Experimental study of the kinetics of transient liquid phase solidification reaction in electroplated gold-tin layers on
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I.
INTRODUCTION
TRANSIENT liquid phase (TLP) bonding is a diffusional bonding process used for joining different alloys. The process utilizes an interlayer, which is introduced between the two metallic surfaces to be bonded. Upon heating and maintaining the trilayer at a constant, sufficiently elevated temperature, the interlayer melts and then solidifies as a result of interdiffusion with the base material. The bonding process is applicable to eutectic and peritectic alloy systems, where the liquidus temperature can be lowered by the interdiffusion of elemental components between the two materials to be joined. In such systems, depending on the rate of heating, the liquid formed initially is a direct result of this interdiffusion. The Au-Sn system is a typical example. In systems such as Au-In, where the lowest melting temperature is that of one of the elemental components (In), the formation of the initial liquid does not depend on the interdiffusion between the two components. Consider a hypothetical phase diagram exhibiting a eutectic transition, as shown in Figure 1(a). The trilayer to be joined is composed of a thin layer of M2 sandwiched between relatively thick layers of M1, as shown schematically in Figure 1(b). Tuah-Poku et al.[1] have identified the following four different stages in the TLP joining process of such a system. (a) Dissolution of the layer M2 at temperature T, which is higher than the eutectic temperature Teu. After dissolution, the composition of the liquid at the interfaces is dictated by the appropriate tie-lines in the phase diagram. (b) Homogenization of the liquid, leading to a widening of the liquid layer. Complete mixing of the components in the liquid can be assumed so that the maximum liqRAMNATH VENKATRAMAN, formerly with IBM Microelectronics Division, is Process Engineer, Motorola Inc., Austin, TX 78721. JAMES R. WILCOX, Development Engineer, and STEPHEN R. CAIN, Technology Assurance Mathematician, are with IBM Microelectronics Division, Endicott, NY 13760. Manuscript submitted March 28, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
uid zone width corresponds to liquid composition c2. This step is controlled both by diffusion of M2 in the liquid and in the solid.[1] (c) Isothermal solidification. Here, the liquid zone decreases in width. The parameter controlling the kinetics of this process is the diffusion of the component M2 through the solid M1, as is evident from the schematic in Figure 2. (d) Homogenization of the solidified joint. One of the candidates for TLP joining is the Au-Sn system. The Au-Sn system and its phase equilibria have been well documented.[2,3,4] The Au-Sn alloy system is used extensively in the electronics packaging industry in joining applications. For example, Au-Sn has been used for the bonding of inner lead bonds on tape-automated bond (TAB) packages. Bonds can be formed between gold bumps on a chip and a tin-plated copper lead on the TAB package.[5,6] Lee and co-workers[7,8] have reported the use of Au-Sn eutectic alloy to bond GaAs die to alum
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