462 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 33, NO. 2, JUNE 2010 Silver Microstructure Control for Fluxless Bonding Success Using Ag–In System Pin J. Wang, Chu-Hsuan Sha, and Chin C. Lee, Fellow, IEEE Abstract —A fluxless bonding process is successfully developed between silicon (Si) chips and copper (Cu) substrates using the silver–indium (Ag–In) binary system. This is a new design concept that utilizes thick Ag plated over the Cu substrate to deal with the large mismatch in coefficient of thermal expansion between semiconductors, such as Si (3 ppm/°C) and Cu (17 ppm/°C). The Ag layer actually becomes a part of the Ag–Cu substrate. Ag is chosen for the cladding because of its superior physical properties of ductility, high electrical conductivity, and high thermal conductivity. Following the thick Ag layer, 5 μm In and 0.1 μm Ag layers are plated. The thin outer Ag layer inhibits oxidation of inner In. After many bonding experiments, we realize that the success of producing a joint relates to the microstructure of the Ag layer. Ag with small grains results in rapid growth of solid Ag 2 In intermetallic compounds through grain boundary diffusion. Thus, a joint is not obtained because of lack of molten phase (L). To coarsen Ag grains, an annealing step is added to the Ag-plated Cu substrate. This step makes Ag grains 200 times coarser compared to the as-plated Ag. The coarsened microstructure slows down the Ag 2 In growth. Consequently, the (L) phase stays at the molten state with sufficient time to react with the Ag layer on the Si chip to produce a joint. Nearly perfect joints are produced on Ag-plated Cu substrates. The resulting joints consist of pure Ag, Ag-rich solid solution, Ag 2 In, and Ag 3 In. The melting temperature exceeds 650 °C. Using the present process, high temperature joints of high thermal conductivity are made between Si chips and Cu substrates at low bonding temperature (200°C). We foresee the Ag–In system as an important system to explore for various fluxless bonding applications in electronic packaging. This system provides the possibilities of producing joints of wide composition choices and wide melting temperature range. This paper provides preliminary but useful information on how the microstructure of Ag affects the bonding results. Index Terms—Fluxless bonding, indium, silver, silver alloys, silver–indium alloys. I. Introduction I N ELECTRONIC products, copper (Cu) has been the most widely used conductor material. Cu is chosen because Manuscript received March 23, 2009; revised October 4, 2009. Date of current version June 9, 2010. Recommended for publication by Associate Editor J. H. L. Pang upon evaluation of reviewers’ comments. P. J. Wang is with Intel Corporation, Chandler, AZ 85226 USA (e-mail: pin.wang@intel.com). C.-H. Sha is with the Materials Manufacturing Technology Program, Uni- versity of California, Irvine, CA 92697 USA (e-mail: csha@uci.edu). C. C. Lee is with the Department of Electrical Engineering and Com- puter Science, University of California, Irvine, CA 92697 USA (e-mail: cclee@uci.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCAPT.2009.2038991 of high electrical conductivity, high thermal conductivity, comfortable rigidity, forging possibility, and low cost [1]–[3]. However, it is always a challenge to bond silicon (Si) chips to Cu substrates or electrodes because of large mismatch in coefficient of thermal expansion (CTE), i.e., 2.7 × 10 -6 /°C for Si and 17 × 10 -6 /°C for Cu [4]. To accommodate the CTE mismatch, tin (Sn)-based solders and silver epoxies are commonly used bonding media in industries. Neither of them has high thermal and electrical conductivities. They cannot sustain operating temperature beyond 220 °C. We previously achieved high temperature joints between Si chips and Cu substrates using the Ag–In system [5], [6]. The resulting joint consists of Ag-rich solid solution phase and pure Ag. The melting temperature is higher than 850 °C. To realize this joint configuration, a 280 μm thick Ag foil is first bonded directly onto the Cu substrate at 250 °C. This thick Ag cladding, serving as a buffer, can relieve shear displacement difference between Si chips and Cu substrates through plastic deformation. Ag is selected because of its low yield strength, only one-sixth of that of copper and 1.1 times of Sn–Ag eutectic alloy [7], [8]. In addition, Ag has the highest electrical conductivity (63 × 10 6 /m·) and highest thermal conductivity (429 W/m·K) among all metals. While direct bonding of Ag foils to Cu substrates works well, we have also looked into the electroplating process as an alternative means of manufacturing thick Ag claddings on Cu substrates. The electroplating technique allows Ag layers to be deposited selectively using a lithographic process. It is applicable to depositing Ag on pads of Cu electrodes where joints are needed. In this paper, the 85 μm Ag layer is electroplated on Cu substrates as a stress buffer. The Ag cladding also be- comes a part of the Ag–Cu substrate. Fluxless bonding is performed between Cu/Ag(plated)/In(plated)/Ag(plated) and Si/Cr/Au/Ag(plated) at 200 °C in 100-millitorr vacuum. The fluxless feature eliminates flux resides that may lead to cor- rosion and degradation of the joint quality [9]. During the process development, it is interesting to discover that the microstructure of Ag plated on Cu substrates can determine the success of joint formation. It is found that the In–Ag chemical reaction rate depends on the Ag grain size. To investigate the effect of Ag microstructure on bonding success, several bonding conditions and structures are performed and evaluated. In what follows, we first present the experiment design and procedures. Experimental results are reported and discussed. A short summary is then given. 1521-3331/$26.00 c  2010 IEEE