On the Asymmetric Growth Behavior of Intermetallic Compound Layers During Extended Reflow of Sn-Rich Alloy on Cu
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Solders serve as electrical and mechanical interconnects in electronic packaging. The need to develop environmentally benign electronic packages has generated great interest in Pb-free solder alloys.[1–6] The demand for smaller portable devices also drives a reduction in solder bump size and pitch. Pb-free solders are typically Sn rich, with small alloying additions of Ag and/or Cu. When solder interconnects are fabricated, the Sn-based alloy is melted between two substrates with metallization layers, such as Cu or Ni. From the reaction between Sn and Cu, a Cu6Sn5 intermetallic compound (IMC) layer is formed at the solder/Cu interfaces. The overall thickness and morphology of the IMC layer is known to have a significant influence on the mechanical behavior of solder joints under quasistatic[7] and mechanical shock conditions.[8–10] Indeed, Deng et al.[7] showed that not only does the IMC layer serve as a stress concentration site, but also this phenomenon can be exacerbated by the morphology of the IMC layer. Therefore, understanding the growth
KYLE E. YAZZIE, formerly Graduate Research Associate, Materials Science and Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 852876106, is now Senior Engineer, Assembly Test and Technology Development, Intel Corporation, Chandler, AZ 85226. JONATHAN TOPLIFF, Undergraduate Research Assistant, and NIKHILESH CHAWLA, Fulton Professor, are with Materials Science and Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University. Contact e-mail: [email protected] Manuscript submitted April 25, 2012. Article published online July 31, 2012 3442—VOLUME 43A, OCTOBER 2012
behavior and morphology of the IMC layer is critical to developing reliable electronic packages. The IMC layer thickness and morphology is affected by multiple factors, including the cooling rate of the solder joint upon solidification, isothermal aging, and extended reflow.[7,11] During extended reflow, the solder joint is held above the melting temperature for extended periods of time, allowing the liquid solder to react with the substrate and form thick IMC layers. In this article, we report on a novel IMC layer growth behavior. Sn-3.9Ag-0.7Cu solder joints were held in extended reflow for up to 168 hours. The resulting IMC layers exhibited an asymmetry with respect to the thicknesses of the top and bottom interfaces. Characterization of this novel growth behavior and discussions of possible mechanisms follow. Sn-3.9Ag-0.7Cu solder (Indium Corp., Utica, NY) was reflowed between two oxygen-free, high-conductivity copper bars (25 mm long and 6.35 mm diameter) to form butt joints. Copper bars were mechanically polished to a 0.05 lm colloidal silica finish. A mildly activated rosin flux was then applied to the end of the copper bars to promote wetting between the solder and Cu during reflow. A jig was used to create a reproducible joint thickness of ~500 lm. Temperatures during reflow and aging were measured using a thermocouple placed near the
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