I have a small PCB that holds 6 LEDs (model, fV 3.2V, 30 mA), 2 parallel lines of 3 LEDs in series. The PCB has wires soldered onto it that go to a connector about 10cm down the line.
In an installation with several of those PCBs, I've experienced that after a while a few of them only light up half the LEDs.
In the proper setup those LEDs are driven with a LED PWM chip. Outside the setup, i test them with a 9V power supply.
What I now want to understand is the behavior I observe when debugging those faulty boards:
- I test connect 9V in connection A (at connector) and only half the LEDs work
- Then I test connect 9V at connection B (directly on the solder joints on the PCB) and now all LEDs work
- I go back to connect the 9V in connection A (at connector) and suddenly all LEDs work from here as well
There is no visible sign of a broken solder joint or lose wire connection. (Obviously, there could be, as the boards are handsoldered. Also, most connection points are covered with conformal coating and I am now suspicious if the coating puts stress on some of the solder joints)
What makes LEDs sometimes turn on and sometimes not? Does this behavior indicate that either one or more of the LEDs are actually damaged, or is it more likely that there's a lose connection somewhere?