Size dependent dewetting and sideband reaction of eutectic SnPb on Au/Cu/Cr multilayered thin film
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Size dependent dewetting and sideband reaction of eutectic SnPb on AuyyCuyyCr multilayered thin film D. W. Zheng, Z. Y. Jia, C. Y. Liu, Weijia Wen, and K. N. Tu Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095-1595 (Received 13 November 1997; accepted 30 January 1998)
˚ ˚ layered structure Dewetting of eutectic SnPb on blank Au(500 A)yCu(1 mm)yCr(800 A) was found to have a solder size dependence. At 250 ±C, if the solder weight fell below 4 mg, dewetting occurred from the center of the solder cap; if the solder weight went beyond 6 mg, dewetting happened from the cap edge. In the latter case, a smaller cap with a higher wetting angle was formed at the center and a ring of solder was left around the edge. Large voids were left in the solder cap after dewetting in both cases. In contrast, all solder caps were found to dewet from the edge on a patterned film at 250 ±C if the solder ball was large enough to wet the whole film initially, irrespective of the solder size. For comparison, pure Sn, eutectic SnAg, and eutectic SnBi caps also dewetted from the edge of the AuyCuyCr thin film, irrespective of the solder size or whether the substrate was patterned. Since eutectic SnPb on blank AuyCuyCr is the only case in which a large sideband growth was found and the dewetting occurred from the center, we postulated sideband to be the main factor which controls the unusual dewetting. The link between them is discussed. Reactive wetting is a very complicated problem of liquid-solid interaction. It has attracted much attention during the past few years1–5 owing to the wide application of solder joints in electronic packaging technology. The formation of a solder joint is due to intermetallic compound formation between the solder and the metals to be joined. Yet the nature of solder wetting involving intermetallic compound formation remains unclear. For example, when eutectic SnPb wets Pd, no equilibrium or stable wetting angle can be found.6 While a stable wetting angle does exist in the wetting of eutectic SnPb on Cu, there is a peculiar and rapid precursor (sideband) formation.7,8 Also when Sn–Pb alloy with different Sn composition wets the Cu surface, there is a strong dependence of wetting angle on Sn composition.9 More importantly, when molten solder consumes a layered thin film structure and reaches a nonwettable substrate surface, what happens then is unclear; it is one of the classical instability problems of fluid flow on a dry surface.10 This is a very serious issue in electronic packaging, because it can lead to dewetting.11,12 It is an urgent problem because at the moment the trend in electronic packaging is to use more and more solder joints in flip-chip technology,13 and there is no clear solution to this dewetting problem yet. In this paper, we report the size dependent dewetting behavior of eutectic SnPb ˚ on unpatterned (blank) and patterned Au(500 A)yCu˚ (1 mm)yCr(800 A) layered thin films and compare this J. Mater. Res., Vol. 13,
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