tldr: 1 half of my split keyboard types certain characters on its own without any input, the same characters don't work on actual key press after I bridged the oled jumper terminals with solder.
My split Lily58 keyboard was working great until recently i decided to add oled displays to it. When I was building the keyboard initially I didn't bridge the oled jumper terminals on the 2 pcbs so I had to do it now. I applied generous amount of liquid flux(Griffon S-39) to the terminals and bridged them with solder.
When I plug in the keyboard now, the left half works perfectly fine. While the right half has issues. Some keys (h, n,m in qwerty layout) don't work at all. Pressing other keys(j) results in multiple (unpressed)characters being typed along with the pressed key. Sometimes a character from the dysfunctional(h,n,m) set of keys will be typed over and over again without any input from me. I didn't do any soldering on the rest of the pcb.
Things I tried so far:
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Cleaning the surface around the jumper terminals thoroughly with water and cotton swabs to get rid of any excessive flux that could be causing shorts. The oled jumpers are right next to the microcontrolelr pins on the pcb (see here: https://user-images.githubusercontent.com/6285554/53293031-d45c6280-380f-11e9-8f1c-1c167b27cfd3.jpg) .
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Leaving the dysfunctional half dry for 2 days in case there was some water causing the issues. - Blow air through the dysfunctional switches and the pcb holes to clean the dust
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Desoldering the bridges on the dysfunctional half of the keyboard in order to revert to the initial state and remove any potential shorts caused by bridging
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Tried the dysfunctional half with a different microcontroller with no effect
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Tried replacing the switches - Tried triggering the dysfunctional keys with a metal strip on the pcb directly with no effect
Generally speaking, avoid water. Use 90% isopropyl alcohol or higher.
Without pictures, best guess is that the flux caused a bridge, and the subsequent water caused another issue.