I have a Pro Micro that PB1 and PB2 seems to be connected.
Long story short, I accidentally created a large pool of solder paste over the Pro Micro PCB. I cleaned it multiple times using isopropyl alcohol and a toothbrush.
Then I noticed PB1 and PB2 are connected. When I use my multimeter to beep when there is a short, and I place the probes on ports 15 (PB1) and 16 (PB2) it indicates a short. I inspected the IC's legs and there is no visible dirt/residue or solder bridge.
Actually I'm not sure if the board came with this defect, or if the solder paste caused this.
I'm thinking about leaving it soaked in isopropyl alcohol for many hours to be sure that all solder paste residue is removed. Do you think it can damage it?
I already bought a new Pro Micro to replace that one, but I'm asking for advice so maybe I can fix it.
I generally use hot water and a bit of soap. Scrub with a soft brush, rinse and put in front of a fan to dry. The flux I use is water soluble. This is the best for me, much better and faster than isopropyl. If you use Isopropyl be sure it is at least 90%. You can check the schematic to be sure they are not connected together. When using your OHM meter and taking the measurements swap the leads so you have read it both ways, you could simply be reading the protection diodes. Reversing the leads will show a large difference in readings.
I measured using the Ohm meter. On the lowest setting (200) it displays 2.1. Reading on the opposite way the same value is displayed.
Another thing is that I'm not sure if what I used is a solder paste or just flux. I think it is not a solder paste because it can't join parts on its own. It has an ember color, and it gets transparent and evaporates with the soldering iron's heat (just like those videos about flux on youtube).
I heard somewhere that flux is conductive, but another person told me that flux is not conductive. But one thing I'm sure is that when the Pro Micro was dirty with the flux/paste, it behaved randomly. And when I cleaned it up it worked as expected (except the PB1 and PB2 ports that seems to be connected).
This might be physically difficult to do, but if you could heat up one of the pins and lift it off the pad, then you could see whether the short is on the board or inside the chip. The danger of trying that is that you may damage the pad or the pin.
eric-kp:
I measured using the Ohm meter. On the lowest setting (200) it displays 2.1. Reading on the opposite way the same value is displayed.
Is this perchance, also the same reading you get when you simply short the multimeter leads together?
eric-kp:
I heard somewhere that flux is conductive, but another person told me that flux is not conductive. But one thing I'm sure is that when the Pro Micro was dirty with the flux/paste, it behaved randomly. And when I cleaned it up it worked as expected (except the PB1 and PB2 ports that seems to be connected).
Some fluxes may be conductive, especially if not brought to soldering temperature to evaporate and harden them. Such resistance will be lower than the impedances you may be connecting to the Arduino pins which are extremely high impedance as inputs so an otherwise unconnected input can easily be pulled to either state.
I tried reflowing the connections, and also cleaned it a few more times. I'm giving up fixing it, and I will save it for a future project that use just a few pins.
This might be physically difficult to do, but if you could heat up one of the pins and lift it off the pad, then you could see whether the short is on the board or inside the chip.
The IC has QFN packaging, and I would have to remove it from the PCB to see underneath. I probably can get it out, but I'm afraid I won't be able to put it back together, because there are no visible pads "sticking out" on the PCB.