Integration techniques and applications of thin chips on low cost foil substrates

Large area flexible foil based 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 systems integrated into food packaging or healthcare and sport monitoring tags as wearable patches or in clothing. Intelligence and communicative capabilities are preferably added to these devices by integrating chips and passives directly on the foil itself. To maintain low thickness and therefore flexibility of these systems, thinned bare die chips, thin passives and unpackaged LEDs are used. For cost effectiveness the preferred substrate material for these large area systems will be low cost polyester materials such as PET (polyethylene teraphtalate) foils. The low thermal stability of the PET foils however puts serious constraints on the integration process and materials, rendering many conventional integration technologies unfeasible. In this paper, several integration techniques are presented that are compatible with low cost foils. These include low temperature flip chip bonding on PET foil substrates, chip embedding in flexible polyurethane boards and a novel face up chip integration technique. This face up chip integration technique is compatible with low cost PET foil processing and screen printing techniques. To attest the extreme flexibility of these systems, flexural strength results of ultrathin silicon dies are presented as well
as lifetime and flexural test results of these chips integrated on foil substrates. Application examples of thin-chip integration are presented in the form of a supply chain monitoring tag, a smart label and a flexible lighting sheet of bare die LEDs on Cu-PET foil. These functional demonstrators consist of an integrated bare die microcontroller and LED chips on low cost foil substrates using flip chip technologies and integration of bare die microcontrollers and passives using double sided embedding techniques in polyurethane flexible substrates.