High curvature bending characterization of ultra-thin chips and chip-on-foil assemblies

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.