Fig. 1. Schematic illustration of the process flow for a local stiffness-variant stretchable substrate consisting of soft PDMS, hard PDMS, and FPCB: (a) attach the acrylicsilicone double-sided tape to FPCB, (b) attach the FPCB to a partially cured hard PDMS using the silicone adhesive of the double-sided tape, (c) fully cure the hard PDMS and process flip chip bonding, and (d) after placing the flip-chip bonded hard PDMS/FPCB into a mold, pour and fully cure the soft PDMS.
Fig. 2. Optical image of the stretchable PDMS/FPCB substrate.
Fig. 3. Schematic illustration of the stretchable substrate consisting of soft PDMS, hard PDMS, and FPCB on which a Si chip is mounted by flip chip bonding.
Fig. 4. Fabrication process of a Si chip: (a) sputtering of Ti/Cu metallization, (b) photoresist patterning, (c) Cu and Au electroplating, (d) photoresist patterning, (e) Ti/Cu electrode formation, and (f) dispensing ACA for flip chip bonding.
Fig. 5. Top-view optical image and cross-sectional scanning electron micrograph of the flip-chip bonded specimen that was processed on the PDMS/FPCB substrate with a bonding pressure of (a) 10 MPa, (b) 50 MPa, and (c) 100 MPa.
Fig. 6. Scanning electron micrographs of (a) Au-coated polymer balls, (b) Ni particle, and (c) SnBi solder particle contained in each ACA.
Fig. 7. Cross-sectional scanning electron micrographs of the flipchip joints processed with the ACA containing (a) Aucoated polymer balls, (b) Ni particles, and (c) SnBi solder particles.
Fig. 8. Optical micrographs of (a) Au-coated polymer balls, (b) Ni particles, and (c) SnBi solder particles contained in each ACA.
Fig. 9. Optical micrographs of the ACA conductive particles entrapped between the chip bump and the glass substrate. ACA conductive particles are Au-coated polymer balls, Ni particles for (c) and (d), and SnBi solder particles for (e) and (f).
Fig. 10. Relative distribution frequency of the number of Ni particles entrapped between a chip bump and the glass substrate for total 42 chip bumps.
Fig. 11. Optical Images of the soft PDMS/hard PDMS/FPCB stretchable substrate, where a Si chip was flip-chip bonded, at an applied strain of (a) 0% and (b) 30%.
Fig. 12. Resistance change ratio, ΔR/Ro, of the Si chip, which was flip-chip bonded to the soft PDMS/hard PDMS/FPCB stretchable substrate, during cycled stretching deformation of the substrate for 10 times in the 0~30% tensile strain interval.
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