Flipchip bonding of thin Si dies onto PET foils: Possibilities and applications

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.