UV LED spot curing

Spot curing concentrates UV light on a small, well-defined area, typically a few millimetres across. It cures a dispensed dot or bead of adhesive exactly where it sits, in seconds, while the rest of the part stays largely unexposed. It is the standard technique for bonding small parts in electronics, optics, fibre optics and medical device assembly.

How it works

A UV LED spot system has three parts:

• The LED head: a small, pen-shaped housing with one high-power UV LED, a heat sink and a lens.
• The cable: connects the head to the controller, so the head can be mounted on a fixture, a robot or a dispensing station.
• The controller: drives each head with its own intensity and exposure time, and connects to the line through I/O signals or a serial interface.

The adhesive is dispensed, the parts are positioned, and the controller switches the LED on for a set time. Because the LED is at full output immediately and switches off without delay, the exposure is defined by the controller alone, with no warm-up and no shutter.

Before UV LEDs, spot curing used a mercury lamp and a flexible light guide. LED heads replace that with a source that runs for tens of thousands of hours, emits no infrared and contains no mercury. Lamp-based UV Lamp Spot systems remain useful for adhesives that need a broad spectrum.

Optics and working distance

The lens shapes the light into a spot of a given diameter at a given distance from the head. Small spots concentrate the LED's output and reach the highest irradiance; large spots spread it over more area and reach less. Each lens is specified at a nominal working distance, measured from the head to the part.

The table shows the trade-off clearly: the 2 mm lens reaches 18,000 mW/cm², the 15 mm lens 650 mW/cm². Choose the smallest spot that still covers the whole adhesive bead with some margin for positioning tolerance.

Moving the head away from its nominal distance changes both the peak and the shape of the spot. The irradiance profiles below show this for six lenses: at the nominal distance the profile is narrow and high; further away it becomes lower and wider. Fix the head in a holder at a repeatable distance, and re-measure if the fixture changes.

Besides the standard round spot, special lenses produce a linear spot for short bond lines or emit at a right angle for heads that have to be mounted parallel to the part.

Irradiance and dose

Two numbers describe the exposure:

• Irradiance (mW/cm²) is the UV power arriving per unit area at the adhesive surface. It depends on the LED, the lens, the working distance and the intensity setting.
• Dose (mJ/cm²) is the energy delivered per unit area: irradiance multiplied by exposure time. 1,000 mW/cm² for 3 s gives 3,000 mJ/cm².

Adhesive data sheets usually state a required dose and often a minimum irradiance. Both matter. Below a certain irradiance, oxygen at the surface can stop the cure, and a longer exposure does not fully compensate. Above it, a higher irradiance shortens the exposure time for the same dose. Very high irradiance can also cure the adhesive so quickly that shrinkage stress builds up, which is why step or pulse irradiation is useful for precision bonds.

Measure irradiance at the working position with a UV Radiometer when you set up the process, and repeat the measurement at regular intervals.

Wavelength

Spot heads are available at 365, 385, 395, 405 and 450 nm. The right choice is set by the adhesive:

• 365 nm suits most general-purpose UV adhesives and many formulations originally designed for mercury lamps.
• 385 to 405 nm suits LED-optimised adhesives, curing through plastics such as polycarbonate that absorb shorter wavelengths, and deeper cure in thicker beads.
• 450 nm (visible blue) suits adhesives formulated for visible-light curing.

Use the wavelength recommended in the adhesive data sheet, and check the transmission of any layer the light passes through.

Typical uses

• Bonding lenses, prisms and camera modules in Optics
• Fixing coils, wires and components, and sealing housings in Electronics
• Tacking fibres, lenses and fibre arrays in Optical Fibre Communication
• Bonding tubes, connectors and needles in Medical devices

Limits

• Small area. A spot covers a few millimetres to about 15 mm. Larger bonds or many parts at once are better served by linear or flood curing.
• Line of sight. UV only cures where it reaches. Adhesive under an opaque part or in a deep gap stays liquid unless it has a secondary cure mechanism.
• Positioning. Spot size and irradiance depend on the head's position, so heads need rigid, repeatable mounting.
• Heat at high power. The LEDs emit no infrared, but absorbed UV still warms the part. At very high irradiance and long exposure, check the part temperature.

How to specify a spot system

1. Number of curing points per part or station, which sets the number of heads and channels.
2. Spot size and working distance, from the bead size and the space available in the fixture.
3. Wavelength, from the adhesive data sheet.
4. Required dose and minimum irradiance, which give the exposure time with the chosen lens.
5. Head form: standard length, long, with additional cooling fins, or with a right-angle lens.
6. Control: touch screen or buttons, PLC triggering, alarm outputs, RS232 and parameter lock.
7. Cable length from controller to head, and whether the head moves on a robot.

Awellcure spot curing systems

Awellcure UV LED Spot controllers drive four or eight heads, each with its own intensity, exposure time and mode, set in steps of 0.1 s. They can be triggered from external signals, output an alarm for a faulty head, lock their settings and log operating hours per head. Each head has a temperature sensor feeding back to the controller, which keeps irradiance within ±3 % at 80 % output. Heads come with a 2 m cable as standard, with 5 m or longer to order, and robot-grade cables are available for moving heads.