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Nano-materials in Anisotropic Conductive Adhesives (ACAs)

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Nanopackaging

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Abstract

Display-related electronics such as smartphones, laptops, and UHD TVs have become an indispensable part of our lives. In line with this, as people are demanding images with higher quality, the number of electrical paths from a display driver chip to a display panel is increasing from HD to full HD to UHD and more, resulting in a reduction of the electrode pitch, which is the center-to-center distance between nearby electrodes. The most critical issue facing current display devices is interconnecting the fine pitch driver chips on the display panels using anisotropic conductive adhesives (ACAs) without an electrical short-circuit problems, since conductive particles in the ACAs can be agglomerated between fine pitch electrodes during the ACA bonding process, thereby causing electrical shorts in the X-Y directions. In this paper, a new concept of nanofiber ACAs has been introduced by incorporating conductive particles into nanofibers to suppress conductive particle movement and obtain stable three-dimensional electrical interconnection properties of fine pitch electronics.

The novel nanofiber ACAs incorporate conductive particles into conductive particle incorporated nanofiber (CPIN) structures to obtain stable electrical properties of fine pitch display devices. The conductive particle movements during ACA adhesive resin flow are fundamentally suppressed by the CPIN structure fabricated by an electrospinning method. The nanofiber ACAs show superior properties compared with conventional ACAs, providing 2.7 times higher particle capture rate and perfect electrical insulation properties at 20 μm fine pitch interconnections.

In addition, the effect of nanofiber material properties on the nanofiber ACA interconnection stability was also investigated in terms of tensile and thermal properties of nanofiber materials. The Nylon 6 nanofiber showed the highest ultimate tensile strength, 19.2 MPa, whereas PVDF and EVOH nanofibers showed values of 16.2 MPa and 9.4 MPa, respectively, with the highest conductive particle capture rate by the Nylon 6 nanofiber. Although the three kinds of nanofiber ACAs showed different capture rates, they all had 100% X-Y axis insulation properties at 20 μm pitch interconnections of chip-on-flex (COF) assembly. The Z-axis contact resistance of all samples rapidly decreased as the nanofibers melted and stabilized at 4~6.4 mΩ above the nanofiber melting temperature.

Furthermore, the fine pitch COF assembly using nanofiber solder (Sn3.0Ag0.5Cu) ACAs was also investigated. The nanofiber solder ACAs offer many advantages such as suppressing micro-solder ball movement during ACAs resin flow, perfect X-Y axis insulation at 25 μm fine pitch, 30% lower electrical contact resistance, and excellent unbiased autoclave reliability. Micro-solder balls were successfully incorporated into a nanofiber structure, and they had good solderability within the nanofiber/epoxy matrix. As a result, (Au,Cu)Sn and Cu3Sn IMCs were formed by interfacial reaction of Sn3.0Ag0.5Cu solder balls, Au bumps, and Sn-finished Cu. The continuous IMCs formed in micro-solder ball joints lowered the Z-axis contact resistances and significantly improved moisture resistance compared with the physical contact-based conventional polymer ball ACAs. Therefore, nanofiber solder ACAs can provide an alternative solution for fine pitch interconnections of various electronic assemblies such as COF, COG, and 3D chip stacks.

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Authors and Affiliations

  1. Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology KAIST, Daejeon, South Korea

    Kyung-Wook Paik & Kyung-Lim Suk

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  1. Kyung-Wook Paik
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  2. Kyung-Lim Suk
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Correspondence to Kyung-Wook Paik .

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  1. Department of Electrical and Computer Engineering, Portland State University, Portland, OR, USA

    James E. Morris

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Paik, KW., Suk, KL. (2018). Nano-materials in Anisotropic Conductive Adhesives (ACAs). In: Morris, J. (eds) Nanopackaging. Springer, Cham. https://doi.org/10.1007/978-3-319-90362-0_12

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