A screen is a stack
When you look at a phone or monitor, it is easy to think of a single sheet of glass with a picture behind it. In reality, the front of a display is a stack of thin layers, and which ones are present depends on the product. A typical modern device may include, from front to back:
- A protective cover. Usually glass or a hard plastic. This is what you touch and what often gets scratched or cracked first.
- A touch layer, if the screen is touch-sensitive. This is a thin network of electrodes, as explained in the article on how touchscreens work. In some designs it is a separate layer; in others it is built into the display itself.
- Bonding material that holds layers together, which can be an adhesive layer or an air gap.
- The display panel. On an LCD this includes thin glass sheets with the liquid crystal layer, color filters, polarizing films and transistors, as described in the article on how liquid crystals control light. On an OLED it is a thin stack with organic emitters and sealing layers.
- Light-handling layers, in an LCD: diffusers, films and a light guide that spread backlight evenly. LCD backlights sit behind these.
- Drive electronics and connections: thin flexible cables and a controller that send signals to the panel.
Exact arrangements vary. Some designs fuse layers tightly and some leave gaps, and flexible screens use different materials, as covered in the discussion of flexible substrates.
Why a crack often goes deeper than the surface
The layers share the force. An impact travels through the stack. A crack in the cover glass shows where it broke, but the panel below may have absorbed some of the shock even when it has no visible crack. Display panels are often made of very thin glass, which can be damaged by pressure without visible fracture.
Cracks can reach the panel. When the break goes through the cover, the panel glass may also crack. On an LCD this can let liquid crystal material leak or be pushed aside, showing up as black or colored blotches that grow. Patches may spread over days as pressure and temperature change.
Electrical connections can break. Tiny conductive lines run across the panel and connect to the edges. A crack, even one that looks small, can cut those lines. The result is often a line or band of dead picture running across or along the screen, or a part of the display that stays black, white or colored. Sometimes lines appear after the impact, not immediately, because stresses continue to affect connections.
The touch layer can fail. If the electrodes are broken, you may find parts of the screen do not respond to touch, or the screen registers touches that did not happen (ghost touches). Understanding capacitive touch helps explain why: the screen senses the position of a finger through changes in an electric field across a grid, and any break in the grid disturbs that. Some people find that a screen works properly until the crack catches skin or until moisture gets in.
Sealing is compromised. On OLED screens, the emissive layers are sensitive to moisture and air, and thin sealing layers protect them. If these are breached, dark spots or lines may start to appear and spread over time. Water or moisture entering through a crack in an LCD can cause staining or corrosion of electrodes.
Layers can separate. Impact can cause bonded layers to peel apart slightly, leaving rainbow patterns, bubbles or cloudy areas.
Cosmetic crack versus functional damage
Not every crack means the display has failed. Several outcomes are possible:
- Cover glass only. The picture and touch work normally, but the surface is broken. This may be the case with a small, shallow crack at the edge.
- Picture partly affected. Lines, blotches or dim areas appear, which commonly signals damage below the cover.
- Touch affected. Parts of the surface are unresponsive or behave erratically.
- Total failure. The screen stays dark or shows only a few colors.
A crack that looks small can be accompanied by functional damage, and a dramatic web of cracks can sometimes leave the picture intact. You cannot always judge by appearance.
Safety and practical points
Broken glass can have sharp edges and lead to cuts. Small fragments can come loose, and a screen with exposed edges is not good to press with a bare finger. Many people cover a cracked screen with clear tape or a protector as a temporary measure to hold fragments in place, although this does not repair anything. A cracked display can also let in moisture and dust, so keeping it away from liquids is wise. If a cracked screen is on a device that holds a battery, such as a phone, and the device is swollen, hot or smells odd, it is a good idea to stop using it and ask a professional; battery damage can be a safety issue.
Continuing to use a cracked screen may allow damage to spread, since flexing and temperature changes keep stressing the broken area.
Repair
Replacing a cracked screen is often more involved than replacing a sheet of glass. In many products the cover, touch layer and panel are bonded together into one unit, so the repair means replacing the whole assembly. Separating just the cover glass is sometimes possible but demanding. Adhesives, thin flexible cables and tight tolerances make it easy to cause more damage. The article on why screens are hard to repair covers the reasons, and the screen materials article describes what goes into the layers.
Costs can be significant relative to the price of the device, so it is sensible to get a quote from a repairer or the maker, and to check any warranty or insurance. Repairs on safety-related equipment, such as vehicle or medical displays, belong with qualified professionals following the maker’s requirements.
Prevention
Prevention is largely about reducing impact and pressure: using a case or protective cover, avoiding putting heavy objects on a screen or in bags with sharp items, and handling large panels with care. Protective films and tempered glass covers can absorb scratches and some impacts, but they do not make a screen unbreakable. Screens used in rough environments are often built differently, as with the rugged displays described elsewhere on this site, and are protected by thicker covers, bonding or enclosures.