Communication On the Asymmetric Growth Behavior of Intermetallic Compound Layers During Extended Reflow of Sn-Rich Alloy on Cu KYLE E. YAZZIE, JONATHAN TOPLIFF, and NIKHILESH CHAWLA When solder interconnects are fabricated, a Sn-based alloy is melted between two substrates with metalliza- tion layers, such as Cu or Ni. From the reaction between Sn and Cu, a Cu 6 Sn 5 intermetallic compound (IMC) layer is formed at the solder/Cu interfaces. The mor- phology of the IMC layer greatly influences the mechanical behavior of the solder joint. Here, we report on the characterization of a novel, asymmetric growth behavior of IMC layers in Sn-3.9Ag-0.7Cu solder joints, based on gravity-induced spalling of the IMC. DOI: 10.1007/s11661-012-1329-8 Ó The Minerals, Metals & Materials Society and ASM International 2012 Solders serve as electrical and mechanical intercon- nects in electronic packaging. The need to develop environmentally benign electronic packages has gener- ated 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 Cu 6 Sn 5 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 quasi- static [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 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-conductiv- ity copper bars (25 mm long and 6.35 mm diameter) to form butt joints. Copper bars were mechanically pol- ished 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 reproduc- ible joint thickness of ~500 lm. Temperatures during reflow and aging were measured using a thermocouple placed near the joint. The jig and solder joints were heated in a box furnace to a nominal reflow temperature of 503 K (230 °C). As-reflowed solder joints were held for 40 seconds above the melting temperature, removed from the oven, and air cooled on an aluminum block to obtain a cooling rate of about ~1 °C/s. The as-reflowed solder joints were an experimental control against which the changes in microstructure for extended reflow solder joints were compared. Extended reflow solder joints were held at the reflow temperature for 3, 12, 24, 96, and 168 hours, then removed from the oven, and cooled at ~1 °C/s. Solder joints were mounted in epoxy and cross sectioned. Solder joint cross sections were polished to a 0.05 lm colloidal silica finish. Scanning electron micros- copy (SEM; FEI-XL30; FEI Corporation, Hillsboro, OR) was used to characterize the solder joint micro- structure. Average intermetallic layer thickness was computed with at least 500 measurements from SEM micrographs. Energy-dispersive X-ray spectroscopy (EDS) was used to identify compositions of the solder constituents. The microstructural evolution of the IMC layers was characterized using backscattered SEM (BSEM) with solder joints that were reflowed for various times. BSEM micrographs of solder joints reflowed for 0 hours (as-reflowed), 3 hours, 12 hours, 24 hours, 96 hours, and 168 hours are shown in Figures 1(a) through (f), respectively. The as-reflowed Cu 6 Sn 5 IMC layer thick- nesses at the top and bottom interfaces were both 2.5 ± 0.6 lm, as shown in Figure 1(a). The IMC layer morphology is nodular, due to a thermal grooving process that occurs in the liquid state. [12] At 3 hours extended reflow, the top IMC layer became slightly irregular, and a continuous Cu 3 Sn IMC layer was detected, as shown in Figure 1(b). At 12 hours, large gaps began to form between the nodules in the top IMC KYLE E. YAZZIE, formerly Graduate Research Associate, Materials Science and Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287- 6106, 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: nchawla@asu.edu Manuscript submitted April 25, 2012. Article published online July 31, 2012 3442—VOLUME 43A, OCTOBER 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A