metals Review Low Melting Temperature Sn-Bi Solder: Effect of Alloying and Nanoparticle Addition on the Microstructural, Thermal, Interfacial Bonding, and Mechanical Characteristics Hyejun Kang, Sri Harini Rajendran and Jae Pil Jung * Department of Materials Science and Engineering, University of Seoul, 163, Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, Korea; [email protected] (H.K.); [email protected] (S.H.R.) * Correspondence: [email protected] Abstract: Sn-based lead-free solders such as Sn-Ag-Cu, Sn-Cu, and Sn-Bi have been used extensively for a long time in the electronic packaging field. Recently, low-temperature Sn-Bi solder alloys attract much attention from industries for flexible printed circuit board (FPCB) applications. Low melting temperatures of Sn-Bi solders avoid warpage wherein printed circuit board and electronic parts deform or deviate from the initial state due to their thermal mismatch during soldering. However, the addition of alloying elements and nanoparticles Sn-Bi solders improves the melting temperature, wettability, microstructure, and mechanical properties. Improving the brittleness of the eutectic Sn-58wt%Bi solder alloy by grain refinement of the Bi-phase becomes a hot topic. In this paper, literature studies about melting temperature, microstructure, inter-metallic thickness, and mechanical properties of Sn-Bi solder alloys upon alloying and nanoparticle addition are reviewed. Keywords: low-temperature solder; Sn-58wt%Bi; melting temperature; microstructure; mechani- Citation: Kang, H.; Rajendran, S.H.; cal property Jung, J.P. Low Melting Temperature Sn-Bi Solder: Effect of Alloying and Nanoparticle Addition on the Microstructural, Thermal, Interfacial Bonding, and Mechanical 1. Introduction Characteristics. Metals 2021, 11, 364. In recent years, lead-free solders having high reliable metallurgical, mechanical, and https://doi.org/10.3390/met11020364 electrical properties have been widely applied not only to consumer appliances but also in industrial electronic products and automobile electronic products [1,2]. Electronic Academic Editor: Jan Vrestal packaging trends move to miniaturization, high-density packaging, and high performance. Fast data communication is required in 5G and IoT (Internet of Things) fields, which need Received: 20 January 2021 the development of a system in the package (SiP), package on a package (PoP), Wafer level Accepted: 17 February 2021 packaging (WLP), 2.5D &3D semiconductor package, system on chip (SOC), and System Published: 22 February 2021 Integration package (SIP) as packaging technologies [3]. Various lead-free solders have been proposed after the restriction of hazardous substances (RoHs) in the electronic industry. Publisher’s Note: MDPI stays neutral Typical lead-free solders include series of Sn-Ag (Sn-3.5wt%Ag) (all the compositions are in with regard to jurisdictional claims in weight percent unless stated otherwise), melting point 221 ◦C), Sn-Cu (Sn-0.7%Cu, melting published maps and institutional affil- point 227 ◦C), Sn-Bi (Sn-58%Bi, melting point 139 ◦C), Sn-In (Sn-52%In, melting point iations. 118 ◦C), Sn-Ni (Sn-0.1%Ni, melting point 231 ◦C), Sn-Zn (Sn-9%Zn, melting point 198 ◦C), Sn-Ag-Cu (Sn-3%Ag-0.5%Cu, melting point 217 ◦C) [4–13]. The difference in thermal expansion between the Si chip and the substrate increases due to the modularization and enlargement of electronic components. The higher the soldering temperature, the more Copyright: © 2021 by the authors. thermal deformation of electronics components, and, resultantly, defects appear frequently. Licensee MDPI, Basel, Switzerland. In addition, in moisture sensitive packaging, defects such as delamination or pop-corning This article is an open access article failure can be accentuated at a higher temperature [14]. To solve these problems, there is an distributed under the terms and increasing need for research on low melting point solders, such as Sn-Bi or Sn-In alloys [15]. conditions of the Creative Commons Sn-Bi solder is cost effective as compared to Sn-In and has better thermal reliability due Attribution (CC BY) license (https:// to its relatively higher melting temperature. The low melting point solder is suitable for creativecommons.org/licenses/by/ temperature-sensitive fabrics [16] and flexible substrates [17]. For example, Sn-Bi solder 4.0/). Metals 2021, 11, 364. https://doi.org/10.3390/met11020364 https://www.mdpi.com/journal/metals Metals 2021, 11, x FOR PEER REVIEW 2 of 26 Metals 2021, 11, 364 2 of 25 (http://creativecommons.org/li- melting point solder is suitable for temperature-sensitive fabrics [16] and flexible sub- censes/by/4.0/). strates [17]. For example, Sn-Bi solder can be used for the bonding of electronic devices oncan a bewearable used for or the flexible bonding substrate, of electronic like devicesa fabric, on as ashown wearable in Figure or flexible 1 for substrate, smart fabrics like a [16].fabric, However, as shown Sn-Bi in Figure eutectic1 for solder smart fabricshas some [ 16 drawbacks]. However, like Sn-Bi low eutectic ductility solder and has poor some fa- tiguedrawbacks properties like low[18]. ductility and poor fatigue properties [18]. Figure 1. Cross-sectional scanning scanning electron electron micrographs micrographs of of the the flip flip-chip-chip joints formed by flip-chip flip-chip bonding of the SnBi solder bumps toto thethe Cu Cu lead lead frame frame of of the the fabric fabric substrate substrate (a) ( withouta) without and and (b) with(b) with a ball-up a ball-up process process of the of SnBi the solderSnBi solder paste, paste,reproduced reproduced from [ 16from], with [16], permission with permission from Journal from Journal of Korean of Korean Microelectronics Microelectronics and Packaging, and Packaging, 2012. 2012. ToTo overcome the problems mentioned above, many studies have been performed, suchsuch as as grain grain refinement refinement by by the the addition addition of ofva variousrious alloying alloying elements elements like like Ag, Ag, Cu, Cu, Ni, Ni,In, Al,In, Sb, Al, and Sb, andso on so [19–22]. on [19– 22Addition]. Addition of nanoparticles of nanoparticles or fibers or fibers like CNT, like CNT, Y2O3, Y Al2O2O3,3, Al ZrO2O23,, TiOZrO2,2 ,Cu, TiO and2, Cu, Ni andto solders Ni to soldersis another is another approach approach [23–28]. [23The–28 epoxy-containing]. The epoxy-containing solders, whichsolders, form which an epoxy form an layer epoxy over layer the solder over the fille soldert, is also fillet, helpful is also to helpful enhance to shear enhance strength shear andstrength drop andreliability drop reliability [29–33]. Therefore, [29–33]. Therefore, in this paper, in this we paper, would we like would to introduce like to introduce the sig- nificantthe significant results resultscorresponding corresponding to the melting to the melting temperature, temperature, spreading spreading and wetting, and wetting, micro- structure,microstructure, inter-metallic inter-metallic thickness, thickness, and mechan and mechanicalical properties properties of Sn-Bi of solder Sn-Bi solderupon alloy- upon ingalloying and nanoparticle and nanoparticle addition. addition. A special A special focus focus has been has beengiven given to Sn-58Bi to Sn-58Bi composition composition due todue their to theirlow melting low melting temperature temperature and their and theireutectic eutectic microstructure. microstructure. 2.2. Melting Melting Temperature Temperature and Wetting Property of Sn-Bi Solders 2.1. Effect of Alloying Addition on the Melting Temperature of Sn-Bi Solder 2.1. Effect of Alloying Addition on the Melting Temperature of Sn-Bi Solder The Sn-Bi-based alloys, which is a low melting point solder that is increasingly applied The Sn-Bi-based alloys, which is a low melting point solder that is increasingly ap- to suppress thermal deformation of electronic parts and to low temperature process in plied to suppress thermal deformation of electronic parts and to low temperature process multi-reflow, should have a low melting point and reliable properties. Among the low- in multi-reflow, should have a low melting point and reliable properties. Among the low- temperature solders of Sn-Bi and Sn-In based, Sn-58%Bi is a representative solder with low temperaturecost, good solderability, solders of Sn-Bi and environmentally-friendly and Sn-In based, Sn-58%Bi characteristics. is a representative Sn-Bi-(0.2–0.5)%Ag solder with lowsolders cost, are good also popularsolderability, for a low-temperature and environmentally-friendly application, which characteristics. has high ductility, Sn-Bi-(0.2– good 0.5)%Agwettability, solders a suitable are also mechanical popular for property, a low-temperature and a low melting application, point [which34–37]. has high duc- tility,Sn-Bi-based good wettability, solders a havesuitable a range mechanical of melting property, temperatures and a low based melting on alloying point [34–37]. elements. Therefore, a detailed and intensive study is required to find suitable applications. Table1 shows the effects of various additive elements such as Ti, Cu, Ag, In, Sb, Co, Zn, and Metals 2021, 11, 364 3 of 25 Rare earth (RE: Ce & La) on the melting point of Sn-58%Bi eutectic solder. Sn-Bi eutectic composition is known as Sn-56.97%Bi and its melting point as 138.8 ◦C by NIST (National Institute of Standards and Technology), and Sn-57%Bi and 139 ◦C in the phase diagram by the ASM International. However, in many publications, Sn-58%Bi with a melting point of 138–139 ◦C is also used as the eutectic composition [19,38,39]. Anyway, this eutectic melting point may have slight variations depending on the heating rate of Sn-58% alloy. As shown in Table1, it is reported that Sn-58%Bi has a melting range of 130.2 ◦C (solidus temperature)– 139 ◦C (liquidus temperature) or higher such as 139.5–147.6 ◦C, 139.4–148.0 ◦C [19,21]. On the other hand, according to Zhou et al.