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DOI: 10.1016/j.jmrt.2018.06.016
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Wetting behavior of Sn–Ag–Cu and Sn–Bi–X alloys: insights into factors affecting cooling rate
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Bismarck L. Silvaa, Amauri Garciab, José E. Spinellic,
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spinelli@ufscar.br

Corresponding author.
a Department of Materials Engineering, Federal University of Rio Grande do Norte-UFRN, 59078-970 Natal-RN, Brazil
b Department of Manufacturing and Materials Engineering, University of Campinas-UNICAMP, 13083-860 Campinas-SP, Brazil
c Department of Materials Engineering, Federal University of São Carlos-UFSCar, 13565-905 São Carlos-SP, Brazil
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Table 1. θ and T˙ for the tested SAC and Sn–34Bi–X solder alloys solidified against a carbon-steel substrate.
Abstract

Based on two experimental approaches: transient directional solidification and drop shape analyses, the measurements of cooling rates and contact angles (θ) of several solders of interest became practical. Three Sn–0.7Cu–(1; 2 and 3Ag) and four Sn–34Bi–(0Cu; 0.1Cu; 0.7Cu; 2Ag) alloys are investigated to determine their wetting behavior, compare to each other and bring to light their ability (or not) to control the initial cooling rate. In the case of SAC alloys, increase in Ag means decrease in both θ and cooling rate. An opposite correlation between cooling rate and θ is observed when Sn–34Bi and the Sn–34Bi–0.1Cu alloys are compared.

Keywords:
Solder alloys
Solidification
Wettability
Thermal analysis
Full Text
1Introduction

Among the candidates to replace Sn–Pb solders, two Sn families of eutectic alloys deserve attention: Sn–Ag–Cu and Sn–Bi–(Ag,Cu) alloys. Sn–Ag–Cu alloys have advantages of good wettability, superior interfacial properties, high creep resistance, and low coarsening rate [1,2]. Sn–Bi based solders in the electronic packaging have shown good joint strength, high creep resistance and low coefficient of thermal expansion [3,4].

Wettability (contact angle – θ) is a key property of solder alloys. It affects the capability of heat to flow across the solder/substrate interface, which directly contributes to the evolution of solidification [5,6]. θ values corresponding to the initial stages of wetting have been related to the interfacial solder/substrate heat transfer coefficient (hi) [7,8]. Consequently, the magnitude of the cooling rate (T˙) in the first stages of solidification may be affected. To the best of the present authors’ knowledge, until recently, no systematic studies relating θ and T˙ of solders were published.

Here T˙ can be analytically described as a function of solder/substrate parameters and other operational conditions [9] and consequently, as a function of hi:

where αSL: thermal diffusivity of the alloy mushy zone, ϕ1 and ϕ2: solidification constants associated with the displacement of solidus and liquidus isotherms, respectively, KS: solid thermal conductivity, TSol: non-equilibrium solidus temperature, T0: room temperature, TLiq: liquidus temperature, Tp: initial melt temperature, m: square root of the ratio of thermal diffusivities of mushy zone and liquid, (αSL/αL)1/2, M: ratio of heat diffusivities of solid solder and substrate material, (kScMρM/kMcSρS)1/2, n: square root of the ratio of thermal diffusivities of solid and mushy zone, (αS/αSL)1/2, and SL: position of liquidus isotherm from the solder/substrate interface.

As described in Eq. (1), beyond hi other alloy properties play a part on T˙, such as the latent heat of fusion, thermal conductivity, density and specific heat. These properties vary with increasing alloy solute content, whereas minor additions are not expected to change them noticeable.

Some previous studies identified that additions of Ag affect the wettability of the Sn–Cu eutectic alloy [10,11]. Higher Ag contents produced better wettability by lowering wetting time and increasing maximum wetting force. Zang et al. [12] showed that the addition of 1.0wt.% Cu did not influence decisively the wettability of the Sn–58Bi solder.

The present contribution aims to determine θ, by using a goniometer, and T˙ by transient directional solidification of various SAC and Sn–Bi based alloys. The results of θ for the wetting first stages will be correlated with T˙. The relative significance of θ and alloy solute content on T˙ will be outlined. Moreover, for both families of alloys, the effects of varying third solute contents (Ag/Cu) on the wetting behavior will be examined.

2Experimental procedure

Three Sn–0.7Cu–(1; 2 and 3Ag) and four Sn–34Bi–(0Cu; 0.1Cu; 0.7Cu; 2Ag) (wt.%) alloys were used in both molten metal shape analysis and solidification experiments. The transient directional solidification experiments allowed T˙ to be determined in the solidification first stages. A water-cooled carbon steel bottom mold allowed heat to flow downwards inside the system, while growth occurred upwards. The transient directional solidification method refers to a well-known experimental setup, as detailed elsewhere [13]. The melt superheat was established as 10% above the liquidus temperature of each alloy at the beginning of each experiment. Once started the cooling procedure, temperatures referring to positions very close to the solder/substrate interface were acquired. For that, a fine thermocouple positioned at 3–4mm from the bottom part of the mold was required. J type thermocouple stainless steel probes with 500mm of length and 1.5mm of diameter were employed.

A goniometer Krüss DSHAT HTM Reetz GmbH model allowed the measurement of θ for each solder alloy/carbon steel substrate (Fig. 1). The surfaces of the substrates used in the goniometer had the same finishing as that employed during directional solidification, that is #1200 grind paper. The data of the tests were all developed in triplicate. The equipment is able to follow continuously the form of the droplet, which is expressed by contact angles. For each alloy, constant heating rate of 10K/min and a natural cooling rate inside the furnace were carried out. Two different periods of the experimental scatter will be considered: initial instants (first 45s) referred θI; and ending part of the curves when an equilibrium regime is achieved (last 45s).

Fig. 1.
(0.23MB).

Goniometer apparatus used to determine the form of metallic molten droplets (t: time; θI: initial contact angle and θe: equilibrium contact angle).

3Results and discussion

Figs. 2 and 3 show the variations of the molten shape during the wetting test performed with the SAC and Sn–Bi based alloys considering two wetting times, which are 15 and 600s. Values of both θI and θe were automatically determined for each alloy through the tangent method 2 of the drop shape analyzer. It is worth noting that the contact angles for higher times are lower than those observed for the first wetting times.

Fig. 2.
(0.22MB).

Advancing contact angles of the SAC alloys droplets on AISI 1020 steel surface showing the molten's shape and size during the wetting tests.

Fig. 3.
(0.33MB).

Advancing contact angles of the Sn–Bi–X alloys droplets on AISI 1020 steel surface showing the molten's shape and size during the wetting tests.

Fig. 4 shows the experimental profiles of θ vs. time for the Sn–Ag–Cu alloys. The values of both θI and θe determined for each alloy are shown in Table 1 related to experimental T˙ values. It is worth noting in Fig. 4 that some fluctuations have occurred in the initial stages when the melt was not able to spread, and convection flows may occur. Also, a period of constancy is achieved for higher wetting times. θI determined for the Sn–0.7Cu–1.0Ag, Sn–0.7Cu–2.0Ag and Sn–0.7Cu–3.0Ag alloys are 36.7°, 33.9° and 32.1°, respectively, that is, the wettability improves as the alloy Ag content is increased. The related T˙ show the same tendency of θI as the Ag content increases. Here, a decrease in θI adheres to a decrease in T˙. Very different amounts of Ag may result in variation of alloy properties such as thermal diffusivity and latent heat of fusion. Consequently, these properties interfere in the evolution of solidification of the alloy, that is, in T˙.

Fig. 4.
(0.12MB).

Evolution of θ between SAC alloys and the AISI 1020 steel substrate.

Table 1.

θ and T˙ for the tested SAC and Sn–34Bi–X solder alloys solidified against a carbon-steel substrate.

Solder alloy  θI (°) – initial  θe (°) – equilibrium  T˙ (K/s)a 
Sn–34wt%Bi  39.2±2.8  23.0±1.1  10.0 
Sn–34wt%Bi–0.1wt%Cu  27.8±4.0  15.8±0.1  12.3 
Sn–34wt%Bi–0.7wt%Cu  34.7±1.3  22.5±0.9  8.0 
Sn–34wt%Bi–2.0wt%Ag  42.9±1.0  35.8±0.1  9.0 
SAC107  36.7±0.2  21.4±0.5  32.0 
SAC207  33.9±0.7  18.8±1.1  16.0 
SAC307  32.1±3.4  22.2±0.2  9.5 
a

Determined based on the nearest thermocouple to the alloy/substrate interface.

The eutectic volume fractions are 35%, 45%, and 61% for the Sn–0.7Cu–1.0Ag, Sn–0.7Cu–2.0Ag and Sn–0.7Cu–3.0Ag alloys, respectively [14]. This explains the presumed variations in parameters/properties denoted in Eq. (1) with the increase in Ag alloying. The same tendency of improving wettability with increasing Ag content has been reported [10,11], however, these studies have not focused on a significant range of Ag alloying in SAC alloys, as is the case of the present investigation.

The results θ vs. time for Sn–34Bi–X alloys can be seen in Fig. 5, and in Table 1 with the corresponding T˙. For facilitate the visualization, detailed inlet plots encompassing the wetting behavior in the first 100s have been inserted. By comparing the measurements for the Sn–34Bi alloy with those for the Sn–34Bi–0.1Cu alloy, it is possible to infer that the Sn–34Bi–xCu alloys are sensitive to small copper additions. Smaller θI (θI=27.8°) associated with higher T˙ (12.3K/s) characterize the Sn–34Bi–0.1Cu alloy. This is because very small additions of Cu are not expected to change the alloy properties (e.g., the latent heat of fusion, thermal conductivity, density and specific heat) and parameters (e.g., solidus and liquidus temperatures) governing T˙. In this case, T˙ is controlled by θI, i.e., by hi (lower θI>higher hi>higher T˙, according to Eq. (1)).

Fig. 5.
(0.14MB).

Evolution of θ between Sn–34Bi–X solder alloys and the AISI 1020 steel substrate.

The additions of 0.7Cu and 2.0Ag decreased and increased θI, respectively, as compared to the binary Sn–34Bi alloy. In contrast, T˙ values are quite close. It appears that a compensation between θI and the thermophysical properties of these modified alloys (i.e., Sn–34Bi–0.7Cu and Sn–34Bi–2.0Ag) occurs, which is conducive to a similar initial solidification progress as compared to that of the binary Sn–34Bi alloy.

4Conclusions

It has been shown that despite the importance of θI as a significant factor affecting soldering cooling rate, with the increase in solute additions to the solder alloys, the thermophysical properties can play a role that can gradually be even more substantial than that exercised by θI. Based on that, a general trend cannot be established between θI and T˙, as shown by the present results:

  • 1.

    Sn–0.7Cu–Ag alloys: Both θI and T˙ were shown to decrease as the Ag content was increased: lower θI>higher hi<lower T˙. According to the analytical Expression (1) for T˙, this was shown to be associated with variations in thermophysical properties with the increase in Ag content, as also supported by previous studies.

  • 2.

    Sn–34Bi–X alloys: A small addition of Cu (X=0.1Cu) was not expected to change the alloy thermophysical properties, however, this has induced a smaller θI, which controlled T˙: lower θI>higher hi>higher T˙, according to Eq. (1) for T˙. The additions of 0.7Cu and 2.0Ag to the Sn–34Bi alloy were shown to decrease and increase θI, respectively, as compared to the binary Sn–34Bi alloy. In contrast, T˙ remained close for these alloys, suggesting that a compensating effect exists between θI (and consequently hi) and the thermophysical properties, which is conducive to similar cooling rate, T˙.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors thank the financial support provided by FAPESP (São Paulo Research Foundation, Brazil: grants 2017/15158-0 and 2017/12741-6), CNPq and the Postgraduate Program in Materials Science and Engineering (PPGCEM-UFRN).

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Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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