Microstructural discovery of Al addition on Sn–0.5Cu-based Pb-free solder design
Graphical abstract
The minor Al additions refined eutectic Cu6Sn5 IMC networks on the Sn–0.5Cu based solder alloys. The microstructure was dramatically changed with the minor Al addition.
Introduction
Sn–Pb solder alloy has long been used for electronic applications due to its outstanding solder properties [1]. In 2002, the European Union passed the RoHS (Restriction of Hazardous Substances) legislation and prohibited the use of Pb in most electronics with the exception of automobile parts. However, the exemption clause in the RoHS for automobile parts will terminate in 2016. Therefore, developing Pb-free solder alloys suitable for automobiles is an urgent matter.
The engine control unit (ECU) box that is placed in engine rooms contains many electronic chips and devices. In the engine rooms, the melting temperature of the Pb-free solder is a critical point from a view of safety. Johnson et al. [2] reported that the ECU box reaches 150–200 °C in gasoline engine vehicle. In addition, hybrid electric vehicle also needs high temperature packaging above 200 °C for their operation [3]. Thus, we should consider a melting temperature of the designed solder alloy higher than 200 °C to ensure its reliability.
Many researchers have studied Pb-free solder systems in a wide temperature range: Sn–Zn (198 °C) [4], Sn–Zn–Bi (189 °C) [5], [6], Sn–Ag (221 °C), Sn–Ag–Cu (215 °C) [7], [8], Sn–Cu (227 °C), Sn–Cu–Pd–Cr–Ca (229 °C) [9], Sn–Sb (239 °C) [10] and Cu–Sn–Sb (230 °C) [11]. The Sn–Ag–Cu (SAC) solder alloy has a fairly high melting temperature of approximately 215 °C [7], [8]. However, this alloy is costly because of its silver content, and there are some reliability concerns related to Ag3Sn intermetallic compound (IMC) growth [12], [13], [14]. This IMC affects the thermal shock and mechanical properties of smaller solder joints due to the trend of the miniaturization of electronic circuits [15]. In addition, to reduce problematic Ag3Sn IMC formation, the use of a lower Ag content reduces the mechanical strength and thermal shock/fatigue properties [16].
In recent papers, the Al addition to the SAC solders was studied due to the resulting enhancement of the mechanical properties. Sabri et al. [17] reported that Al addition (0.1–0.5 wt.%) increased the ductility and toughness for the Sn–1Ag–0.5Cu solder alloy. Alam and Gupta [18] reported that the Al addition (0.4–0.6 wt.%) significantly improved the ductility when compared to commercial Sn–0.7Cu solder. Li et al. [19] observed that Al (1 wt.%) slows down Sn–Cu IMC growth on a Cu substrate due to its effectiveness in forming a barrier layer at the interface. Most researchers added less than 1 wt.% Al to the Pb-free solder alloys because of the oxidative properties of Al. Al is a very oxidative and light metal [20]. Thus, the excessive Al content can cause a non-homogeneous microstructure between the surface and core due to floated Al during the alloying and soldering process [21]. However, using a small Al addition to avoid the side effects of excessive Al content, it is difficult to trace the Al effects in Pb-free solder alloys.
In this study, we developed Sn–0.5Cu wt.% Pb-free solder alloys for automotive parts that are exposed to temperatures above 200 °C. We adopted a minimal Al addition to avoid the reliability and cost problems of Ag addition in Pb-free solder alloys.
Section snippets
Casting of designed alloy
Pure Sn (99.9%), Cu (99.9%) and commercial Al alloy were used as raw materials. The Al contents was controlled in the range of the ppm level (0.01–0.05 wt.%). Pure Al has poor machinability to weight and alloy during the casting process. Here, the commercial Al alloy 4047 was selected to easily alloy minor Al contents due to its high machinability. In addition, it decreases the alloying cost and reliability issue together with eliminating the use of Ag. First, pure Sn and Cu were melted at
Microstructures
To compare the overall microstructure, the cross-sectioned microstructures of solder balls with different Al contents were observed using an optical microscope (OM) (Fig. 2). The dark gray small particles indicated mostly eutectic β-Sn and Cu–Sn IMC (in the text indicated as “eutectic β-Sn + Cu–Sn IMC”), which were continuously formed in the Sn–0.5Cu and Sn–0.5Cu–0.01Al solder alloys (Fig. 1(a–b)). However, with the increasing Al content (0.03–0.05 wt.%), refined eutectic β-Sn + Cu–Sn IMC
Conclusions
The Al addition induced the formation of the Cu–Al (δ-Cu33Al17) IMC, which effectively suppressed the formation of eutectic β-Sn + Cu6Sn5 networks in the Sn–0.5Cu-based solder matrix. We thermodynamically calculated the order of the solidified phases during the solidification. The δ-Cu33Al17 is solidified first, resulting in refined eutectic β-Sn + Cu6Sn5 networks with Al addition. Then, the primary β-Sn and eutectic β-Sn + Cu6Sn5 were continuously solidified. Based on the thermodynamic
Acknowledgments
This work was supported by the Global Technology Innovation Program (N02130092) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). We appreciate the Korea Basic Science Institute (KBSI, Daejeon) for the use of EPMA and EBSD. We appreciate the KAIST Analysis Center for Research Advancement (KARA, Deajeon) for the use of ion polisher, SEM, BSE, EDX, XRD, TEM and FIB.
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