By Max Baum, director, Product Management Consumer Assembly & Equipment, DELO

As display technologies evolve toward ever‑brighter, thinner and more energy‑efficient architectures, the methods used to assemble these systems must adapt at the same pace. Miniaturization in LED backlight and direct‑emissive systems challenges long‑established joining processes, such as soldering, which no longer can meet design and reliability demands at microscopic scales. Through two recent studies, DELO has explored high‑tech adhesives as a practical and reliable alternative for connecting miniLEDs. The same principles may one day support R2R processing.

Over the past two years, DELO has investigated high‑tech adhesives as a practical and reliable alternative for connecting miniLEDs – and, by extension, for paving the way to future microLED production.

While the studies discussed here explore adhesive bonding of miniLEDs from a semiconductor‑packaging perspective – feasibility and reliability testing was carried out using pick‑and‑place assembly for practical reasons – the same principles may one day support roll‑to‑roll (R2R) processing. Such an approach would depend on the specific application but is particularly promising for flexible and transparent display concepts. A potential use case is a transparent windshield display: What is realized today through projection could tomorrow be achieved by a directly emissive display integrated onto the glass, improving contrast, brightness and design freedom. Although outside the scope of this article, DELO’s internal trials with directional conductive adhesives on flexible substrates already have shown encouraging results.

From feasibility to functional proof

The 2024 feasibility study investigated whether conductive adhesives could successfully replace solder in connecting miniLED dies (see Figure 1). A specially designed three‑section test board simulated the flip‑chip configuration common in next‑generation display assemblies. Sections of the board featured common‑ground and resistor‑array layouts, with pad gaps ranging from 80 to 100 µm – representative of typical miniLED contact geometries – and included both direct resistivity and daisy‑chain measurement circuits to assess continuity and electrical behavior.

Different joining materials were compared: isotropic conductive materials (ICMs), such as solder, and directional conductive adhesives (DCAs), which are conductive only through the bond line while remaining insulating laterally. This key property allows DCAs to bridge larger stencil openings without creating shorts between adjacent pads.

Three stencil‑printing cases illustrated the difference. Case 01 shows an isotropic material with standard stencil openings confined to the pads; Case 02 shows the same material but with wider openings, bridging adjacent contacts; and Case 03 shows a directional conductive adhesive dispensed through wider stencil openings without shorting, because current flow was limited to the vertical axis.

MiniLED dies were attached using DELO MONOPOX AC268, then cured by thermode at 180° C for 20 seconds. In both single‑die and daisy‑chain configurations, the bonded LEDs immediately illuminated without electrical failure, confirming mechanical integrity and connectivity. The conclusion was clear: Adhesives can function as soldering alternatives even for pitch dimensions where solder begins to fail. Beyond feasibility, adhesive bonding also simplifies mask design; larger stencil openings reduce squeegee pressure, lowering the risk of substrate damage and improving print uniformity.

At the conclusion of these feasibility tests, additional next steps were defined: measurement of current–voltage (U‑I) characteristics, long‑term reliability under heat and humidity, and evaluation of die‑shear strength – all of which became the focus of the 2025 follow‑up study.

Reliability proven under stress

In the 2025 program, the central question shifted from “Can it work?” to “Will it last?” Using the same test‑board concept and flip‑chip layout, researchers evaluated how these adhesive bonds behave under environmental stress. The investigations included light‑on testing, die‑shear measurements and long‑term storage at 85° C / 85% relative humidity and at 120° C for up to 500 hours. Directional conductive adhesives were applied via stamping – chosen for its precision and reproducibility – and cured with a thermode cycle.

Across all conditions, the results remained stable. Initial die‑shear values of roughly 25 to 30 N per die decreased only marginally after 500 hours at 120° C, and the measured U‑I characteristics aligned almost perfectly with the reference data from the LED manufacturer. The forward voltage showed less than a 1% shift compared with initial measurements. Under the combined heat‑and‑humidity scenario (conditions that traditionally cause solder joints to degrade), adhesive connections maintained functionality without measurable drift. No short circuits or delamination effects were observed.

This consistent performance demonstrates that conductive adhesives can meet the industry’s long‑term reliability standards. By confirming both mechanical robustness and electrical stability, the research verifies that adhesives can fulfil the same critical connectivity role as solder but with added flexibility. They enable assemblies at smaller pitches and facilitate new display architectures that place greater emphasis on thin, lightweight and thermally gentle bonding.

Advantages over soldering in miniaturized assembly

Soldering remains the dominant joining technique in electronics, but with miniaturization comes a clear set of limitations:

• Pitch constraints. When pad spacing approaches 100 µm, surface tension and paste spreading make it nearly impossible to avoid bridging.
• Thermal stress. Melting temperatures above 200° C can damage thin substrates or distort polymer films.
• Stencil complexity. Fine mesh and small apertures slow down throughput and increase cost.

Conductive adhesives resolve many of these issues. Because the polymer matrix cures instead of melting, processing temperatures remain lower and thermal load on the component is minimized. The adjustable rheology of adhesives allows them to conform to uneven surfaces, ensuring planar placement even on flexible films. Furthermore, anisotropic conductivity eliminates the risk of lateral shorts, while permitting enlarged stencil openings or high‑throughput dispensing techniques. The impact cascades upstream into mask design, print speed and tool life – all key advantages when scaling production.

Integration into roll-to-roll concepts

Although the published studies were performed on rigid substrates using discrete die‑bonding equipment, the same chemistry can support roll‑to‑roll integration. The curing reaction of DCAs can be activated locally by thermodes or adapted to continuous heat sources compatible with web handling.

In practice, R2R suitability depends on how the miniLED devices are transferred and aligned onto the flexible film. Because the adhesives remain semi‑structured before cure, they can accommodate minor planarity deviations and ensure precise contact once pressure and heat are applied. This makes them attractive for flexible or transparent displays, where conventional solder reflow is incompatible with polymeric or glass‑laminated substrates. A typical example could be a windshield‑integrated heads‑up display, currently realized through projection systems but potentially achievable with direct‑emitting transparent pixel arrays.

While details of these experiments have not yet been published, initial internal testing at DELO with directional conductive adhesives on flexible webs has yielded promising processability and stable electrical performance.

Looking ahead

Taken together, the 2024 and 2025 investigations demonstrate a clear evolution: Adhesive bonding has progressed from “feasibility confirmed” to “reliability proven.” Directional conductive adhesives not only eliminate the short‑circuit risks of solder at finer pitches but also offer lower process temperatures, reduced mechanical stress and compatibility with emerging flexible substrates.

For engineers seeking to modernize interconnection methods in LED display manufacturing, whether for mass miniLED backlights or the next generation of self‑emissive microLED panels, adhesives represent a serious engineering option. Their combination of mechanical stability, anisotropic conductivity and process flexibility aligns well with the industry’s push toward thinner, lighter and more versatile display modules. And while soldering may remain entrenched for conventional board assemblies, the horizon for advanced displays looks increasingly adhesive. 

Max Baum is the director, Product Management Consumer Assembly & Equipment for DELO. More information can be found at www.delo-adhesives.com.