The digitiser, and why touch fails when the picture is perfect
The short answer
The digitiser is the touch layer, a grid of transparent electrodes between the glass and the display that senses your finger by the change it makes to a tiny electrical field. It is a separate component from the panel, which is why touch can fail completely on a screen that looks perfect.
Three layers people call one thing
What everybody calls the screen is at least three parts stacked and bonded together. On top is the cover glass, which is what cracks. Underneath is the digitiser, the touch layer. Underneath that is the display panel itself, the part that makes the picture.
On almost every current phone those three are laminated into one assembly and cannot be separated without destroying at least one of them. That is why a repair replaces all three even when only one has failed, and it is the single biggest reason a screen costs what it does.
Knowing they are separate still matters, because it explains symptoms that otherwise make no sense: a perfect picture with dead touch, or touch that works everywhere except one strip.
How it senses a finger
A capacitive digitiser is a grid of transparent conductive lines, usually indium tin oxide, running horizontally and vertically with a tiny voltage across them. Where the lines cross they hold a small electrical charge. A finger is conductive and slightly grounded, so putting one near a crossing point changes the charge there measurably.
The controller scans that grid many times a second, works out where the change is, and reports a coordinate. Multi-touch is the same thing happening at several crossings at once.
Two consequences follow. A glove or a dry, calloused fingertip conducts poorly, which is why touch sometimes gets unreliable in cold weather. And electrical noise from a poor-quality charger can look to the controller exactly like a finger, which is where ghost touch on charge comes from.
What a failing digitiser looks like
The last one is the one worth trying at home first. If touch misbehaves only on charge, swap the plug and the cable for known good ones before booking anything. Noise from a cheap charger produces exactly this and it costs nothing to rule out.
The others do not recover. A damaged digitiser does not heal and the affected area does not shrink.
- Touch dead across the whole screen while the picture is perfect
- A horizontal or vertical band that never registers anything
- Taps landing slightly to one side of where you touched
- Ghost touch, where the phone opens things nobody pressed
- Multi-touch failing while single taps still work, so typing goes wrong
- Touch that only fails while the phone is charging
Why a crack you can barely see can kill touch
The conductive lines are microns thick and printed across the whole panel. A crack does not have to be visible to sever some of them; it has to be in the wrong place. That is why a hairline fracture at one corner sometimes takes out a strip of touch two thirds of the way across the screen, and why a phone that looks shattered sometimes has perfect touch.
It also explains the phones that come in weeks after a drop with touch that has slowly got worse. The crack travels with everyday flexing and temperature changes, and it takes more lines with it as it goes.
What we check before quoting one
Whether the whole panel is dead or only part of it, because a strip points at the digitiser and a total failure could be the connector or the board.
Whether it fails on charge, on a specific cable, or all the time.
Whether the assembly has been replaced before, because a poorly seated connector after a rushed repair presents identically to a failed digitiser and costs a great deal less to fix.
Whether there is a swelling battery underneath lifting the assembly, which is a different and more urgent repair.
Common questions.
On almost every current phone, no. The glass, the digitiser and the display panel are laminated into one assembly, and separating them destroys at least one. On a handful of older LCD models they are separate layers and a glass-and-digitiser repair is possible.
Most likely the digitiser or its connector. Both sit behind glass that can be entirely undamaged. If it started after a drop it is usually the layer itself; if it started after a repair somewhere else it is often the connector, which is a much smaller job.
Electrical noise from a poor-quality charger looks to the touch controller like a finger. Try a different plug and cable first, because if that fixes it there is nothing to repair.
Usually, because the digitiser is part of the assembly being replaced. Tell us when it happens, though, since ghost touch only on charge points at the charger rather than the phone.
The damage does not heal and cracks travel with everyday flexing, so it tends to get worse rather than better. There is nothing to gain by waiting.
Where this leads.
Next in screens and displays.
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