Flip Chip Process on the Local Stiffness-variant Stretchable Substrate for Stretchable Electronic Packages

신축성 전자패키지용 강성도 국부변환 신축기판에서의 플립칩 공정

Abstract

A Si chip with the Cu/Au bumps of $100-{\mu}m$ diameter was flip-chip bonded using different anisotropic conductive adhesives (ACAs) onto the local stiffness-variant stretchable substrate consisting of polydimethylsiloxane (PDMS) and flexible printed circuit board (FPCB). The average contact resistances of the flip-chip joints processed with ACAs containing different conductive particles were evaluated and compared. The specimen, which was flip-chip bonded using the ACA with Au-coated polymer balls as conductive particles, exhibited a contact resistance of $43.2m{\Omega}$. The contact resistance of the Si chip, which was flip-chip processed with the ACA containing SnBi solder particles, was measured as $36.2m{\Omega}$, On the contrary, an electric open occurred for the sample bonded using the ACA with Ni particles, which was attributed to the formation of flip-chip joints without any entrapped Ni particles because of the least amount of Ni particles in the ACA.

강성도가 서로 다른 polydimethylsiloxane (PDMS) 탄성고분자와 flexible printed circuit board (FPCB)로 이루어진 PDMS/FPCB 구조의 강성도 국부변환 신축기판에 $100{\mu}m$ 직경의 Cu/Au 범프를 갖는 Si 칩을 anisotropic conductive adhesive (ACA)를 사용하여 플립칩 본딩 후, ACA내 전도성 입자에 따른 플립칩 접속저항을 비교하였다. Au 코팅된 폴리머 볼을 함유한 ACA를 사용하여 플립칩 본딩한 시편은 $43.2m{\Omega}$의 접속저항을 나타내었으며, SnBi 솔더입자를 함유한 ACA로 플립칩 본딩한 시편은 $36.2m{\Omega}$의 접속저항을 나타내었다. 반면에 Ni 입자를 함유한 ACA를 사용하여 플립칩 본딩한 시편에서는 전기적 open이 발생하였는데, 이는 ACA내 Ni 입자의 함유량이 부족하여 entrap된 Ni 입자가 하나도 없는 플립칩 접속부가 발생하였기 때문이다.

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/R_o, 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.