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Low-pressure solid-state bonding technology using fine-grained silver foils for high-temperature electronics

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Abstract

A solid-state bonding technique using fine-grained silver (Ag) foils is presented. The Ag foils are manufactured using many runs of cold rolling and subsequent annealing processes to achieve the favorable microstructure. X-ray diffraction and pole figure measurement are performed to examine the crystal structure and grain orientations. Si chips are bonded to bare Cu substrates using the Ag foil as the bonding medium at 300 °C in 0.1 torr vacuum assisted by 6.9 MPa static pressure, which is much lower than that used in conventional thermal compression bonding. Cross sections prepared by focus ion beam show clear bonding interfaces with only a few voids smaller than 100 nm. The bonded structures do not crack after cooling down to room temperature, indicating that the ductile Ag layer is able to manage the strain induced by the large coefficient of thermal expansion mismatch between Si and Cu. The average shear strength of as-bonded samples is 29 MPa. High-temperature storage tests are conducted, and slight increase in strength can be observed after 300 °C aging. Fracture analyses show that the breakage occurs within the Ag foil rather than on the bonding interface. Transmission electron microscopy and energy-dispersive spectroscopy (TEM/EDX) are conducted for Ag/Cu interface after 200-h aging, and the result shows that slight diffusion proceeds during the aging. Since Ag has the highest electrical and thermal conductivities among metals, therefore the bonded structures reported in this paper probably represent the best possible design for high-temperature and high-power electronic packaging applications.

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Acknowledgement

XRD, in-plane pole figure measurement, and SEM/FIB and TEM/EDX work were performed at UC Irvine Materials research Institute (IMRI).

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Authors and Affiliations

  1. Electrical Engineering and Computer Science, University of California, Irvine, CA, 92697-2660, USA

    Jiaqi Wu & Chin C. Lee

  2. Materials and Manufacturing Technology, University of California, Irvine, CA, 92697-2660, USA

    Jiaqi Wu & Chin C. Lee

Authors
  1. Jiaqi Wu
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  2. Chin C. Lee
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Correspondence to Jiaqi Wu.

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Wu, J., Lee, C.C. Low-pressure solid-state bonding technology using fine-grained silver foils for high-temperature electronics. J Mater Sci 53, 2618–2630 (2018). https://doi.org/10.1007/s10853-017-1689-y

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  • DOI: https://doi.org/10.1007/s10853-017-1689-y

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