Roel Kusters

Large area flexible lighting foils using distributed bare LED dies on polyester substrates

Integration of LEDs on flexible foil substrates is of interest for flexible lighting applications and for backlights for flexible displays. Such a large area lighting device can be made by integrating a matrix of closely spaced LEDs on a flexible foil substrate. Preferably, these LEDs are integrated unpackaged, i.e. as bare dies, as this reduces footprint, thickness and cost. As substrates, low cost materials like polyethylene terephthalate (PET) should preferably be used. However, the use of these materials also imposes limitations. Especially, their low thermal stability limits the maximum temperatures during the processing and the thermal dissipation of the LED during operation will pose constraints on the thermal design. This paper describes the results of research on possibilities for integrating bare die LEDs with such low cost flexible PET foils. Bonding of LED dies on PET substrates with copper circuitry using conductive adhesives was performed. Both anisotropic conducting adhesives and isotropic conducting adhesives were investigated. An experimental comparison is made between the different techniques based on temperature/humidity reliability and flexural stability of the bonded LEDs. Additionally, finite element (FE) thermal modeling results of adhesively bonded LED-on-foil configurations are presented. The role of the different materials and the effect of their geometries on the temperature distribution in the simulated devices are discussed. The results are compared to experimentally observed temperature distributions using infrared thermal imaging in LED on PET foil reference devices. Finally a demonstrator device of 64 LEDs on flexible copper-PET substrate is presented.

4.5.2 Development of printed RFID sensor tags for smart food packaging

Sensors integrated into food packages could benefit consumers by ensuring freshness and quality while allowing retail industry to more efficiently manage food stocks and product authenticity. Here we present smart radio-frequency labels with sensors able to measure temperature, humidity and the presence of volatile amine compounds. The labels are made via high quality screen printing and lamination technologies on low cost foils in combination with pick and place technology. As a case study the smart labels are used to quantify the freshness of fish.

Mechanical and electrical properties of ultra-thin chips and flexible electronics assemblies during bending

Ultra-thin chips of less than 20 μm become flexible, allowing integration of silicon IC technology with highly flexible electronics such as food packaging sensor systems or healthcare and sport monitoring tags as wearable patches or even directly in clothing textile. The ultra-thin chips in these products will be bent to a very high curvature, which puts a large strain on the chips during use. In this paper a modified four-point bending method is presented, which is capable of measuring chip stress at high curvatures. The strength of several types of ultra-thin chips is evaluated, including standalone ultra-thin test chips and back-thinned 20 μm thick microcontrollers, as well as assemblies containing integrated ultra-thin microcontroller chips. The effect of chip thickness, bending direction and backside finish on strength and minimum bending radius is investigated using the modified four point bending method. The effect of bonding ultra-thin chips to flexible foils on the assembly strength and minimum bending radius is evaluated as well as the effect of bending on electrical properties of the bonded microcontroller dies.

A comparative study of via drilling and scribing on PEN and PET substrates for flexible electronic applications using excimer and Nd:YAG laser sources

A study on via drilling and channel scribing on PEN and PET substrates for flexible electronic application is discussed in this paper. For the experiments, both KrF excimer laser (248 nm) and frequency tripled Nd:YAG (355 nm) laser are used. Different measurement techniques like optical microscopy, Dektak profilometer, Confocal microscopy and scanning electron microscopy (SEM) were employed to characterize the quality of the channels and vias. The patterned structures were filled by three different methods using a conductive paste or ink that is cured in an oven at an elevated temperature. The cross-sectional measurements of channels and vias were carried out using SEM to study the uniformity of filling.

A chip embedding solution based on low-cost plastic materials as enabling technology for smart labels

Expanding the current smart packaging solutions to individual products requires improvement for several of the following properties: cost, thickness, weight, flexibility, conformability, transparency, and even stretchability. This paper focusses on an embedding technology that targets the first four properties, with an emphasis on cost reduction. The progress on the development work for this foil-based embedding is reported, along with a detailed failure analysis of the process flow. A functional demonstrator is realized in the form of a smart sensing label, including an embedded micro controller. Practical and technological shortcomings of the current technology are used as a starting point for proposing an improved embedding technology based on low-cost plastic materials. The first developments for lamination and via interconnection are described.

Flipchip Bonding of Si Chip on Flexible PEN Foil using Novel Electronic 100 μm Pitch Fan-out Circuitry

A novel, cost effective technology to manufacture high density embedded electronic circuitry is demonstrated. The process consists of laser photoablation of the circuitry into a substrate through a mask and subsequent filling using a polymer thick film paste. Because the volume of the substrate is used it is possible to make thick and thereby highly conductive lines using low cost materials and processes. The process is demonstrated for a fan out circuitry in 100 µm thick polyethylene naphthalate (PEN). The fan out circuitry has linewidths of 50 µm and line spacings of 100 µm. The usability of the circuitry is demonstrated by the successful flipchip bonding of a thinned Si daisy chain dummy chip with 176 IOs.

Photonic Flash Soldering of Thin Chips and SMD Components on Foils for Flexible Electronics

Ultrathin bare die chips and small-size surface mount device components were successfully soldered using a novel roll-to-roll compatible soldering technology. A high-power xenon light flash was used to successfully solder the components to copper tracks on polyimide (PI) and polyethylene terephthalate (PET) flex foils by using a lead-free solder paste. Results are compared with oven-reflowed solder joints on PI substrates. The delicate PET foil substrates were not damaged owing to the selectivity of light absorption, leading to a limited temperature increase in the PET foil while the chip and copper tracks were heated to a temperature high enough to initiate soldering. The microstructure of the soldered joints was investigated and found to be dependent on the photonic flash intensity. Reliability of the photonically soldered joints during damp heat testing and dynamic flexing testing was comparable with the reflowed benchmark and showed increased reliability compared with anisotropic conductive adhesives bonded on PET foils.

Bonding bare die LEDs on PET foils for lighting applications: Thermal design modeling and bonding experiments

Integration of LEDs on flexible foil substrates is of interest for flexible lighting applications and flexible photonic devices. A matrix of LEDs on a foil combined with a diffuser can be a potential alternative for flexible OLED lighting devices. Preferably, these LEDs are integrated in an unpackaged, bare die form as it reduces cost, footprint and thickness. As a substrate, preferably low cost materials like polyesters (PET) are being used, especially for large area devices. However, the use of these materials imposes some limitations. Most notably, the low temperature stability (<100ºC continuous use temperature) of these materials limits the maximum temperatures during the manufacturing process and poses constraints on the thermal design of the device. The current paper describes the results of research on possibilities for integrating bare die LEDs with low cost flexible PET foils. Finite element (FE) thermal modeling has been performed of possible designs of adhesively bonded LED-on-foil and laminated LED-in-foil configurations. The role of the different materials and the effect of their geometries on the temperature distribution in the simulated devices are discussed. The results give insight in measures that can be taken to keep the temperature of all the components of the device within operational limits. For LEDs bonded on Cu-PET foil the modeled temperature distributions are compared to experimentally observed temperature distributions in LED on PET foil reference devices using infrared thermal imaging. Besides this, initial studies on directly bonding LEDs on etched Cu on PET substrates using anisotropic conducting adhesives and isotropic conducting adhesives were performed. An experimental comparison is made between the different techniques based on temperature/humidity reliability and flexural stability of the bonded LEDs, based on these preliminary results.

Hybrid integration on low-cost flex foils using photonic flash soldiering

Soldering of packaged electronic components using industry standard Sn-Ag-Cu (SAC) lead-free solders on low-cost foils, which are often the substrate of choice for flexible electronics, is challenging. This is mainly originating from the fact that the reflow temperatures of these solder alloys are normally higher than the maximum processing temperature of the low-cost flex foils. To enable component integration on the low-cost foils a novel method for soldering has been introduced by Holst Centre as an alternative to oven reflow, termed “photonic soldering”. In this method high intensity photonic flashes are used to deliver the thermal energy required for soldering. By taking advantage of the selectivity of light absorption, the required energy for soldering is delivered to the components and circuit tracks while excessive heating of the foils is avoided. This paper presents successful photonic flash soldering of packaged LED components on low-cost polyethylene terephthalate (PET) foils using conventional SAC solders as a demonstration of the capabilities of this novel soldering technology.