Kwang-Seong Choi | Electronics and Telecommunications Research Institute: What happens to micro-LED electrical properties after laser bonding, and how much light is absorbed?
10:27 - 12:23
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Summary of the clip:
What happens to micro-LED electrical properties after laser bonding, and how much light is absorbed?
Local heating is one of the key issues. The team measured the glass and PDMS interposer absorbance across wavelengths. At 980 nanometres, only 0.4% was absorbed. That low figure means you do not need to worry about CT variation during the bonding process. The SiTrap material can fill the whole area between the micro-LED and the backplane, which matters for both optical and mechanical integrity of the joint.
Solder choices include tin, tin-silver and indium, and they are very soft, so underfill is required to make the solder joint mechanically reliable. Alternatively, if you apply gold tin, you may think you do not want underfill or such an encapsulation process. That area can be filled by encapsulation, but voids may form inside the gap. Perfect filling of the underfill material is therefore needed.
The work also demonstrates RGB devices. Electrical properties of the micro-LED were checked before and after the SiTrap process. As shown, there is no variation. So the laser-driven, locally heated bonding route preserves the micro-LED's electrical behaviour while the SiTrap material fills the gap, addressing both the thermal budget concern and the mechanical reliability of the joint in one process flow.
In this short video, you can learn:
* At 980 nanometres the glass and PDMS interposer absorbed only 0.4%, so CT variation during bonding is not a concern.
* Tin, tin-silver and indium solder joints are soft and need underfill, while gold tin may need encapsulation that risks voids in the gap.
* Micro-LED electrical properties showed no variation before and after the SiTrap process, with RGB devices demonstrated.
📋 **Clip Abstract** Local heating is a key issue for micro-LED transfer and bonding, so absorbance of the glass and PDMS interposer was measured, with only 0.4% absorbed at 980 nanometres, removing concern over CT variation. SiTrap fills the area between micro-LED and backplane, and micro-LED electrical properties showed no variation before and after processing.
About the speaker:
* Speaker: Kwang-Seong Choi
* Company: Electronics and Telecommunications Research Institute
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#SiTrapFilling, #UnderfillVoidControl, #LaserBonding980nm, #SolderJointReliability, #MicroLEDBonding, #ARDisplays
This is a highlight of the presentation:
From Materials to OSAT: SITRAB Technology for Scalable Micro-LED Transfer and Bonding
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03:13 - 05:07
Why is bonding, not transfer, the real bottleneck for scalable micro-LED displays?
Why is bonding, not transfer, the real bottleneck for scalable micro-LED displays?
Bonding is more important than transfer in packaging. The commercially accepted transfer technology is the stamp, but the stamp has low throughput. That forces heavy investment in transfer equipment. Because of yield, AOI equipment must follow transfer and bonding. If a gold thin case is adopted, underfill equipment is also needed. The whole line becomes very large, so capital expenditure must increase.
The explanation for linking transfer and bonding is that people usually apply thermal compression bonding. The micro LED is usually under PDMS, and PDMS has a CT over 300 ppm. Because of conductive thermal energy, it expands, so alignment accuracy is lost. The material always requires hot staging. That leads to another misalignment. Control becomes very difficult.
The result is a packaging bottleneck: transfer throughput and yield drive AOI, and gold thin case drives underfill. Thermal compression bonding with PDMS at over 300 ppm CT, plus hot staging, makes alignment hard to control. So scaling micro-LED displays is not just a transfer problem. Bonding determines alignment, equipment count, and capital expenditure, which is why packaging and bonding are more important than transfer.
In this short video, you can learn:
* The accepted commercial transfer method, the stamp, has low throughput.
* AOI follows transfer and bonding, and a gold thin case requires underfill, enlarging the line and increasing capital expenditure.
* Thermal compression bonding under PDMS and hot staging cause expansion, misalignment, and lost alignment accuracy.
📋 **Clip Abstract** Bonding is more important than transfer in micro-LED packaging because the commercial stamp transfer has low throughput, requiring large transfer equipment investment. Yield adds AOI after transfer and bonding, gold thin case adds underfill, and thermal compression bonding with PDMS over 300 ppm CT plus hot staging makes alignment very difficult to control.
About the speaker:
* Speaker: Kwang-Seong Choi
* Company: Electronics and Telecommunications Research Institute
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#ThermalCompressionBonding, #PDMSAlignmentLoss, #AOIEquipment, #UnderfillEquipment, #MicroLEDPackaging, #MicroLEDDisplays
14:32 - 16:34
Can SITRAB process RGB micro-LEDs repeatedly, and is the supply chain commercially ready?
Can SITRAB process RGB micro-LEDs repeatedly, and is the supply chain commercially ready?
Building an interposer from RGB is very difficult, yet the material can be done many times. In the data, as the number of laser irradiations increased, the curing — the conversion — is less than 20%, even at 100 degrees. That means you can SITRAB process for RGB. You can optimise the process, or take another option.
The key lies in a material property: it does not react with the laser. The chemical reaction starts only with temperature. That is why the SITRAB process can be done as many times as we want, as shown. The laser does not trigger conversion on its own — temperature does — so repeated passes remain controlled, and the RGB interposer can be built up step by step without premature reaction.
On the ecosystem side, the material has already been transferred to a Korean material manufacturer, Prime Material, founded on this material platform and commercially available now. Laser bonding is already applied in mass production at an OSAT. That means SITRAB-capable equipment has already been adopted by that OSAT company, taking the process beyond demonstration.
In this short video, you can learn:
* Conversion stays under 20% as laser irradiation count rises, even at 100 degrees, so SITRAB can process RGB repeatedly.
* The material does not react with the laser; only temperature starts the chemical reaction, so the process can be done as many times as needed.
* The material has been transferred to Korea's Prime Material and is commercially available, while laser bonding and SITRAB-capable equipment are already running in OSAT mass production.
📋 **Clip Abstract** Repeated laser irradiation raises conversion by less than 20% even at 100 degrees, showing SITRAB can process RGB interposers many times. The material is now commercialised through Korea's Prime Material and relies on temperature-only chemistry, while laser bonding and SITRAB-capable equipment are already in OSAT mass production.
About the speaker:
* Speaker: Kwang-Seong Choi
* Company: Electronics and Telecommunications Research Institute
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#SITRAB, #RGBInterposer, #LaserBonding, #TemperatureTriggeredBonding, #MicroLED, #AdvancedPackaging
05:18 - 07:32
How does a 980 nm laser bond micro-LEDs through glass and PDMS without damaging them?
How does a 980 nm laser bond micro-LEDs through glass and PDMS without damaging them?
The interposer captures the micro-LED. Solder can be tin, tin AG or indium. A SiTrap film is applied at that stage on the back plane. Pressure is applied, and a homogenized laser irradiates the whole silicon back plane. The wavelength is 980 nanometer. Glass, PDMS and even the micro-LED are transparent, so the laser is only absorbed by the electrode and solder.
When the laser is applied, it is absorbed in this region, generating local heating. Because of that, the material is activated and reduces the oxide layer of the solder. As temperature increases, the film or adhesive loses its viscosity and is squeezed out. Solder and the metal pad, usually UBM, become wet. The SiTrap material is squeezed out, meaning you have metallic bonding.
The adhesion strength of this metallic bond is much higher than the adhesion strength between PDMS and micro-LED. That PDMS/micro-LED interface is just Van der Waals force. So you just release to achieve the transfer process. This enables release and transfer. In this scheme, you bond first time and then perform a second transfer.
In this short video, you can learn:
* A 980 nm homogenized laser passes through glass, PDMS and micro-LED, and is absorbed only by the electrode and solder.
* Local heating activates the SiTrap material, reduces the solder oxide, lowers viscosity, squeezes out adhesive, and creates metallic bonding.
* Metallic bond adhesion is much higher than the PDMS/micro-LED Van der Waals force, enabling release and a bond-first, second-transfer scheme.
📋 **Clip Abstract** A 980 nm homogenized laser irradiates the silicon back plane through transparent glass, PDMS and micro-LED, so only the electrode and solder absorb energy. Local heating activates SiTrap film, reduces solder oxide and squeezes out adhesive to form metallic bonding; because this bond is stronger than the PDMS/micro-LED Van der Waals force, release occurs for a bond-first, second-transfer process.
About the speaker:
* Speaker: Kwang-Seong Choi
* Company: Electronics and Telecommunications Research Institute
* Event: Eindhoven 2026
* Location: High Tech Campus, Eindhoven
#SiTrapFilm, #HomogenizedLaser, #SolderOxideReduction, #MetallicBonding, #MicroLEDTransfer, #HeterogeneousIntegration