fried in a project on a perf board. Eight of the outputs are PWM, and aside from the power lines, there are no other outputs from the MCU. The PWM frequency is 40 kHz. All these output lines are routed to a connector with long, beefy solder tracks under the board. After going through the connector, each line meets a 2.2k resistor (series) and drives some transistor circuitry.
Two of the PWM channels worked on their own for quite a while before connecting the other six, so I first thought my troubles could be the result of continuous overcurrent. The MCU is rated for 200 mA max, but 5 V/2.2 kohms * eight channels only comes out to 18 mA.
I am inclined to say that the fast switching frequency combined with the beefy solder tracks pushed too much current in and out of the parasitic capacitance before the series resistors.
How much parasitic capacitance can lead solder introduce? I realize I could put the resistors closer to the MCU BEFORE the long tracks, but there is always a degree of parasitic capacitance in everything.
I would like to know if the following reasoning is correct:
If I solder a wire to a large copper ball (say apple-sized) and then plug the end of the wire onto my MCU PWM output pin, I will fry the unit as high current quickly rushes in and out of the ball's parasitic capacitance. If I did the same thing with just a bare wire, the capacitance would not be great enough to fry the controller. I have done several times in projects with no issues.
If the reasoning is correct (I would not like to test it :)) and all capacitors can draw huge momentary current, why does the ball fry the MCU and the wire doesn't?
Is there a calculation to help me understand this better?