UV Bonding, Potting and Encapsulation

In UV bonding, what matters is not how much radiation the source emits, but how much of it actually reaches the bondline. Between the lamp and the photoinitiator there is almost always a component in the way – glass, plastic, a coating – and that component filters the light. In electronics, sensor and optics manufacturing, and in e-mobility, a second variable comes into play: layer thickness. Potting compounds and encapsulants are rarely just a few micrometres thick; they range from fractions of a millimetre to several millimetres – and that is exactly where it is decided whether a batch cures through or stays liquid at the bottom. Designing a bonding or potting process therefore starts with two questions: which wavelengths does the joining part actually transmit, and how deep does that radiation penetrate the layer?

The market for UV-curing adhesives is estimated at around USD 3.2 billion for 2025 and is expected to grow to about USD 7.2 billion by 2035 (CAGR approx. 8.6 %); electronics applications already account for the largest single segment, at roughly 32.5 %. Growth in adhesives for electromobility specifically is expected to be markedly faster – one report puts the CAGR at around 40.5 % over its forecast period, driven by electrification and new material combinations in battery modules.

A second, regulation-driven trend is accelerating the shift from mercury vapour lamps to UV-LED systems: from 2027, the EU's implementation of the Minamata Convention will restrict certain mercury-containing UV lamps [5], further speeding up the conversion of existing plant to LED technology. For adhesive formulators, this means more systems tuned specifically to one of the four LED peaks – 365/385/395/405 nm – instead of the broad mercury spectrum. In parallel, UV/VIS-curing and dual-cure formulations are becoming more established in applications with partially transparent substrates and shadow areas, as typically found in densely populated electronics and e-mobility assemblies. A third trend: process monitoring is moving from spot-check radiometer readings to continuous inline capture that reports failures in real time instead of at the next quality check.

UV bonding, potting and encapsulation are used across industries wherever a joint needs to cure quickly, precisely and without thermally stressing the component:

• Optical lenses and prisms
• Camera modules
• Displays and touchscreens
• Sensors
• Connectors
• Printed circuit boards and electronic assemblies
• IC encapsulation
• Microsystems technology
• Medical device components
• Glass-to-metal and glass-to-plastic joints
• Microfluidics
• Potting of electronic components

UV- and light-curing adhesives consist of monomers or oligomers and a photoinitiator. When light of the right wavelength hits the photoinitiator, it breaks down into reactive species that trigger polymerisation. Two basic mechanisms are distinguished here [1]:

• Free-radical curing (mostly acrylates): The photoinitiator breaks down into free radicals that link monomers into long chains. These systems cure very quickly but are sensitive to oxygen.
• Cationic curing (mostly epoxies): The photoinitiator generates a photoacid that opens epoxy rings and starts crosslinking. This reaction keeps running after the lamp is switched off (dark cure) and is insensitive to oxygen – one reason cationic systems are preferred for electronic encapsulation and optical bonding.

A practically relevant effect in free-radical systems is oxygen inhibition: at the surface, atmospheric oxygen reacts with the radicals being formed and disrupts crosslinking, leaving a tacky residual layer. This is mitigated by high irradiance (often above 1,000 mW/cm²), a broader spectrum, longer exposure, or blanketing the surface with nitrogen or argon [2]. In a covered bondline or under a potting layer, this effect plays almost no role, since little oxygen diffuses in there anyway.

A coating is exposed to ambient air; there, oxygen at the surface is the main issue. A bondline, by contrast, is usually covered – it is irradiated through the component or from the side, with correspondingly less oxygen ingress. What matters more here is the spectral transmission of the component and the geometry of the joint. The underlying relationships between spectrum, dose rate and crosslinking are summarised under UV Curing & Photopolymerization.

Photoinitiators only absorb within a limited band. An initiator tuned to 365 nm is barely excited by a 405 nm source – even if that source has the higher optical power. The industrial UV-LED peaks at 365, 385, 395 and 405 nm are different chemical offerings, not interchangeable power classes [3]. Selection starts with the adhesive datasheet and ends with a measurement at the actual source. A common pitfall: a type designation states the nominal peak wavelength, not the spectral full width at half maximum (FWHM), manufacturing tolerances, or the shift with junction temperature. More on this under UV-LEDs for UVA, UVB and UVC.

Note: The table describes general tendencies from the technical literature. Which wavelength actually fits depends on the photoinitiator in use and the transmission of the joining part, and should be measured on the real assembly – not inferred from the nominal LED designation.

The transmission of the component decides which part of the spectrum even reaches the adhesive. Optically clear does not mean UV-transparent: many plastics that look water-clear in the visible range fall off steeply below roughly 380 nm; float glass is nearly opaque in the UVB range; and UV-blocking additives in housing materials exist for exactly that purpose. In practice, this means the transmission curve of the joining part and the absorption band of the initiator must overlap. Where that overlap is narrow, the options shift towards a longer wavelength, irradiating from the joint side, dual-cure systems that offer a second curing path via heat or moisture, and light-activated adhesives that keep reacting after exposure.

Small dots of adhesive (connectors, sensors, populated PCBs) suit a UV-LED spot source with high irradiance over a few mm². Longer seams or several parts cured at once suit area sources or modular line-array systems along a conveyor. The real design question is not spot versus area, but shadowing: housing edges, metal frames, opaque components and undercuts create zones that cannot be reached. It pays to calculate the irradiance field before building the equipment via optical simulation, which works on imported component geometries. An unreachable zone is not a design detail to be tolerated – it calls for a second curing mechanism, covered in the section on dual-cure systems.

In electronics and semiconductor manufacturing, UV adhesives are increasingly taking on tasks that were traditionally reserved for soldering: component bonding, conformal coating and display assembly can be achieved at lower process temperatures and shorter cycle times than soldering or thermally curing systems. In sensors, tight fit tolerance for small components matters most; in optics, it is dimensional stability – which is why cationically curing epoxy systems are more often used instead of acrylates there, thanks to lower shrinkage, particularly for bonding glass, metal and polymer in optical assemblies.

E-mobility brings its own constraints: rising power density alongside shrinking installation space, higher requirements for temperature, chemical and mechanical resistance, and increasingly flexible substrates and flex-PCBs on curved surfaces. For battery modules, power electronics and sensors in drivetrains, this means UV adhesives must not only fixture quickly but remain stable over the vehicle's service life – one reason the market for adhesives in electromobility is currently growing markedly faster than the UV adhesives market as a whole.

Adjacent application fields: automation and process integration, electronics and semiconductor manufacturing, and optics and precision components.

Potting compounds and encapsulants have a layer thickness that a bondline does not. Radiation is absorbed and scattered over depth – the surface reacts first and the bottom zone last, and with filled, pigmented or turbid compounds, sometimes not at all. Penetration depth depends simultaneously on filler, colour, photoinitiator concentration and layer height, and cannot be derived from surface irradiance alone. In electronic assemblies, the thermal side adds to this: a UV-LED carries almost no infrared load, but part of its optical power still converts to heat in the material, and crosslinking itself is exothermic. For temperature-sensitive components, component temperature must therefore be tracked alongside dose – shorter exposure at higher power is not automatically the gentler option.

The cause of this attenuation is absorption by the matrix material and the photoinitiator: every additional increment of thickness consumes a constant fraction of the remaining radiation. For clear, homogeneously absorbing systems, this can be approximated with the Beer-Lambert law:

I(z) = I₀ · e^−αz
where I₀ = incident irradiance, I(z) = irradiance at depth z, α = effective absorption coefficient.

At constant exposure time, the cumulative dose D(z) follows the same exponential form – the worked example below uses the more common coefficient µ instead of α, physically the same quantity.

For filled potting compounds, this simple model is often not enough, because scattering at pigments, fillers and interfaces adds to the picture – the radiation is not only absorbed but also weakened further along a longer, diffuse path.

Attenuation of the radiation over layer thickness can be estimated in simplified form with the Lambert-Beer law:

D(z) = D₀ · e^−µz
where D₀ = surface dose, D(z) = dose at depth z, µ = material-dependent extinction coefficient.

Illustrative assumptions for a 2 mm thick, lightly filled epoxy potting compound at 365 nm:

• Required minimum dose at the bottom of the joint (per adhesive datasheet): D_min = 1,500 mJ/cm²
• Measured residual transmission of the compound at 2 mm layer thickness: T ≈ 30 % (values for similar filled systems typically fall in the 20–40 % range depending on filler content and should be measured on the real material)

This gives the required surface dose:

D₀ = D_min / T = 1,500 mJ/cm² / 0.30 ≈ 5,000 mJ/cm²

If the equipment, within the allowed process window (time, temperature budget), delivers only, say, 3,000 mJ/cm² at the surface, the bottom of the joint is left at roughly 900 mJ/cm² – below the minimum dose. The batch cures through at the surface but not at the bottom. Options in that case: reduce layer thickness, switch to an adhesive with higher transmission at the chosen wavelength, add irradiation from the side, or move to a dual-cure system. The actual transmission of a potting material can be measured on a material sample with a spectroradiometer, rather than estimated from datasheet values.

UV radiation travels in straight lines and cannot bend around obstacles. Tall components on a PCB (electrolytic capacitors, connectors, coils), undercuts, cavities, or a closed housing irradiated from outside all create shadow areas where the adhesive stays liquid without an additional measure. Dual-cure adhesives solve this via a second, UV-independent curing path:

Practical tip for detection: a UV indicator film at the bond site, exposed under production conditions, makes shadow areas visible as unexposed spots before the first production run.

A cured bond cannot be checked visually. Because the decisive process variable is invisible, every approved bonding process needs a measured value: the irradiance at the joint position and the resulting dose. For initial process design and comparing sources, the UVpad delivers the spectrum at component level; for recurring checks, a broadband radiometer such as the RMD Pro is sufficient. Matching sensor and source matters: how far a broadband sensor can deviate against a narrowband LED spectrum is shown by the spectral mismatch.

Alongside optical dose, temperature belongs in the same measurement chain: commercial UV radiometers do not capture infrared irradiance, so the thermal effect is usually captured as a rise in component temperature via a non-contact pyrometer [4]. Heat can be a benefit or a risk depending on the application, but it is always part of the process window – for temperature-sensitive components (electronics, optical cements) it should be released and monitored just as rigorously as the dose itself.

In series production, measurement moves into the line: UV curing sensors stay permanently in the beam path, and curelog records up to 2,000 measurements per second across four spectral ranges, making the profile of a fast-running curing line visible. Both approaches answer the same question – is the source still operating within its approved window? That it does not stay there automatically over time is described under ageing of UV lamps and UV-LEDs: peak wavelength and output power shift, often slowly enough to go unnoticed during ongoing operation until parts fall outside the approved window. More broadly, this trend towards continuous inline monitoring is also visible among other manufacturers of UV curing systems in the context of Industry 4.0.

For trial series and material approvals away from the line, irradiation chambers with defined geometry are available, plus, for the process side, control units with timer, dimming and trigger functions, and the rest of the UV-LED light source range.

Our own customer publications also show UV bonding in several facets – from surface pretreatment for bonded joints to encapsulation of photovoltaic modules.

Examines how a UVO pretreatment changes the interlaminar toughness of co-extruded epoxy-LMPAEK bonded joints – evidence that UV treatment does not just cure, but can affect joint strength itself.
Hybrid epoxy-LMPAEK composite co-cured joints: A study on UVO pretreatment and its effect on interlaminar toughness
Katz, L., et al. "Hybrid epoxy-LMPAEK composite co-cured joints: A study on UVO pretreatment and its effect on interlaminar toughness." Composites Part B: Engineering 323 (2026): 113727.

Monitors a silicone adhesive under UV exposure in high vacuum with a single multiparameter sensor – an application for the same kind of process monitoring that runs via a UVpad on the ground.
Multiparameter Single Sensor for Space Silicone Adhesive Monitoring Under High-Vacuum Ultraviolet Exposure
Fazzi, Luigi, et al. "Multiparameter Single Sensor for Space Silicone Adhesive Monitoring Under High-Vacuum Ultraviolet Exposure." Journal of Spacecraft and Rockets 60.3 (2023): 740-752.

Compares four encapsulant materials for photovoltaic modules under UV exposure in the Atacama Desert – encapsulation under extreme UV load, similar to what e-mobility assemblies can reach.
Comparing the effects of ultraviolet radiation on four different encapsulants for photovoltaic applications in the Atacama Desert
Correa-Puerta, Jonathan, et al. "Comparing the effects of ultraviolet radiation on four different encapsulants for photovoltaic applications in the Atacama Desert." Solar Energy 228 (2021): 625-635.

Examines the oxygen-affected layer in multilayered photopolymers – the effect that leaves the surface tacky while the core is already fully crosslinked.
Effect of the oxygen affected layer in multilayered photopolymers
Pierrel, J., et al. "Effect of the oxygen affected layer in multilayered photopolymers." Polymer Chemistry 8.31 (2017): 4596-4602.

Shows, for ultrafast-curing acrylates, how comonomer choice and amount shift the thermomechanical properties of the crosslinked network.
Influence of co-monomer-type and amount on thermomechanical properties of ultrafast-curing acrylic resins using a triacrylic crosslinker
Böhm, Michael, et al. "Influence of co-monomer-type and amount on thermomechanical properties of ultrafast-curing acrylic resins using a triacrylic crosslinker." Journal of Applied Polymer Science 136.13 (2019): 47294.

Further work from the photochemistry and polymer research, and stereolithography, crosslinking and coatings topic fields is listed under Publications by customers – by topic.

[1] : PMC – Free-Radical Photopolymerization

[2] : Dymax – Sauerstoffinhibierung überwinden

[3] : ScienceDirect – Specific photoinitiating systems at 365, 385, 395 and 405 nm

[4] : uvmeasurement.org – Radiometric Methods for UV Process Design and Process Monitoring

[5] : Fraunhofer IOSB-AST – Possible ban on some types of UV lamps in the EU from 2027

How does the curing of UV adhesives work chemically?
A photoinitiator absorbs light of the right wavelength and breaks down into reactive species (radicals for acrylates, a photoacid for epoxies) that trigger polymerisation of the monomers and crosslink into a solid network.

Which wavelength suits which photoinitiator?
That depends on the specific initiator – common offerings sit at 365, 385, 395 and 405 nm. What matters is the overlap between the LED emission spectrum and the initiator's absorption band, not the raw power of the source. The adhesive datasheet is binding, backed up by a measurement at the real source.

What is the difference between 365, 385, 395 and 405 nm UV-LEDs?
365 nm offers the highest photon energy and often the greatest penetration depth into clear materials, 405 nm the shallowest penetration but efficient surface curing. 385 and 395 nm sit in between and offer broad material compatibility with long LED lifetime and energy efficiency.

How deep does UV light penetrate a potting compound?
Penetration depth roughly follows the Lambert-Beer law and depends on filler, colour, photoinitiator concentration and wavelength. It cannot be derived from surface irradiance alone and must be measured on the real material, for example with a spectroradiometer.

What are dual-cure adhesives and when are they needed?
Dual-cure adhesives combine fast UV fixturing with a second, UV-independent curing path (heat, moisture or anaerobic cure). They are used when shadow areas – for example under tall components or inside closed housings – cannot be sufficiently irradiated.

How is the UV dose measured correctly for bonding?
Via a radiometer or spectroradiometer at the joint position, capturing irradiance and, from it, dose (irradiance × time). What matters is the spectral match between sensor and source – a poorly matched broadband sensor can give strongly deviating readings with narrowband LEDs.

Which UV adhesives suit electronics manufacturing and e-mobility?
Depending on requirements, electronics and e-mobility use either free-radical curing acrylates (fast fixturing) or cationically curing epoxies (low shrinkage, high temperature and chemical resistance), often as a dual-cure system for shadow areas on densely populated assemblies.

How can a UV bonding process be monitored in series production?
Via UV curing sensors permanently installed in the beam path and continuously logging instruments that compare irradiance and dose against the approved process window at all times, reporting deviations – for example from source ageing – in real time.

Not sure whether your UV dose still reaches the bottom of the joint? We measure your spectrum and irradiance on the real component and advise you on the right wavelength, geometry and process monitoring – send us your question or request a UVpad measurement directly.