Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel Shear properties of In-Bi alloy joints with Cu substrates during thermal aging Sanghun Jin a,b, ⁎ , Min-Su Kim a , Shutetsu Kanayama c , Hiroshi Nishikawa a a Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan b Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 560-0871, Japan c Connected Solutions Company, Panasonic Corporation, 1-15 Matsuo-cho, Kadoma, Osaka 571-8504, Japan ABSTRACT This paper describes the shear properties of solder joints using In-Bi alloys for flexible and wearable electronic devices. The effect of thermal aging on the shear strength of the In-Bi joints on Cu substrates prepared under various aging conditions was examined. In addition, the cross-sectional microstructures of the Cu/In-Bi alloy/Cu joints were investigated after shear testing. 1. Introduction The emergence of flexible and wearable electronic devices has created a demand for highly compact high-performance devices [1]. Flexible electronic applications require bendable and flexible substrates such as polypropylene (PP) and polymethyl methacrylate (PMMA) [2]. These substrates have low-melting temperatures below 160 °C [3]. Therefore, researchers have proposed various low-temperature alloys such as Sn-Zn and Sn-Bi systems for use as solder [4]. However, these alloys have some limitations as they are brittle and susceptible to oxi- dation [5]. Furthermore, they still have high-melting temperatures, which make it difficult to apply them to flexible substrates such as PP and PMMA. Thus, it is necessary to develop filler materials which can be soldered at temperatures below 100 °C to avoid thermal damage to the temperature-sensitive components in flexible electric devices. We therefore propose In-Bi alloys as a candidate for low-temperature Pb- free solder alloys. Indium-based alloys exhibit low-melting tempera- tures, extreme softness, and ductility [6]. In our previous research, the basic thermal and mechanical properties of In-Bi binary alloys were investigated. In-Bi alloys with melting temperatures of approximately 72.7 and 88.6 °C were selected for use in this study. The purpose of this study is to evaluate the shear strengths of the Cu/In-Bi alloy/Cu joints and analyze the interfacial reactions as a function of isothermal aging to evaluate their long-term reliability. 2. Experimental procedures 2.1. Heating by reflow oven We selected four alloy compositions with 50, 60, 66.3, and 70 mass % In, denoted as In-50Bi, In-40Bi, In-33.7Bi, and In-30Bi, respectively, having melting temperatures of approximately 72.7 and 88.6 °C based on the In-Bi binary phase diagram. The In-Bi alloys were prepared from indium (99.99% pure, Nilaco) and bismuth (99.99% pure, Nilaco) as starting materials. Table 1 shows the chemical compositions of the al- loys used in this study. Each alloy was melted in a furnace at 500 °C for 5 h before being cast in a steel mold. The alloying process was per- formed in air and the molten alloys were frequently stirred to ensure homogeneity. Alloy sheets were fabricated by casting and cold rolling processes. Then, the alloy sheets were sandwiched between Cu discs. Before making a Cu/In-Bi alloy/Cu joint sample, 10-mm diameter Cu discs were cleaned using 4 vol% HCl and ethanol. Alloy sheets were attached to the surfaces of the Cu discs using a mildly activated 12 wt% rosin flux. Then 3-mm diameter Cu discs, which were cleaned using the aforementioned method, were placed on top of the alloy sheet, as shown in Fig. 1(a). The samples were placed inside the reflow oven, preheated at 50 °C for 90 s, and heated to 100 °C for 600 s in a nitrogen atmosphere. The heating profile of this process is shown in Fig. 1(b). In this study, the samples were used as the average over six samples for each alloy composition. 2.2. Thermal aging and IMC thickness measurement The Cu-Cu joint specimens were aged at 40 and 60 °C for 168, 504, and 1008 h in an oil bath to evaluate the morphology and intermetallic compound (IMC) growth near the interface of the joints. The IMC thickness at the interface between the alloy and the Cu disc was mea- sured and analyzed to identify the phases in a cross-section of the specimen. The thicknesses of the IMC layers were determined using ImageJ quantitative analysis software. The average thickness was cal- culated by measuring the area of the compound and dividing it by the total length of the compound. In this work, the thickness of the IMC https://doi.org/10.1016/j.microrel.2018.07.046 Received 1 June 2018; Received in revised form 27 June 2018; Accepted 4 July 2018 ⁎ Corresponding author at: Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. E-mail address: passionista82@gmail.com (S. Jin). Microelectronics Reliability 88–90 (2018) 795–800 0026-2714/ © 2018 Elsevier Ltd. All rights reserved. T