UV glue: how it works, and can it re-laminate a phone screen, fix traces under a BGA or a windscreen chip

In short

How UV glue works

The bottle holds a mixture of liquid molecules (usually acrylates) and a small amount of a photoinitiator. Without light of the right wavelength nothing happens: the glue can sit on the part for an hour, and you can adjust it, wipe it off and apply it again. That is exactly why people like it.

When ultraviolet reaches the glue, the photoinitiator absorbs it and splits into radicals, fragments of molecules with an unpaired electron. A radical grabs a neighbouring acrylate molecule, that one grabs the next, and long chains grow in a fraction of a second. The chains cross-link into a network and the liquid turns solid. The light does not "dry" the glue: there is no solvent to evaporate. It is a chemical reaction that the light merely starts.

Why ultraviolet and not ordinary light

Light comes in packets called photons, and the shorter the wavelength, the more energy each photon carries. The photoinitiator only splits when it absorbs a photon with enough energy. It simply doesn't absorb the red or green light of a room lamp, so no amount of it will start the reaction. The opaque bottles are there for a different reason: sunlight through a window contains ultraviolet and will slowly cure it.

Hence the practical rule: match the torch to the glue. The bottle or the description usually states the wavelength it needs: 365 nm, 395 nm, 405 nm or a range. A glue made for 365 nm will cure slowly or incompletely under a 405 nm torch.

Why the glue sometimes doesn't set

Kinds of light-curing glues and resins

Wavelength: 365, 395, 405 nm and what glass lets through

Ultraviolet is divided into three bands. UV-A (315–400 nm) is the softest, and most glues and torches work in it. UV-B (280–315 nm) causes tanning and sunburn. UV-C (100–280 nm) from the sun doesn't reach the ground at all, because the ozone layer stops it; it is produced artificially by germicidal ("quartz") lamps.

The important part at the bottom: ordinary glass lets 365–395 nm ultraviolet through, but a car windscreen barely does. A PVB film lies between the two layers of windscreen glass and absorbs nearly all ultraviolet. Many plastics with a UV stabiliser behave the same way: clear to the eye, but no ultraviolet reaches the glue through them.

Choosing a torch for glue:

A filtered 365 nm torch suits a wider range of glues but costs more. A cheap "395" has no filter, so part of its light is already visible violet.

Can you re-glue the glass of a phone or tablet screen

First find out how your screen is built. The touch sensor can sit in one of three places.

Separate touch panel (many tablets, older and budget phones). The touch layer is on the back of the cover glass, so glass and sensor are one part. If the glass cracks, the whole touch panel is replaced and stuck to the housing frame or the display with double-sided tape around the edge, leaving an air gap. UV glue isn't needed here: don't fill the gap with glue, because it will seep under the frame and into the backlight.

Fully laminated LCD. The glass is bonded to the display with a continuous layer of clear adhesive, either OCA film or liquid LOCA. If only the glass is cracked while the picture and touch work, repair shops do what is called refurbishing: separate the glass, remove the old adhesive and bond new glass with LOCA. You can do this at home too, but it is risky:

1. the glass is separated by heating the module to 80–100 °C and cutting through the adhesive with a thin wire; without heat the glass cracks further and the panel may break;
2. the old adhesive must be removed completely, because any speck will leave a bubble;
3. the edges are sealed first: a bead of LOCA around the perimeter is tacked with a few seconds of light to form a dam;
4. LOCA is applied, the glass is laid on, the glue is allowed to spread without bubbles, and only then is the whole area cured.

⚠ The main risk is glue seeping past the edge of the panel into the backlight. The LCD's films and light guide soak it up, and the screen gets stains that can never be removed. So use just enough glue that none escapes, and keep the edges behind the dam.

OLED. The glass is bonded to a thin flexible panel, and the sensor is often deposited right on it. Separating the glass without breaking the panel is only possible with special equipment: separators, freezing tables and very thin wire. At home the chance of ruining an expensive panel is very high.

And gluing the screen to the frame?

UV glue won't do: the frame is opaque and the ultraviolet can't reach the glue under it. Screens are fixed to the housing with double-sided tape cut to the shape of the frame, or with glues that cure in air (repairers commonly use clear gel-like glues in tubes with a fine nozzle, such as B-7000). In phone repair, UV glue is used specifically for laminating the glass to the display, where the light passes through the glass.

Ripped-out traces and pads under a BGA

When a BGA chip is removed carelessly, a copper pad can come off with the solder ball, sometimes with a piece of trace. Glue alone won't fix this: no UV glue conducts electricity. Only copper restores the electrical contact; UV solder mask is a helper here.

How it's done:

1. Find out whether the pad is connected at all. Some pads under a BGA lead nowhere and only hold the chip. If such a pad is ripped out, you can leave it. The chip's documentation or the board schematic shows what leads where.
2. Find the end of the trace. From where the pad was, the trace runs under the mask to a via. Carefully scrape the mask off the tip of the trace with a scalpel to expose the copper.
3. New pad and jumper. Make a pad from thin copper foil or the flattened end of a copper wire and solder a jumper to the exposed trace. Ready-made adhesive-backed copper pads are also sold.
4. UV mask. Apply a thin layer of mask over the jumper and the edges of the new pad and cure it with a 365–395 nm torch (usually from several tens of seconds to a few minutes, depending on the mask and the torch). Keep the top of the pad clear: either don't put mask there or carefully scrape it off after curing.
5. Check. With a multimeter, check continuity from the new pad to the via and make sure there is no short to the neighbouring pads. Only then reball and fit the chip.

⚠ The mask must be thin and must not rise above the pads: the gap between a BGA chip and the board is a fraction of a millimetre, and a bump of mask will stop the balls from seating.

A real case with such pads on a memory module is described in the article about the Corsair Vengeance DDR5 module.

A crack or chip in the windscreen

A windscreen is a sandwich: two layers of glass about 2 mm each with a PVB film of about 0.76 mm between them. A stone usually damages only the outer layer, and that is what gets repaired with resin that cures under ultraviolet.

How the repair is done:

1. The sooner the better. A fresh chip has no dirt or water in it yet, so the resin fills it better and the mark is less visible. Until the repair, cover the chip with clear tape.
2. Cleaning and drying. Water and dirt in the cracks get in the resin's way. The glass must be dry and not hot, because a temperature shock can make the crack run further.
3. Injector. A bridge on suction cups is placed on the glass with a resin cylinder above the chip. Vacuum (pulls air out of the cracks) and pressure (pushes resin in) are alternated several times.
4. Curing. The injector is removed, one more drop of resin goes on the chip, it is covered with clear film and cured with a UV lamp for a few minutes, or the car is left in the sun. The film keeps oxygen from leaving a tacky layer on top.
5. Finishing. The cured excess is shaved flush with a razor blade and polished if needed.

The chip doesn't disappear completely after the repair: a faint mark usually remains. The main thing is that the crack usually stops spreading from frost or vibration.

When a repair isn't suitable:

• damage larger than a coin, or a crack longer than a few centimetres (chips up to 2–2.5 cm and cracks up to 15 cm are typically repaired; DIY kits are mostly meant for chips only);
• a crack starting at the edge of the glass: the edge is under stress and the crack will keep going;
• the inner glass layer or the film is damaged;
• a chip right in the driver's line of sight: even repaired, it distorts the view. In many countries inspection rules and insurers don't allow repairs in this zone, so check how it works where you live.

To stop a long crack spreading, technicians drill its end during the repair: a hole of about 1 mm made with a diamond bit, in the outer layer only.

What the dentist does with that "lamp"

To place a filling, the dentist applies a paste called composite in layers and cures each layer with a blue lamp. That isn't ultraviolet: in modern composites the photoinitiator (most often camphorquinone) absorbs blue light around 460 nm best. The lamp is an LED and very bright, and each layer is usually cured for 10 to 40 seconds.

Why in layers: blue light penetrates the composite only a few millimetres, so a normal layer is up to 2 mm. Some composites can be placed in layers up to 4–5 mm.

Why not ultraviolet: the first such filling systems of the 1970s really were cured with ultraviolet, but it penetrated the material only shallowly, and eyes had to be protected from it. Blue light turned out to be safer and penetrates deeper. Some lamps also add a violet band around 405 nm for composites with other photoinitiators.

The orange shield on the lamp or the dentist's orange glasses protect the eyes: bright blue light up close harms the retina.

Where else ultraviolet is used for glues and resins

Is it safe to look at a UV torch

No. And the main trap is that ultraviolet looks dim. The eye barely sees 365 nm, so the pupil doesn't constrict and you don't squint as you would at a bright lamp, yet the energy still enters the eye.

395 and 405 nm torches fall right on the border of the last two rows: part of their light is absorbed by the lens, part reaches the retina. A brief accidental glance at a small torch is usually harmless, but don't look straight into one, up close or for long, especially a powerful one.

Can a UV torch damage a phone camera

A typical glue-curing torch won't. A phone lens is made of several elements with coatings, and there is a filter in front of the sensor as well. Between them they block 365 nm ultraviolet almost completely. Light at 395–405 nm is already on the edge of visible violet, and the camera records it like any bright light. But that is not enough power for the sensor: a few-watt LED is far too diffuse to heat or "burn" it the way a laser can.

What you will see in photos and videos:

• the torch spot comes out violet-blue and often overexposed: the camera renders 395–405 nm light differently from how the eye sees it;
• autofocus and white balance can get confused, so it helps to add some ordinary light;
• the glow under ultraviolet (white paper, stains, glue if it has a fluorescent tracer) shows up well on camera, which is a handy way to check that the glue went on evenly.

What not to do, though: holding a powerful torch right against the lens for minutes. Plastic lens elements could in theory yellow over the years under long, strong ultraviolet, like any plastic left in the sun. A quick photo or clip does no harm.

There is a useful side effect too: watching the curing on the phone screen is safer than looking with your eyes. Your eyes then see the screen, not the ultraviolet, as long as the shield still blocks the spot itself.

Protecting eyes and skin while working

How to use a torch on a board without harming your eyes

1. Everything ready first, then the light. Apply the glue, position the part, check it, and only then pick up the torch. While the glue is liquid you can adjust as much as you like. Rushing with the torch already on is exactly what makes you look at it.
2. Torch in a holder, head down. A "helping hands" clamp will do. The beam goes into the board rather than sideways, and your hands are free.
3. Aim it while it's off. Set the torch 1–3 cm above the drop and only then switch it on. The closer it is, the faster the glue cures and the less light scatters around.
4. Switch on, look away. Don't look at the spot even with glasses on: glasses reduce the dose, they don't make looking safe. To see what's happening, put an orange UV-blocking plastic shield between the spot and your eyes (you can look through it), or watch it on your phone screen through the camera.
5. A timer, not guesswork. Cure for as long as the glue or mask requires (usually from 10–30 seconds to a few minutes). Run a countdown on your phone so you don't keep glancing to check.
6. Check with it off. Test whether the glue has set with a toothpick or tweezers after switching off.
7. Shiny things reflect. Fresh solder, copper, metal shields and glass reflect ultraviolet sideways, towards where your face may be. So the shield goes on the side where you sit.
8. Never under a microscope or magnifier. A microscope or magnifier gathers light from the whole lens and sends it straight into your eye. Apply the glue under the microscope, but cure it with your eyes away from the eyepieces. A microscope with a camera and a screen is a different matter: looking at the screen is fine.
9. The torch gets hot. Powerful UV LEDs heat both themselves and whatever they shine on up close. Don't cure small plastic parts and connectors for minutes at point-blank range.
10. Afterwards, into the drawer. A UV torch looks like an ordinary one, and children or guests can easily shine it into their own eyes.

Can a UV torch be used as party lighting

The effect is familiar: under "black light" white clothes, teeth, paper, fluorescent paints and posters glow brightly. It isn't magic: these substances absorb ultraviolet and emit visible light. White fabrics and paper contain optical brighteners for exactly that.

The answer depends on the source:

• a weak 395–405 nm LED strip on the ceiling or behind furniture, away from the eyes, is quite tolerable for a party. But it gives off a lot of visible violet light that tires the eyes quickly and makes things look blurred;
• fluorescent "black light" tubes (365–370 nm, dark violet glass) are what clubs use. They are meant to light a room, not to be stared at up close;
• a powerful torch for glue is no good as party lighting: it shines a narrow, strong beam, and sooner or later someone will look into it;
• ⚠ a germicidal ("quartz") lamp, never. Its 254 nm ultraviolet burns eyes and skin within minutes, and some lamps also produce ozone.

Keep in mind too that children like to look at "magic" light up close, and their eyes let more ultraviolet through to the retina. Constant ultraviolet also fades fabrics, paper and paint.

A UV torch and a "quartz" lamp are not the same thing

The UV disinfection done in hospitals and schools (known in Eastern Europe as "quartzing") uses exactly these germicidal lamps, and people leave the room while it runs. Such a lamp would cure glue only on the very surface: 254 nm is absorbed in the thinnest top layer and doesn't penetrate, and working next to it is dangerous. For glue, use only a 365–405 nm torch or lamp.

There are also "ozone-free" germicidal lamps: their doped glass blocks 185 nm, so there is no ozone, but 254 nm remains and is just as dangerous for the eyes.

How to remove UV glue

• Liquid: wipe it off with a tissue dampened with isopropyl alcohol before curing. Glue left in shadow doesn't set either and is wiped off the same way.
• Cured: mechanically, with a blade or a scraper. Many acrylate glues soften when heated (hot air gun, 100–150 °C) and come off more easily. Glass bonded with LOCA is also separated with heat, but gentler: up to 80–100 °C, as in the screen section.
• Solvents such as acetone only make cured glue swell, after which it is easier to scrape off. But acetone damages many plastics and polarisers, so it isn't used near screens.