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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.

This paper describes the successful process investigations on ultrathin flip-chip bonding for fine-pitch applications on foil, using a novel bonding process involving Isotropic Conductive Adhesives (ICA). A Ag-based B-stage curing ICA was printed using state-of-the-art electroformed stencil on printed Ag circuitry, pre-cured and flip-chip bonded at low bonding force and short duration. The interconnection resistance measurements, performed before and after a stabilizing underfilling step, revealed low interconnect resistances down to 150 μm bond pad pitch. Finally, the reliability of the specimens prepared using this process was successfully evaluated by means of standard reliability test procedures such as thermal shock testing and accelerated humidity testing.

Ultra-thin chips of less than 20μm become flexible, allowing integration of silicon IC technology with highly flexible electronics. This combination allows for highly intelligent products of unprecedented thinness, flexibility and cost. Examples include sensor systems integrated into food packaging or healthcare and sport monitoring tags as wearable patches or even directly in clothing textile. During use the ultra-thin chips in these products can be bent to a very high curvature, which puts a large strain on the chips. In this paper the strength of ultra-thin chips at very high curvatures is evaluated, using a modified four-point bending method. Stand-alone ultra-thin chips are evaluated which achieve a minimum bending radius below 1mm, as well as assemblies containing integrated ultra-thin chips. The effect of chip thickness, bending direction and backside finish on strength and minimum bending radius is investigated using the developed method. The difference between blank ultra-thin silicon dies and daisy chain ultra-thin chips with bond pads and bumps is highlighted. Finally the high curvature behaviour is investigated of ultra-thin chips that were integrated on low-cost polyester foil substrates using several different low-temperature integration techniques. The excellent suitability of these ultra-thin chips on low-cost foil assemblies for highly flexible large area devices is shown. © 2013 IMAPS.

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. © 2013 Elsevier Ltd. All rights reserved.

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 (

Low cost large area flexible electronic products are expected to be used in a wide range of applications and in large quantities in our society. Examples of this include sensor packages added to food or conformal intelligent patches that monitor a patient's well-being. Because of their large area, the preferred substrate material for these applications will be low cost materials like polyesters (PEN/PET). Intelligence or communicative capabilities are preferably added to these devices by integrating the chips directly on the low cost foil itself. To maintain the flexibility of the package and not to add too much to the thickness, the Si chip needs to be integrated into the product as a bare, thinned die. Flip chip bonding is currently the most mature, widely available technology to integrate these thin chips. The low temperature stability of the PET foils however puts serious constraints on the materials and the process. The current paper specifically addresses the challenges associated with this. Initial results from a finite element model will be discussed. The model is being developed to understand the influence of the bonding process and material parameters on the final stresses and warpage of the chip. Additionally, lifetime and flexural test results will be discussed of ultrathin chips bonded on Cu and Ag-based screen printed circuitry. Finally, some applications of the technology will be shown: a microcontroller integrated on a Cu-PET foil and a supply chain monitoring tag. © 2011 IMAPS.

In order to develop an advanced polymer based package technology to obtain small and flexible medical implants, diffusion barriers are essential to protect electronic chips and components for corrosion caused by leaching of body fluids. These barriers need also to stop diffusion of harmful products such as Cu from the device into the body. Furthermore, the barrier layers need to be biocompatible, biostable, compatible with all process steps of the package fabrication and sterilization. Parylene is under investigation as candidate barrier material. WVTR tests are performed to characterize diffusion of water through the Parylene layer. Adhesion tests of Parylene-C on various substrates are performed and if needed adhesion promotion treatments are optimized, since good adhesion is crucial for corrosion prevention. Dedicated corrosion tests have shown that 5um Parylene-C is protecting Cu lines for corrosion at 70 degrees C for at least 170 hours, more testing needs to be performed. Cell culture test have shown that Cu diffusion is not sufficiently stopped by 5um Parylene-C or -N, a thicker Parylene layer or a combination with other barriers might be essential. Also dedicated cell culture protocols with non-adherent cell types will have to be developed to perform reliable tests on these hydrophobic and cytophobic barrier layers.