Transient Unidirectional Solidification, Microstructure and Intermetallics in Sn-Ni Alloys

Materials Research. DOI: http://dx.doi.org/10.1590/1980-5373-MR-2017-1099 Transient Unidirectional Solidification, Microstructure and Intermetallics in Sn-Ni Alloys† Clarissa Barros da Cruza, Rafael Kakitania, Marcella Gautê Cavalcante Xavierb, Bismarck Luiz Silvac , Amauri Garciaa, Noé Cheunga, José Eduardo Spinellib* Departamento de Engenharia de Materiais e Manufatura, Universidade de Campinas - UNICAMP, 13083-860, Campinas, SP, Brasil b Departamento de Engenharia de Materiais, Universidade Federal de São Carlos - UFSCar, 13565905, São Carlos, SP, Brasil c Departamento de Engenharia de Materiais, Universidade Federal do Rio Grande do Norte - UFRN, 59078-970, Natal, RN, Brasil a Received: December 16, 2017; Revised: February 23, 2018; Accepted: February 28, 2018 The present research work examines the microstructural arrangements formed during the transient solidification of eutectic Sn-0.2wt.%Ni and hypereutectic Sn-0.5wt.%Ni alloys. Also, it examines their respective correlations with solidification thermal parameters: eutectic growth rate (VE) and eutectic cooling rate (ṪE); length scales of matrix and eutectic phases: microstructural spacings and the corresponding tensile properties: ductility and strength. Both alloys were directionally solidified upwards under unsteady-state regime, and characterized by optical and scanning electron microscopy. Concerning the hypereutectic Sn-0.5wt.%Ni, the increase in Ni content is shown to influence both thermal behavior and cellular spacing (λC). The NiSn4 intermetallics is present in the eutectic mixture of both alloys, whilst in the Sn-0.5wt.%Ni alloy the primary phase has been identified by SEM-EDS as the Ni3Sn4 intermetallics. A β-Sn morphological cellular/dendritic transition occurs in the 0.2wt.%Ni eutectic alloy for ṪE> 1.2K/s. Despite that, regular cells in the hypereutectic alloy (0.5wt.%Ni) turns into plate-like cells for Ṫ E> 1.4K/s. If considered a reference cellular spacing about 20μm (i.e., λc-1/2=0.22), the samples associated with the Sn-0.5wt.%Ni alloy are shown to be associated with higher tensile strengths, but much lower ductility as compared with the corresponding results of the eutectic alloy. Keywords: Lead-free alloys, Transient solidification, Microstructure, Intermetallics, Tensile properties, Wettability. 1. Introduction Nickel (Ni) is one of the main alloying elements of Sn-based lead-free solder alloys. It has been largely added as third or as fourth element in Sn-Ag or Sn-Ag-Cu alloys, respectively. It is worth noting that the additions of minor alloying elements to Sn-based lead-free solders has received a lot of attention recently in order to boost or fine-tune various application properties1-6. For example, Wang et al.7 found that Ni concentrations higher than 0.01 wt.% added to a Sn-2.5Ag-0.8Cu alloy may retard the growth of deleterious Cu3Sn particles layers at the alloy/copper interface even after an aging time of 2000 h. According to Nishikawa et al.8 the addition of 0.1 wt.% Ni to a Sn-3.5Ag alloy does not affect the joint mechanical strength with a Cu pad, but it changes the fracture mode. The sustainable development of soldering processes experiences a change towards lead-free electronic products, the so called green electronics9,10. Consequently, manufacturers are required to set their furnaces and *e-mail: †Article presented in the ABM Week 2017, October 2nd to 6th, 2017, São Paulo, SP, Brazil procedures for the application of lead-free solder alloys. This is explained by the recent legal restrictions put in place for lead-based solders. Sn-Ni alloys are considered possible substitutes, even though there is a lack of studies devoted to their microstructures and application properties11,12. Understanding the microstructure evolution in these alloys as well as establishing correlations between microstructural features and solidification thermal parameters (i.e., cooling rate and solidification velocity) is fundamental for the preprogramming of final application properties of as-soldered joints. Determination of solidification conditions might yield either a specified β-Sn morphology and size or a required intermetallic particle in eutectic and hypereutectic Sn-Ni alloys. Such lack of knowledge regarding Sn-Ni alloys is quite unexpected considering that Ni is a very common substrate in electronic packaging. Ni is largely employed since soft soldering applications involving Ni and Sn represent slower growth kinetics as compared, for example, with that occurring between Cu and Sn12. The existing research efforts dedicated to Sn-Ni solder alloys put emphasis on examining and identifying the formed intermetallics in compositions of interest. One of the few 2 Cruza et al. studies has shown that the NiSn4 intermetallics (IMC) can be formed by either primary precipitation during solidification or eutectic transformation 11-13. Overall, the Sn-NiSn 4 eutectic structure has been identified as the predominant one regardless the tested solidification conditions, which varied from 120K/s to 0.022K/s11. The same research showed that for hypereutectic compositions and solidification cooling rates in the range 1-10K/s, a very large fraction of primary Ni3Sn 4 IMC grew to the detriment of the primary NiSn4 IMC. Various studies devoting to develop alternative and new solder alloys have been recently performed14-20. The aim was to characterize microstructure features and determine their correlations with solidification thermal parameters, that is, the eutectic growth rate (V E) and the eutectic cooling rate (ṪE). This kind of knowledge ought to be expanded for Sn-Ni alloys. Within this approach, the main priority is to compare the final microstructures and how they could be induced via solidification cooling rate. In addition, in what manner the Ni content can affect the microstructure features and properties such as wettability and tensile properties. In general, the soundness of an alloy depends on the morphology/size of the β-Sn matrix and on the size/nature of phases surrounding it21,22. The microstructure features of directionally solidified Sn-0.7Cu alloys containing minute additions of Ni were recently investigated. It has been reported that Ni modified alloys with 500 ppm and 1000 ppm of Ni developed both the (Cu,Ni)6Sn 5 phase and the Ni3Sn 4 primary IMC for specimens experiencing fast cooling conditions 23. Within this work, the mean equilibrium contact angles determined for the Sn-0.7Cu, Sn-0.7Cu0.05Ni and Sn-0.7Cu-0.1wt%Ni alloys were 19.1°, 12.9° and 36.0° respectively. These values can be considered compatible to those reported for Sn-Pb alloys. According to Kong et al.24, the increase in Sn content of Sn-Ni alloys can lead to better wettability, and such alloys are often required in practical applications mainly because of such essential quality. In order to gain insight into microstructure formation and solidification thermal parameters of eutectic Sn0.2wt.%Ni and hypereutectic S (...truncated)