Research Papers

40 μm Silver Flip-Chip Interconnect Technology With Solid-State Bonding

[+] Author and Article Information
Chu-Hsuan Sha, Chin C. Lee

Electrical Engineering and Computer Science, Materials and Manufacturing Technology,  University of California – Irvine, Irvine, CA 92697-2660

J. Electron. Packag 133(3), 031012 (Sep 30, 2011) (4 pages) doi:10.1115/1.4004660 History: Received January 27, 2011; Revised June 13, 2011; Published September 30, 2011; Online September 30, 2011

Formation of pure silver (Ag) flip-chip interconnect of silicon (Si) chips on copper (Cu) substrates is reported. Arrays of Ag columns, each 36 μm in height and 40 μm in diameter, are fabricated on 2-in. Si wafers which are first coated with chromium (Cr)/gold (Au) dual layers. The Si wafers are diced into 6 mm × 6 mm chips, each having 50 × 50 Ag columns. The Si chip with Ag columns is directly bonded to Cu substrate at 260 °C in 80 mTorr vacuum to inhibit oxidation. The static bonding pressure is as low as 680 psi (4.69 MPa), corresponding to a load of 0.021 oz (0.60 g) per column. During bonding, the Ag columns deform and conform to the Cu substrate. They are well bonded to the Cu. No molten phase is involved in the bonding process. The joints consist of pure Ag only. The ductile Ag joints are able to accommodate the thermal expansion mismatch between Si and Cu. It is well known that in nearly all soldering processes used in electronic industries, intermetallic compound (IMC) formation is essential to make a solder joint. In the pure Ag interconnect, no IMCs exist. Thus, reliability issues associated with IMCs are eliminated. Compared to tin-based lead-free solders, pure Ag joints have superior electrical and thermal properties.

Copyright © 2011 by American Society of Mechanical Engineers
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Grahic Jump Location
Figure 1

SEM images of Ag columns after photoresist stripping, viewed at 45 deg angle at (a) low magnification (×200), (b) medium magnification (×500), and (c) high magnification (×2000)

Grahic Jump Location
Figure 2

Low magnification SEM images on the cross section of the samples bonded under pressure of (a) 960 psi (6.62 MPa), (b) 760 psi (5.24 MPa), and (c) 680 psi (4.69 MPa), respectively

Grahic Jump Location
Figure 3

SEM images of the samples bonded under pressure of 680 psi (4.69 MPa), (a) at low magnification of (×800), showing the Ag joint has roughly maintained the shape of Ag column, (b) at medium magnification (×2000), revealing a single Ag joint, (c) at high magnification (×10000), showing Si/Ag interface, where the thin Cr/Au layers between Si and Ag cannot be seen, and (d) at high magnification (×10000), Ag/Cu interface is revealed



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