Comparison of Flip-Chip Bonding Characteristics on Rigid, Flexible, and Stretchable Substrates: Part II. Flip-Chip Bonding on Compliant Substrates

Donghyun Park*, Kee-Sun Han* and Tae Sung Oh

Department of Materials Science and Engineering, Hongik University, Seoul 04066, Republic of Korea

The contact resistance and microstructure of the �ip-chip joints processed using anisotropic conductive adhesive (ACA) were character-ized on �exible printed-circuit-board (FPCB) and stretchable FPCB/polydimethylsiloxane (FPCB/PDMS) substrates. On the FPCB substrate, the contact resistance was remarkably reduced from 56.9 mΩ to 13.3 mΩ when the bonding pressure increased from 10 MPa to 200 MPa. However, at a bonding pressure of 300 MPa, it substantially increased to 74.8 mΩ with an excessive deviation of ±37.7 mΩ. On the more compliant FPCB/PDMS substrate, the contact resistance decreased from 43.2 mΩ to 31.2 mΩ when the bonding pressure increased from 10 MPa to 50 MPa. Severe distortion of the FPCB/PDMS substrate occurred at bonding pressures above 50 MPa because of the softness of the PDMS. A more compliant substrate has a lower appropriate bonding pressure for the �ip-chip process. [doi:10.2320/matertrans.M2017066]

(Received February 27, 2017; Accepted May 11, 2017; Published June 30, 2017)

Keywords:　 stretchable electronic packaging, wearable device, substrate stiffness, �ip chip, contact resistance

1.　 Introduction

Recently, much attention has been focused on wearable devices for various new applications that could not be possi-ble with conventional rigid electronics.1–9) To ensure the suf-�cient functionality, wearable comfortability, and aesthetical acceptability of wearable devices, stretchable electronic packaging technologies with an island-bridge or archipelago con�guration, which consists of mechanically disparate soft and hard materials in a single structure, have been pro-posed.5,8,10–14)

Flip-chip bonding has been used in the electronic packag-ing industry for high-density and �ne-pitch chip intercon-nection on �exible printed circuit boards (FPCBs) and rigid substrates, such as printed circuit boards (PCBs) and liq-uid-crystal-display glass substrates.15–17) However, it is nota-bly dif�cult to �nd any reports of the �ip-chip process on a stretchable substrate that consists of polydimethylsiloxane (PDMS), which is considered the most suitable elastomer to use as the substrate material for stretchable electronic pack-aging because of its superior stretchability, �exibility, bio-compatibility, easy processability, low curing temperature, low dielectric constant, low loss tangent, and high dielectric strength.18–22)

The �ip-chip process can be accomplished by using either solder-bump re�ow or an anisotropic conductive adhesive (ACA). Compared to the �ip-chip technology with solder bump re�ow, the process using an ACA or anisotropic con-ductive �lm (ACF), where conducting particles are random-ly distributed in an adhesive, has the signi�cant advantages of low-temperature bonding and �uxless bonding.15–17) PDMS is deformed by solvent-induced swelling and is not recommended for use at temperatures above 200°C.23–26) The �ip-chip process using ACA is more applicable for stretchable packaging using the PDMS substrate than the �ip-chip process using solder re�ow because the re�ow tem-peratures of typical Pb-free solders, such as Sn-Ag-Cu and

Sn-Ag, are above 200°C, and solvent cleaning for �ux re-moval is required after the solder re�ow.15)

To develop chip interconnection technology for stretch-able electronic packaging from the established technologies for conventional and �exible electronic packages, the �ip-chip bonding behavior on substrates of different stiffness values must be compared. We sequentially measured the av-erage contact resistance of �ip-chip joints processed on the rigid Si substrate, �exible FPCB substrate, and stretchable and compliant FPCB/PMDS substrate. In this paper, which is the second part of our study, we investigated the �ip-chip bonding behavior on the �exible FPCB substrate and stretchable FPCB/PMDS substrate.

2.　 Experimental Procedure

The fabrication process of a Si chip to mount on the FPCB and FPCB/PDMS substrates was described in detail in our previous report regarding the �ip-chip behavior on rigid Si substrates.27) The 78-μm-thick �exible FPCB sub-strate consisted of 45-μm-thick polyimide, 18-μm-thick Cu with the daisy-chain pattern, and 15-μm-thick photosol-der-resist (PSR). Forty-two pads of 135-μm-diameter and 600-μm-pitch were fabricated by treating the Cu surface, which was exposed through the PSR, with the 4-μm-thick electroless Ni and 30-mm-thick immersion gold (ENIG).

Figure 1 shows the process �ow to fabricate the FPCB/PDMS composite substrate by locally attaching the more rigid FPCB to the more compliant PDMS using silicone/acrylic double-coated tape, which consists of silicone pres-sure-sensitive adhesive coated on one side of a polyester �lm carrier and acrylic pressure-sensitive adhesive coated on the other side of the carrier. The FPCB part, where the acryl-ic-adhesive side of the silicone/acrylic double-coated tape was bonded, was attached to the half-cured PDMS using the silicone-adhesive side of the double-coated tape. The half-cured PDMS was formed by mixing the base of Dow Corn-ing Sylgard 184 with its curing agent at a weight ratio of 10  :  1 and curing at 60°C for 25 min. Then, the PDMS, * Graduate Student, Hongik University

Materials Transactions, Vol. 58, No. 8 (2017) pp. 1217 to 1222 ©2017 The Japan Institute of Metals and Materials

where the FPCB was attached, was fully cured at 60°C for 12 hr to complete the formation of the FPCB/PDMS com-posite substrate of 40 mm ×  18 mm ×  1 mm-size. Figure 2 shows the optical images of the �exible FPCB substrate and stretchable FPCB/PDMS substrate.

After dispensing the ACA, where Au-coated polymer beads of 4-μm-diameter were dispersed as conductive parti-cles, the Cu/Au chip bumps were aligned onto the substrate pads and bonded together by curing the ACA at 160°C for 150 sec with a bonding pressure of 10–300 MPa. The aver-age contact resistance of the �ip-chip joint was analyzed by measuring the daisy-chain resistance. Cross-sectional micro-structures of the �ip-chip joints were observed using scan-ning electron microscopy (SEM).

3.　 Results and Discussion

The average contact resistances of the �ip-chip joints that formed on the �exible FPCB and stretchable FPCB/PDMS substrates are shown in Fig. 3. For comparison, the average contact resistance measured on the rigid Si substrates27) is also plotted in Fig. 3. On the FPCB substrate, the contact re-sistance remarkably reduced from 56.9 mΩ to 13.3 mΩ when the bonding pressure increased from 10 MPa to 200 MPa. However, the contact resistance substantially in-creased to 74.8 mΩ with an excessive deviation of ±37.7 mΩ at the bonding pressure of 300 MPa.

Cross-sectional micrographs of the �ip-chip joints formed on the FPCB substrate are shown in Fig. 4. The conductive particles became severely deformed with increasing the

bonding pressure. However, unlike the case for the rigid Si substrate,27) their intercalation into the Au layer of the chip bump was not observed on the FPCB substrate even at bond-ing pressures above 100 MPa, which can be due to the cush-ioning effect of the compliant FPCB substrate. Contrary to other specimens processed at a bonding pressure up to 200 MPa, complete cracking at the ACA resin between the chip and the substrate was observed for four samples bonded at 300 MPa after their mounting and polishing for micro-structural observation. The elastic deformation of the com-pliant FPCB substrate increased at higher bonding pressure

Fig. 2　Optical images of the (a) �exible FPCB substrate and (b) stretch-able PDMS/FPCB substrate.

Fig. 3　Average contact resistances on the �exible FPCB and stretchable FPCB/PDMS in comparison with the contact resistance on the rigid Si substrate.

Fig. 4　Cross-sectional scanning electron micrographs of the �ip-chip joints formed on the FPCB substrate with a bonding pressure of (a) 10 MPa, (b) 50 MPa, (c) 100 MPa, (d) 200 MPa, and (e) 300 MPa.

Fig. 1　Fabrication process of a PDMS/FPCB composite substrate: (a) at-tachment of the double-coated adhesive tape to FPCB; (b) attachment of FPCB to half-cured PDMS; (c) full curing of the FPCB-attached PDMS.

1218 D. Park, K.-S. Han and T. S. Oh

and caused more downward de�ection at the area under the chip bumps and further upward displacement in other re-gions. When bonded at a bonding pressure of 300 MPa, the restoring force of the elastically deformed FPCB substrate to its original shape became suf�ciently large to cause a large deviation in the measured contact resistance and completely separated the chip from the substrate during the microstruc-tural sample preparation. The downward de�ection of the FPCB substrate at the area under the chip bump made the edge of the chip bump touch the FPCB substrate, which hin-dered the full compression of the ACA conductive particles under the chip bump. Hence, the contact resistance increased with increasing bonding pressure from 200 MPa to 300 MPa, as in Fig. 3.

As shown in Fig. 5, the FPCB/PDMS substrate severely distorted with the chip burrowed into it at 50 MPa and 100 MPa because of the softness of the PDMS. Although the contact resistance of the �ip-chip joints on the FPCB/PDMS substrate was measured at 50 MPa and 100 MPa, as in Fig. 3, the appropriate bonding pressure for the �ip-chip process on this stretchable substrate of soft PDMS can be considered as low as 10 MPa. The contact resistance de-creased from 43.2 mΩ to 31.2 mΩ when the bonding pres-sure increased from 10 MPa to 50 MPa, but it slightly in-creased to 35.0 mΩ at the bonding pressure of 100 MPa. This contact resistance variation vs. the bonding pressure is similar to that observed on the FPCB substrate, whereas the critical bonding pressure, beyond which the contact resis-tance increased, decreased to 50 MPa for the softer FPCB/PDMS substrate from 200 MPa for the less compliant FPCB substrate. In Fig. 6, the cross-sectional micrographs of the �ip-chip specimens on the FPCB/PDMS substrate show that the local downward de�ection of the substrate under the bump prevented the full compression of the ACA particles at the central region of the bump compared to those under the edge of the bump. The comparison of the contact resistance data on the FPCB and FPCB/PDMS substrates with their microstructures shows that the critical bonding pressure, be-yond which the contact resistance increases, decreases for

the more compliant substrate.To characterize the deformation behavior of the rigid Si,

�exible FPCB, and stretchable FPCB/PDMS substrates as a function of the bonding pressure, the bondline thickness (BLT) was measured for the �ip-chip samples on each sub-strate. Here, the BLT was de�ned as the actual thickness of the ACA resin that remains between the chip and the sub-strate.28,29) The BLT is the distance between the chip elec-trode and the SR layer of the FPCB substrate, but it is the distance from the chip electrode to the top of the Si sub-strate, where the SR layer is absent. The deviation of the BLTs measured at different positions of a �ip-chip sample becomes more noticeable with more non-uniform local de-formation of the substrate during the �ip-chip bonding. To clarify the difference in BLT on each substrate, the BLTs were measured adjacent to the chip bump, where the local bonding pressure was most severely applied, and at the mid-dle of two chip bumps, where the bonding pressure was least applied, and ΔBLT was calculated.

Figure 7 shows ΔBLT on the Si, FPCB, and FPCB/PDMS

Fig. 5　Top-view and side-view optical images of the �ip-chip specimens that were processed on the FPCB/PDMS substrate with a bonding pres-sure of (a) 10 MPa, (b) 50 MPa, and (c) 100 MPa.

Fig. 6　Cross-sectional scanning electron micrographs of the �ip-chip joints formed on the FPCB/PDMS substrate with a bonding pressure of (a) 10 MPa, (b) 50 MPa, and (c) 100 MPa.

1219Comparison of Flip-Chip Bonding Characteristics on Rigid, Flexible, and Stretchable Substrates: Part II.

substrates as a function of the bonding pressure. On the Si substrate, ΔBLT was notably small and less than 0.35 μm at all bonding pressures, which is af�rmed with the cross-sec-tional microstructures in Fig. 8. On the FPCB substrate, ΔBLT increased from 0.72 μm to 4.07 μm when the bonding pressure increased from 10 MPa to 200 MPa. For the sample bonded at 300 MPa, the BLT could not be measured because of the delamination between the chip and the substrate during the specimen preparation for microstructural observa-tion. The cross-sectional microstructures of the �ip-chip specimens bonded at 200 MPa and 300 MPa are compared in Figs. 9(d) and (e), which show the more downward de-�ection of the substrate under the bump for the specimen bonded at 300 MPa. Thus, ΔBLT is larger with more upward warpage of the substrate between the chip bumps. For the FPCB/PDMS substrate, which has the largest compliance among the three substrates, ΔBLT was 2.4 μm even at 10 MPa and rapidly increased to 11.9 μm and 12.9 μm at 50 MPa and 100 MPa, respectively, with severe deformation of the substrate, as shown in Fig. 10. The elastic moduli of Si, FPCB, and PDMS are 110 GPa, 2.8 GPa, and 1.7 MPa, respectively.19,30) The more signi�cant deformation of the softer substrate during �ip-chip bonding increased ΔBLT in the order of FPCB/PDMS, FPCB, and Si substrate, as shown in Fig. 7.

In our previous work,27) we correlated the average contact resistance of the �ip-chip joints on the rigid Si substrate with the average contact area of ACA conductive particles trapped between the chip bump and the rigid glass substrate. However, this correlation cannot be directly applied to the average contact resistance of the �ip-chip joints processed on the soft FPCB and FPCB/PDMS substrates because of their cushioning effect. To characterize the different defor-mation of the ACA conductive particles on the Si, FPCB, and FPCB/PDMS substrates at identical bonding pressures, we measured the gap distance between the chip bump and the substrate pad on each substrate. In Fig. 11, the bump-pad

gap on each substrate decreased with increasing the bonding pressure because of the more severe deformation of the trapped ACA conductive particles. On the rigid Si substrate, the bump-pad gap was smaller than those on the FPCB and

Fig. 7　Difference in bondline thickness (ΔBLT) of the �ip-chip specimens processed on the Si, FPCB, and FPCB/PDMS substrates as a function of the bonding pressure.

Fig. 8　Cross-sectional scanning electron micrographs of the �ip-chip specimen processed on the Si substrate at a bonding pressure of (a) 10 MPa, (b) 50 MPa, (c) 100 MPa, (d) 200 MPa, and (e) 300 MPa.

Fig. 9　Cross-sectional scanning electron micrographs of the �ip-chip specimen processed on the FPCB substrate at a bonding pressure of (a) 10 MPa, (b) 50 MPa, (c) 100 MPa, (d) 200 MPa, and (e) 300 MPa.

1220 D. Park, K.-S. Han and T. S. Oh

FPCB/PDMS substrates and became zero at the bonding pressures of 200 and 300 MPa because of the direct contact of the chip bump to the substrate pad. The larger bump-pad gap measured on both FPCB and FPCB/PDMS substrates could be attributed to the cushioning effect of the compliant substrates, which made the deformation of the ACA conduc-tive particles less severe than that on the rigid Si. The varia-tion of the bump-pad gap vs. the bonding pressure was con-sistent with the change in contact resistance, as shown in Fig. 3.

4.　 Conclusion

On the FPCB substrate, the contact resistance was re-markably reduced from 56.9 mΩ to 13.3 mΩ when the bonding pressure increased from 10 MPa to 200 MPa. How-

ever, it substantially increased to 74.8 mΩ with an excessive deviation of ±37.7 mΩ at the bonding pressure of 300 MPa, which could be attributed to the severe cushioning effect of the compliant FPCB substrate. On the stretchable FPCB/PDMS substrate, the contact resistance was reduced from 43.2 mΩ to 31.2 mΩ with increasing bonding pressure from 10 MPa to 50 MPa; then, it slightly increased to 35.0 mΩ at a bonding pressure of 100 MPa. The bonding pressure, be-yond which the contact resistance was increased, ap-proached 50 MPa on the FPCB/PDMS substrate because of the larger cushioning effect of the softer but more elastomer-ic PDMS at 200 MPa on the less compliant FPCB substrate. The deformation behavior of the trapped ACA conductive particles during �ip-chip bonding was also differentiated by the stiffness of the substrate because a more compliant sub-strate exhibited more cushioning effect, which hindered their deformation. The FPCB/PDMS substrate was severely dis-torted at bonding pressures of 50 MPa and 100 MPa, where the chip burrowed into it because of the softness of the PDMS. Thus, a low bonding pressure is more adequate for �ip-chip bonding on a more compliant substrate. The varia-tion of the bump-pad gap distance with the changing bond-ing pressure on the rigid Si substrate, �exible FPCB sub-strate, and stretchable FPCB/PMDS substrate is consistent with the contact-resistance-change behavior of the �ip-chip joints processed on each substrate.

Acknowledgement

This work was supported by the ICT R&D programs of MSIP/KEIT of Korea (Project No. B0101-16-0420, Devel-opment of Transformational and Slap-on Wearable Device and UI/UX).

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Fig. 10　Cross-sectional scanning electron micrographs of the �ip-chip specimen processed on the FPCB/PDMS substrate at a bonding pressure of (a) 10 MPa, (b) 50 MPa, and (c) 100 MPa.

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