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j m a t e r r e s t e c h n o l . 2 0 1 9;8(1):1581–1586

www.jmrt .com.br

Available online at www.sciencedirect.com

hort Communication

etting behavior of Sn–Ag–Cu and Sn–Bi–X alloys:nsights into factors affecting cooling rate

ismarck L. Silvaa, Amauri Garciab, José E. Spinelli c,∗

Department of Materials Engineering, Federal University of Rio Grande do Norte-UFRN, 59078-970 Natal-RN, BrazilDepartment of Manufacturing and Materials Engineering, University of Campinas-UNICAMP, 13083-860 Campinas-SP, BrazilDepartment of Materials Engineering, Federal University of São Carlos-UFSCar, 13565-905 São Carlos-SP, Brazil

r t i c l e i n f o

rticle history:

eceived 8 March 2018

ccepted 13 June 2018

vailable online 5 November 2018

eywords:

older alloys

a b s t r a c t

Based on two experimental approaches: transient directional solidification and drop shape

analyses, the measurements of cooling rates and contact angles (�) of several solders of inter-

est 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 � and cooling rate. An opposite correlation between

cooling rate and � is observed when Sn–34Bi and the Sn–34Bi–0.1Cu alloys are compared.

olidification

ettability

hermal analysis

© 2018 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier

Editora Ltda. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

. Introduction

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

3,4].

Wettability (contact angle – �) is a key property of sol-

er alloys. It affects the capability of heat to flow across theolder/substrate interface, which directly contributes to the

∗ Corresponding author.E-mail: spinelli@ufscar.br (J.E. Spinelli).

ttps://doi.org/10.1016/j.jmrt.2018.06.016238-7854/© 2018 Brazilian Metallurgical, Materials and Mining Assocrticle under the CC BY-NC-ND license (http://creativecommons.org/lic

evolution of solidification [5,6]. � values corresponding to theinitial stages of wetting have been related to the interfacial sol-der/substrate heat transfer coefficient (hi) [7,8]. Consequently,the magnitude of the cooling rate (T) in the first stages of solid-ification may be affected. To the best of the present authors’knowledge, until recently, no systematic studies relating � andT of solders were published.

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

T =[

m(Tp − TLiq)√

�.˛SL�2[1 − erf (m�2)] exp(m�2)2

]

[2˛SL�2

2

((2KS�2(TSol − T0))/(n√

�(TLiq − T0) exp(�21)[M + erf (�1)]hi)) + SL

]2

(1)

iation. Published by Elsevier Editora Ltda. This is an open accessenses/by-nc-nd/4.0/).

o l .

1582 j m a t e r r e s t e c h n

where ˛SL: thermal diffusivity of the alloy mushy zone, �1 and�2: solidification constants associated with the displacementof solidus and liquidus isotherms, respectively, KS: solid thermalconductivity, TSol: non-equilibrium solidus temperature, T0:room temperature, TLiq: liquidus temperature, Tp: initial melttemperature, m: square root of the ratio of thermal diffusivitiesof mushy zone and liquid, (˛SL/˛L)1/2, M: ratio of heat diffusivi-ties of solid solder and substrate material, (kScM�M/kMcS�S)1/2,n: square root of the ratio of thermal diffusivities of solid andmushy zone, (˛S/˛SL)1/2, and SL: position of liquidus isothermfrom the solder/substrate interface.

As described in Eq. (1), beyond hi other alloy properties playa part on T, such as the latent heat of fusion, thermal conduc-tivity, density and specific heat. These properties vary withincreasing alloy solute content, whereas minor additions arenot expected to change them noticeable.

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

The present contribution aims to determine �, by usinga goniometer, and T by transient directional solidification ofvarious SAC and Sn–Bi based alloys. The results of � for thewetting first stages will be correlated with T. The relative sig-nificance of � 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 beexamined.

Controller andsamplethermocouples Thermal insula

Temperaturecontroller

Data logger

PC

Opticalsystem

Fig. 1 – Goniometer apparatus used to determine the form of me�e: equilibrium contact angle).

2 0 1 9;8(1):1581–1586

2. Experimental 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 metalshape analysis and solidification experiments. The transientdirectional solidification experiments allowed T to be deter-mined in the solidification first stages. A water-cooled carbonsteel bottom mold allowed heat to flow downwards insidethe system, while growth occurred upwards. The transientdirectional solidification method refers to a well-known exper-imental setup, as detailed elsewhere [13]. The melt superheatwas established as 10% above the liquidus temperature of eachalloy at the beginning of each experiment. Once started thecooling procedure, temperatures referring to positions veryclose to the solder/substrate interface were acquired. For that,a fine thermocouple positioned at 3–4 mm from the bottompart of the mold was required. J type thermocouple stainlesssteel probes with 500 mm of length and 1.5 mm of diameterwere employed.

A goniometer Krüss DSHAT HTM Reetz GmbH modelallowed the measurement of � for each solder alloy/carbonsteel substrate (Fig. 1). The surfaces of the substrates used inthe goniometer had the same finishing as that employed dur-ing directional solidification, that is #1200 grind paper. Thedata of the tests were all developed in triplicate. The equip-ment is able to follow continuously the form of the droplet,which is expressed by contact angles. For each alloy, constantheating rate of 10 K/min and a natural cooling rate inside the

furnace were carried out. Two different periods of the exper-imental scatter will be considered: initial instants (first 45 s)

tion chamberOn-off valveand flowmeter

Argoncylinder

Alloy sample

1020 carbonsteel substrate

Solder flux

t1 t2 t3

θI

t1 < t2 < t3

θe

tallic molten droplets (t: time; �I: initial contact angle and

j m a t e r r e s t e c h n o l . 2 0 1 9;8(1):1581–1586 1583

SAC107 SAC107

SAC207

SAC307SAC307

SAC207

Carbon steel substrate

Carbon steel substrate

Carbon steel substrate

Carbon steel substrate

Carbon steel substrate

Carbon steel substrate3mm

3mm

3mm

3mm

3mm

3mm

15s

15s

600s

600s

600s

F on Ad

rr

3

FiaVeat

Seuossfab

ig. 2 – Advancing contact angles of the SAC alloys droplets

uring the wetting tests.

eferred �I; and ending part of the curves when an equilibriumegime is achieved (last 45 s).

. Results and discussion

igs. 2 and 3 show the variations of the molten shape dur-ng the wetting test performed with the SAC and Sn–Bi basedlloys considering two wetting times, which are 15 and 600 s.alues of both �I and �e were automatically determined forach alloy through the tangent method 2 of the drop shapenalyzer. It is worth noting that the contact angles for higherimes are lower than those observed for the first wetting times.

Fig. 4 shows the experimental profiles of � vs. time for then–Ag–Cu alloys. The values of both �I and �e determined forach alloy are shown in Table 1 related to experimental T val-es. It is worth noting in Fig. 4 that some fluctuations haveccurred in the initial stages when the melt was not able topread, and convection flows may occur. Also, a period of con-

tancy is achieved for higher wetting times. �I determinedor the Sn–0.7Cu–1.0Ag, Sn–0.7Cu–2.0Ag and Sn–0.7Cu–3.0Aglloys are 36.7◦, 33.9◦ and 32.1◦, respectively, that is, the wetta-ility improves as the alloy Ag content is increased. The related

ISI 1020 steel surface showing the molten’s shape and size

T show the same tendency of �I as the Ag content increases.Here, a decrease in �I adheres to a decrease in T. Very differentamounts of Ag may result in variation of alloy properties suchas thermal diffusivity and latent heat of fusion. Consequently,these properties interfere in the evolution of solidification ofthe alloy, that is, in T.

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

The results � vs. time for Sn–34Bi–X alloys can be seenin Fig. 5, and in Table 1 with the corresponding T. For facili-tate the visualization, detailed inlet plots encompassing thewetting behavior in the first 100 s have been inserted. By

comparing the measurements for the Sn–34Bi alloy withthose for the Sn–34Bi–0.1Cu alloy, it is possible to infer thatthe Sn–34Bi–xCu alloys are sensitive to small copper addi-tions. Smaller �I (�I = 27.8◦) associated with higher T (12.3 K/s)

1584 j m a t e r r e s t e c h n o l . 2 0 1 9;8(1):1581–1586

Sn-34wt.%BiSn-34wt.%Bi

Carbon steel substrate3mm

Carbon steel substrate3mm

15s

Sn-34wt.%Bi-0.1wt.%Cu Sn-34wt.%Bi-0.1wt.%Cu

Sn-34wt.%Bi-0.7wt.%Cu

Sn-34wt.%Bi-0.2wt.%Ag Sn-34wt.%Bi-0.2wt.%Ag

Sn-34wt.%Bi-0.7wt.%Cu

Carbon steel substrate Carbon steel substrate

Carbon steel substrate Carbon steel substrate

Carbon steel substrate Carbon steel substrate

3mm

3mm 3mm

3mm3mm

3mm

15s

15s

15s

600s

600s

600s

600s

Fig. 3 – 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.

characterize the Sn–34Bi–0.1Cu alloy. This is because verysmall additions of Cu are not expected to change the alloyproperties (e.g., the latent heat of fusion, thermal conductiv-ity, density and specific heat) and parameters (e.g., solidus andliquidus temperatures) governing T. In this case, T is controlledby �I, i.e., by hi (lower �I > higher hi > higher T, according toEq. (1)).

The additions of 0.7Cu and 2.0Ag decreased and increased

�I, respectively, as compared to the binary Sn–34Bi alloy. Incontrast, T values are quite close. It appears that a compen-sation between �I and the thermophysical properties of thesemodified alloys (i.e., Sn–34Bi–0.7Cu and Sn–34Bi–2.0Ag) occurs,

which is conducive to a similar initial solidification progressas compared to that of the binary Sn–34Bi alloy.

4. Conclusions

It has been shown that despite the importance of �I as a signif-

icant factor affecting soldering cooling rate, with the increasein solute additions to the solder alloys, the thermophysicalproperties can play a role that can gradually be even moresubstantial than that exercised by �I. Based on that, a general

j m a t e r r e s t e c h n o l . 2 0 1

0 100 200 300 400 500 600 700 8005

10

15

20

25

30

35

40

45

50

55

60

0 10 20 30 40 50 60 70 80 90 10020

25

30

35

40

45

50

First 100s

θ, C

onta

ct A

ngle

(°)

Time (s)

Sn-0.7Cu-1.0Ag - SAC 107Sn-0.7Cu-2.0Ag - SAC 207Sn-0.7Cu-3.0Ag - SAC 307

Fig. 4 – Evolution of � between SAC alloys and the AISI 1020steel substrate.

Table 1 – � and T for the tested SAC and Sn–34Bi–Xsolder alloys solidified against a carbon-steel substrate.

Solder alloy �I (◦) – initial �e (◦) – equilibrium T (K/s)a

Sn–34wt%Bi 39.2 ± 2.8 23.0 ± 1.1 10.0Sn–34wt%Bi–0.1wt%Cu 27.8 ± 4.0 15.8 ± 0.1 12.3Sn–34wt%Bi–0.7wt%Cu 34.7 ± 1.3 22.5 ± 0.9 8.0Sn–34wt%Bi–2.0wt%Ag 42.9 ± 1.0 35.8 ± 0.1 9.0SAC107 36.7 ± 0.2 21.4 ± 0.5 32.0SAC207 33.9 ± 0.7 18.8 ± 1.1 16.0SAC307 32.1 ± 3.4 22.2 ± 0.2 9.5

a Determined based on the nearest thermocouple to thealloy/substrate interface.

0 100 200 300 400 500 600 700 8005

10

15

20

25

30

35

40

45

50

55

60

65

0 10 20 30 40 50 60 70 80 90 10 020

25

30

35

40

45

50

55First 100 s Sn-34Bi

Sn-34Bi-0.1Cu Sn-34Bi-0.7Cu Sn-33Bi-2Ag

θ, C

onta

ct A

ngle

(°)

Time (s)

Fig. 5 – Evolution of � between Sn–34Bi–X solder alloys andt

tp

1

r

he AISI 1020 steel substrate.

rend cannot be established between �I and T, as shown by theresent results:

. Sn–0.7Cu–Ag alloys: Both �I and T were shown to decreaseas the Ag content was increased: lower �I > higher hi < lowerT. According to the analytical Expression (1) for T, this wasshown to be associated with variations in thermophysical

9;8(1):1581–1586 1585

properties with the increase in Ag content, as also sup-ported by previous studies.

2. Sn–34Bi–X alloys: A small addition of Cu (X = 0.1Cu) was notexpected to change the alloy thermophysical properties,however, this has induced a smaller �I, which controlledT : lower �I > higher hi > higher T, according to Eq. (1) for T.

The additions of 0.7Cu and 2.0Ag to the Sn–34Bi alloy wereshown to decrease and increase �I, respectively, as com-pared to the binary Sn–34Bi alloy. In contrast, T remainedclose for these alloys, suggesting that a compensatingeffect exists between �I (and consequently hi) and thethermophysical properties, which is conducive to similarcooling 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 inMaterials Science and Engineering (PPGCEM-UFRN).

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