I was fiddling around with a newly designed shield for Arduino and I just can not get the shield to work properly after a few tries. Then I grabed my DVM and measured power supply pin of one of the ICs on the shield. The shield is supposed to be powered by the 3.3V pin on Arduino. Superisingly, I found it was 5V!
Then I quickly disconnected the USB cable and pulled off the shield from Arduino. I measured the 3.3V and 5V pins on the shield. There was no short. Then I measured the 3.3V and 5V pins on the Arduino. There was no short either!!!
I powered the Arduino alone and measured the 3.3V pins, and it gives a good value of 3.35V.
I realized that there may be something wrong with the 6-pin female header of Arduino, and then I tear off the housing of the header and I found there is a weakly contacted solder bridge as shown below:
It is a quite old device, I bought it online back in year 2011 or 2010. I had been using this device for years and the symptom shows up today! The problem only happens after a shield is applied to the Arduino.
It seems that the soldering bridge is due to excess solder, excess diameter of the through hole and the unsealed buttom of the header.
This is just another story of how bad soldering affects the long term reliability of an electronics device.