Four years ago I got a folding Bluetooth keyboard for my then-newish Pixel phone:
A few days ago, the 2 W S X Win keys stopped working, suggesting a problem with the matrix scan of that column.
The trim cover over the fold on the back of the keyboard disengages from the hinge with gentle prying at the obvious places, exposing a flex cable pressed against a disturbingly right-angled edge:
Unfolding the keyboard makes the acute bend against the case obvious, even though it’s hidden under the cable:
Some tedious poking around with a continuity meter revealed not only a broken trace, but a crack in the flex cable:
Protip: when you have nothing to lose, poke a pin through the flex cable into the trace to localize the break. The point leaves little holes, but so what?
I scraped off the black coating and the insulation over the traces with an Xacto knife under the microscope, which definitely reveals my need for a tiny Waldo manipulator.
Coating the exposed copper with solder and bridging the crack with one strand of the finest wire in my collection produced a truly horrific scene:
The glop on the left is flux applied before soldering. The rugged terrain on the right is the exceedingly gummy adhesive holding the cable to the keyboard, which turned out to be surprisingly heat-sensitive.
Fairly obviously, those patches will not survive much more flexing, so wrap the cable with Kapton tape and apply a stiffening layer of thick plastic tape:
Apply more reinforcing tape and button it up again:
I stuck the flex cable down with the repaired joint about a millimeter under that sharp edge, with double-sided sticky tape underneath to help immobilize the bruised area.
While I had the covers off, I also reinforced the same section of the cable on the other side of the keyboard, in the hopes of preventing a crack.
I have little faith in the long-term survival of this repair. Similar keyboards routinely emerge from the quantum froth of randomly named Amazon sellers, most of which have negative reviews reporting the failure of entire key columns; there’s no indication of any design improvement.
The alert reader will have noted the cable has eight traces, enough for a 3×5 matrix of 15 keys, but the folding wing has 16 keys: the second row has four keys. I have no idea how they made that work, other than perhaps resistive coding for some of the keys.