Hello, i faced this trouble twice in a week with two different iphone7 with very badly damaged screen (and one with water damaged). They have been previously opened so i don t know if i did the mistake at least for one but for the water damaged the trouble was already present: the flex cable from the button (not from the touch id) is cut near the grounding pad
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as a capacitive sensor the button sensor is driven using specific pulse to sense the capacity change due to finger: with the track cut… it sense using the second half of flex board. Most of such circuit have a automatic calibration mode and without real sensor area, the button become crazzy. I will try two have my button repair in microsoldering surgery…