UV Adhesive Bonding, Potting & Encapsulation
Structural bonds between glass, metal, and engineering plastics rarely tolerate the clamps-and-oven cycle of a heat-cure epoxy. Fixtures tie up work-in-progress for hours, dissimilar-material joints build stress as they cool, and throughput is capped by rack space. UV-curable adhesives replace that cycle with a timed exposure measured in seconds: the part is fixtured, the light comes on, and the joint is at handling strength before the operator reaches for the next assembly.
The catch is that light has to reach the adhesive. Getting bonding, potting, and encapsulation right is mostly a matter of designing the joint, the dam, and the illumination so that it does, and knowing what to do about the adhesive it can't reach. This guide covers those fundamentals and the equipment choices that follow from them.
Joint design basics: the light is part of the joint
A UV cure is line-of-sight, so a bondable joint is one the light can see into. That drives a few design rules:
- Put a transmissive member in the path. Glass passes UVA well, and many clear plastics transmit long-wave UV even when they filter shorter wavelengths. A lap joint with one transparent substrate cures through that substrate; an opaque-to-opaque butt joint must be reached from an exposed fillet or edge.
- Expose the fillet. Where light enters at the bond-line edge, design the fillet geometry so a light-guide tip has a clear view of it. Shadowed adhesive stays wet, angling two guide poles at opposing 45° positions reaches around vertical walls.
- Control the gap. A consistent bond-line gap gives a consistent cure and consistent strength. Shims, molded standoffs, or glass spacer beads all work; what matters is repeatability.
- Prepare the surface. Solvent-wipe both substrates, and abrade or plasma-treat low-surface-energy plastics for reliable adhesion. No cure schedule rescues a contaminated surface.
Dam-and-fill encapsulation
Encapsulating a sensor, bare die, or wire-bonded component is a two-material job. First dispense the dam, a high-viscosity bead around the perimeter, and gel it with a short exposure so it holds its shape. Then dispense the fill, a lower-viscosity encapsulant that flows over the component and self-levels inside the dam, and cure it through with a longer timed exposure. The dam sets the boundary in seconds instead of waiting on a thixotropic material to hold position through an oven cycle, and the whole encapsulation is done in one fixture at one station.
The practical constraint is depth: the fill has to cure through millimeters of material without cooking the component underneath. That's where source selection matters most.
Depth of cure: what actually limits it
As light enters the adhesive, the photoinitiator absorbs it, that's the mechanism of cure, but it also means intensity falls with depth. Three levers govern how deep a single exposure reaches:
- Spectrum. Longer wavelengths are absorbed less strongly near the surface and penetrate deeper, while shorter UVB content finishes the oxygen-inhibited surface layer that otherwise stays tacky. A broad-spectrum source does both in one exposure.
- Intensity and time. Dose at depth scales with what you deliver at the surface. High irradiance at the guide tip buys cure depth and shorter cycle times.
- The formulation itself. Fillers and pigments scatter and absorb light; heavily filled or opaque encapsulants cure shallower than water-clear ones. The adhesive datasheet's cure-depth guidance is the starting spec.
In practice, plan on curing to roughly 5 mm per exposure with a strong broad-spectrum source. For deeper pots, fill and cure in two passes, and verify the dose at depth with a radiometer during qualification rather than assuming it.
This is the job the SunSpot 2 was built for. Its 200 W arc delivers more than 18,000 mW/cm² of UVA plus UVB and visible content across a 275–650 nm spectrum, penetrating thick encapsulant sections and fully curing surface-inhibited and dual-cure chemistries that single-wavelength LEDs can leave tacky.
Dual-cure chemistries: the answer to shadows
Some adhesive will always end up where light can't follow, under a component, inside a blind bore, behind an opaque wall. Redesigning the joint is the first option; when it isn't available, specify a dual-cure adhesive. These formulations cure by UV wherever light reaches, then finish the shadowed regions by a secondary mechanism, typically moisture or heat. The UV exposure still does the production work, fixing the part in seconds and skinning every exposed surface; the secondary cure completes the hidden volume over the following hours without a dedicated process step.
Note that many dual-cure and older broadband formulations expect spectral content beyond a single LED line, another reason the broad 275–650 nm arc output is the safe default for bonding and potting work.
Our pick for bonding, potting, and encapsulation: the SunSpot 2 200 W arc system ($4,220). Broad 275–650 nm spectrum for depth, surface finish, and dual-cure chemistries; more than 18,000 mW/cm² UVA at the tip; integral timer and shutter for repeatable exposures; and one lamp splits across up to four light-guide poles (UV2168 dual-pole or UV2587 four-pole splitter) so a full dam perimeter cures in a single timed exposure. Request a quote with your adhesive and part geometry and we'll configure the guides and spares.
For guide selection: the UV0570 (1 m × 5 mm) covers general bond points, the UV0571 adds reach at 1.5 m, and the UV0774 high-flux guide puts an 8 mm spot on larger fillets and fill areas. Keep a spare UV1866 lamp module, a quick-change 200 W cartridge rated for more than 2,000 hours, on the shelf so a production cell never waits on lamp end-of-life.
Common questions
How deep can a UV encapsulant cure in one exposure?
With a high-intensity broad-spectrum source and a clear formulation, plan on roughly 5 mm per pass. Filled or pigmented materials cure shallower. For deeper pots, fill and cure in two passes or specify a dual-cure chemistry, and confirm cure at depth during your own qualification.
Why is my cured surface still tacky?
Oxygen at the surface inhibits the polymerization of many acrylates, leaving a wet-feeling skin. Shorter-wavelength UVB content, higher dose, or an inert-gas blanket all defeat it. The SunSpot 2's spectrum includes that UVB content, which is why broad-spectrum arc systems are the standard cure for tack-free finishes on these chemistries.
What if part of my joint is permanently shadowed?
First try illumination: two poles at opposing angles reach around most vertical features. If geometry truly blocks the light, use a dual-cure (UV plus moisture or heat) adhesive so the shadowed volume finishes on its own after the UV fixture step.
LED or arc lamp for this work?
For deep sections, surface-inhibited chemistries, and dual-cure formulations, the broad-spectrum arc is the safer choice. If your adhesive is specified for a single LED wavelength and your sections are thin, a 365–405 nm LED system like the SkyBeam cures it with instant on/off and no lamp changes. The side-by-side comparison lays out the trade.
