Short communicationRecommended values for the βSn solidus line in Sn-Bi alloys
Graphical abstract
Introduction
There is growing interest in adding bismuth to electronic solders [1], [2], [3], [4], [5], [6], [7], [8]. At relatively low Bi levels of 1–4wt%Bi, bismuth increases solder joint strength and often improves reliability in both thermal cycling and drop impact loading [1], [2], [3], [4], [5]. At Bi levels near the binary eutectic point (∼57 wt% Bi), the strongly reduced melting temperature allows for low temperature soldering which protects temperature-sensitive electronics and reduces production costs [6], [7]. Many of the new solders under development are multicomponent alloys and, therefore, require alloy design within a CALPHAD framework where the multicomponent equilibria are calculated from assessed experimental unary, binary and ternary data. Therefore, accurate experimental data for binary systems is important for the accurate prediction of high order multicomponent solders.
There has been a large body of experimental work devoted to the binary Sn-Bi system dating from 1908 [9] (Table 1). This is a relatively simple eutectic system with no intermediate phases. However, while there is generally good agreement on the βSn-(Bi) eutectic point (57 wt%Bi, ∼138 °C) [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], the reported maximum solubility of Bi in βSn varies quite dramatically from 6 to 11 wt% [9], [18], [23] to up to ∼31 wt% [15], [16] as summarised in Table 1 and, therefore, the solidus slope and Bi partition coefficient are also strongly different between the different studies. Even the most recent work is in significant disagreement over the βSn solidus line and commercial thermodynamics databases have adopted these discrepancies. For example, in some of the latest experimentally determined phase diagrams, the maximum solubility of Bi in βSn is 10wt%Bi in [18] and 21wt%Bi in [19]. And, in assessed Sn-Bi phase diagrams, the maximum solubility of Bi in βSn is ∼21 wt% in [21] and ∼11 wt% in [22], [23] with similar eutectic point.
To gain some insight into the discrepancies, Fig. 1 is a plot of past experimental and assessment work on the maximum solubility of Bi in βSn from Table 1, where the data are sorted in order of the reported maximum solubility of Bi in βSn. Four studies give highly variable data in the range 6.5–18wt%Bi, five studies give similar data of 20.5 ± 0.5wt%Bi and two studies give >30wt%Bi. From this, it might be hypothesised that: (i) the lower data come from studies where the alloys had not been fully equilibrated so that a Bi concentration gradient existed in the βSn and the measured ‘solidus temperatures’ were the incipient melting point of alloys that had been homogenised only partially and to different degrees; (ii) the studies that gave similar data were performed on equilibrated solid solutions and the equilibrium maximum solid solubility is 20.5 ± 0.5wt%Bi; and (iii) the upper data originates from some exotic studies, for instance when nano-scaled Sn-Bi films were investigated and where “dimensional effects” alter the physical propertites of the system [16]. However, these hypotheses cannot be confirmed since full details of the equilibration/homogenisation procedures are not given in all the papers as sumarized in Table 1.
The aims of this work were (i) to test the conditions required for the full equilibration of Bi solute in βSn, (ii) to re-measure the equilibrium βSn solidus line, and (iii) to present a case study on the potential pitfalls of solidus temperature measurements.
Section snippets
Methods
Alloys were prepared by mixing 99.99mass%Bi (DS Metals Ltd., Wolverhampton, UK) with ∼150 g of 99.95mass%Sn (DKL Metals Ltd., Grangemouth, UK) in a premium grade (g-grade) graphite crucible (Tokai Carbon Europe Ltd.) and heating in a resistance furnace to 450 °C in air. Samples were then produced by drawing the liquid into preheated quartz tubes of 4 mm inner diameter under vacuum in air. The residual pressure was not measured. The obtained rods were then cut into 300 ± 50 mg DSC samples. The
Results and discussion
It was confirmed by analytical SEM (EDS and EBSD) that, at room temperature (23 ± 2 °C) after experiments, all samples contained βSn phase and samples with ≥2 wt% Bi contained both βSn and (Bi) phases where the (Bi) phase had a morphology consistent with eutectic growth, with solid state precipitation or with both.
Fig. 3 illustrates examples of DSC heating curves for Sn-8Bi in the as-solidified condition (prior cooling rate 3 K/min) and after different equilibration times (0.5–860 h) at 180 °C. As can
Conclusions
The βSn solidus line has been re-measured due to significant inconsistencies in the experimental literature and differences in which data are accepted in thermodynamic assessments and models. The maximum solubility of Bi in βSn at the eutectic temperature was confirmed to be ∼20.6wt%Bi and recommended values for the solidus line have been given. This study reaffirms the importance of the prior homogenisation step in solidus measurements and the value of using multiple homogenisation times to
Acknowledgements
This research was funded by Nihon Superior Co., Ltd. and EPSRC grant no. EP/M002241/1. We thank K.C. Mills for fruitful discussions and Yuchen Hsu for translation of Japanese References.
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2019, Journal of Alloys and CompoundsCitation Excerpt :The heating rate was 0.17 K/s, the peak temperature was 240 °C and the cooling rate was 0.33 K/s. Every solder ball/joint was given two and/or multiple cycles and at least 10 balls/joints of each solder/substrate combination were measured. To determine the liquidus temperature of β-Sn in each solder/substrate combination, a similar cyclic method to Ref. [32] was used which is overviewed in Fig. 1 (C). In the first cycle in the DSC, the sample was held at the melting onset temperature (as shown in DSC curves in Fig. 1(A) and (B)) for 30 min before being heated up to 250 °C, and cooled down to 180 °C.
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