Materials Transactions, Vol. 45, No. 3 (2004) pp. 606 to 613 Special Issue on Lead-Free Soldering in Electronics #2004 The Japan Institute of Metals

Thermal Characteristics and Intermetallic Compounds Formed at Sn-9Zn-0.5Ag/Cu Interface Tao-Chih Chang1; * , Min-Hsiung Hon1 and Moo-Chin Wang2 1

Department of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 80782, Taiwan 2

The thermal characteristics of various Sn-based solder alloys and intermetallic compounds (IMCs) formed at the Sn-9Zn-0.5Ag/Cu interface have been investigated by using differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy, energy dispersive spectrometry, transmission electron microscopy and electron diffraction. The melting ranges of the Sn-37Pb, Sn-9Zn and Sn-3.5Ag alloys are 179:5
191:0, 195:5
208:1 and 220:4
227:8 C, and the heats of fusion are 104.2, 163.9 and 151.0 J/g, respectively. When 0.5 mass% Ag is added to the Sn-9Zn alloy, the melting temperature of the solder alloy increases from 195.5 to 196.7 C, but the melting range and heat of fusion decrease from 12.6 to 11.3 C and 163.9 to 74.7 J/g, respectively. The IMCs formed at the Sn-9Zn-0.5Ag/Cu interface are determined as a scallop-shaped Cu6 Sn5 near the solder alloy, a flat Cu5 Zn8 close to the Cu substrate and Ag3 Sn particles between the Cu substrate and Cu5 Zn8 layer. The Cu6 Sn5 is bi-structural, namely, hexagonal and monoclinic, which is caused by the Ag dissolution in the Cu6 Sn5 layer. (Received July 4, 2003; Accepted October 28, 2003) Keywords: thermal characteristics, intermetallic compounds, melting point, fusion heat, bi structure

1.

Introduction

The Sn-37Pb eutectic solder alloy has been used widely as an interconnecting material in electronic packaging industry, but is a source for lead pollution.1,2) The major electronic industrialized countries such as Japan, Europe, and the United States of America have created up a roadmap in limiting the use of Sn-37Pb alloy since 2001.3–5) Even Japan has established strict regulations to ban Pb-contaminated imports.6) Therefore, the development of lead-free solders is one of the urgent subjects for the electronics industry. The metallurgical characteristics of the Sn-37Pb solder alloy have been studied intensively.7–9) The Sn-9Zn and Sn3.5Ag are the two important binary lead-free solders investigated,10,11) but inferior wettability and oxidation resistance of the Sn-9Zn alloy and high melting point (221 C) of the Sn-3.5Ag alloy, limit the usage of these solder alloys.12–15) Chen et al.16) have determined the melting and solidification characteristics of various solder alloys using DSC. Besides the binary alloys, ternary solder alloys are also considered as a substitute for the Sn-37Pb solder alloy. The suitable eutectic composition of the Sn-Ag-Cu solder alloy has been determined as 3.5 mass% Ag, 0.9 mass% Cu and the balance Sn by Loomans and Fine,17) but the melting point of the solder alloy is 217 C which is too high to use in practice. Yoon et al.18) has been investigated phase equilibrium in the Sn-Bi-In system. However, there are few lead-free solders which can substitute Sn-37Pb completely. Hence, the need to build a database of lead-free solders is urgent. It has been shown that adding Ag to the Sn-9Zn solder alloy can improve wettability between the solder alloy and the Cu substrate by Takemoto et al.,19) because the addition of Ag reduces the potential difference between the base metal and the solder alloy. Chang et al.20,21) have demonstrated that *Corresponding

author, E-mail: [email protected]

the addition of Ag offers better solder joint reliability and hinders the formation of Kirkendall voids. However, only a few thermal characteristics of the solder alloy have been studied. The objectives of this study are (1) to build a database of thermal characteristics for the Sn-9Zn-0.5Ag solder alloy and (2) to determine the IMCs formed at the Sn9Zn-0.5Ag/Cu interface after soldering. 2.

Experimental Procedure

The compositions of the solder alloys used in this study were Sn-37Pb, Sn-3.5Ag (commercial grade, 99.9 mass% in purity). The Sn-9Zn and Sn-9Zn-0.5Ag solder alloys (in mass%) were melted with pure Sn, Zn and Ag metals (99.9 mass%). The pure metals were weighed and pickled in a 5 vol% HCl solution to remove oxides on the surface. Then they were mixed and melted at 600 C in a stainless steel crucible with stirring to homogenize. The molten solder alloys were cast into a 3 mm diameter cylinder and the microstructures of the solder alloys were observed with an optical microscope. A differential scanning calorimeter (DSC, Universal V2.6D TA Instruments) was used to estimate the melting points of the solder alloys with In metal as a base material. The ramping rate was 3 C/min and the data was recorded from 100 to 270 C. Pure Sn was utilized as a standard to estimate the calibration coefficient K of the instrument. The area under the peak was integrated with a commercially available software to calculate the fusion heat of the solder alloys. An oxygen-free and high conductivity (OFHC) Cu substrate of 60 mm  25 mm  0.5 mm was pickled in a 5 vol% HCl solution for 15 and then rinsed in de-ionized water for 10 s. After rinsing, the Cu substrate was degreased in a 5 mass% NaOH solution for 15 s at 70 C, rinsed in deionized water for 10 s again and finally immersed in a flux of

Thermal Characteristics and Intermetallic Compounds Formed at Sn-9Zn-0.5Ag/Cu Interface

607

close to the previous published results.16,22) When 0.5 mass% Ag is added to the Sn-9Zn solder alloy, the Tonset of the solder alloy increases from 195.5 to 196.7 C and the increment of 1.2 C is acceptable. Only one endothermic peak is found in the DSC curve, showing that the chemical composition of the Sn-9Zn-0.5Ag solder alloy is near eutectic. The melting ranges (Tend  Tonset ) of the Sn-37Pb, Sn-9Zn, Sn-9Zn-0.5Ag and Sn-3.5Ag solder alloys are 11.5, 12.6, 11.3 and 7.4 C, respectively. A narrow melting range can avoid the occurrence of segregation and hot tear in materials as reported by Vianco and Rejent.23) Hence, the Sn-3.5Ag solder alloy has excellent resistance to avoid segregation. The addition of Ag decreases the melting range of the Sn-9Zn solder alloy from 12.6 to 11.3 C, which is comparable to that of the Sn-37Pb solder alloy. The heat of fusion,
H, is given by eq. (1):24)
H ¼ Fig. 1

Dipping furnace used in this study.

3.5 mass% dimenthylammonium chloride (DMAHCl) to avoid reoxidation. After fluxing, the Cu substrate was held in a dipping furnace as shown in Fig. 1. The dipping temperature was 250 C and the dipping times were 10, 20 and 30 s, respectively, at a dipping rate of 11.8 mm/s. The unreacted solder alloy on the dipped sample was removed with sandpaper and an etching solution. An X-ray diffractometer (XRD, D-MAX III B, Rigaku, Japan) tool was employed to identify the phases in bulk solder and the IMCs formed at the Sn-9Zn-0.5Ag/Cu interface at a scanning rate of 1 /min for 2
from 20 to 80 and Si powders as a standard for calibration. The dipped sample was sticked together with epoxy, then ground and polished with sandpaper and 0.3 mm Al2 O3 powder paste. Afterwards, it was etched with an etchant (2 vol% HCl + 3 vol% HNO3 + 95 vol% C2 H5 OH). A scanning electron microscope (SEM, JXA-840, JEOL, Japan) was used to observe the morphology of the Sn-9Zn-0.5Ag/ Cu interface, and energy dispersive spectrometer (EDS, AN10000/85S, LINKS, England) was utilized to determine the chemical compositions of the IMCs. The dipped sample was sealed in a 3 mm Cu tube with G1 epoxy and baked at 120 C for 1 h to harden the epoxy. After baking, the sample was cross-sectioned and ground to a thickness below 100 mm, then thinned by using an ion-miller. A transmission electron microscope (TEM, HF-2000, HITACHI, Japan) with EDS (Voyager 1000, Noran) was utilized to observe the morphology of the Sn-9Zn-0.5Ag/Cu interface and selected area electron diffraction (SAED) was used to identify the structure of the IMCs. 3.

KA m

ð1Þ

where K is the calibration coefficient depending on the shape of a crucible and regarded as a constant in the DSC system,25) m is the mass of a sample, and A is the area under the curve peak. The heat of fusion of pure Sn is 60.6 J/g26) and the A/m value is 22.96 J/g as shown in Fig. 2(e). From eq. (1), the calibration coefficient K of the DSC instrument is obtained as 2.64. The DSC result of the various solder alloys shows that the heats of fusion,
Hf , for the Sn-37Pb, Sn-9Zn and Sn-3.5Ag solder alloys are 104.2, 163.9 and 151.0 J/g, respectively, showing that the Sn-37Pb solder alloy needs the lowest energy for melting. Chen et al.16) have demonstrated that the Sn-37Pb solder alloy melts at 183 C and the enthalpy of fusion is 45.2 J/g. The melting point of the solder alloy reported by Chen et al.16) is close to our result, but the fusion heat is only one half of ours, because the K value of the DSC instrument has not been calibrated in the former study. When 0.5 mass% Ag is added to the Sn-9Zn solder alloy, the heat of fusion of the solder alloy decreases dramatically from 163.9 to 74.7 J/g, showing that the Ag addition is beneficial to decrease the heat of fusion of the Sn-9Zn solder alloy. The heat of fusion of the Sn-9Zn-0.5Ag solder alloy is even lower than those of the Sn-37Pb and Sn-3.5Ag solder alloys and is a useful material for saving energy. The XRD patterns of the Sn-9Zn and Sn-9Zn-0.5Ag bulk solders are shown in Figs. 3(a) and (b), in which the Sn-9Zn solder alloy is composed of Sn-rich and Zn-rich phase. However, the Zn-rich phase disappears in the bulk solder of Sn-9Zn-0.5Ag, showing that the Ag addition promotes the dissolution of Zn in Sn. Because Zn has a higher heat of fusion of 115.79 J/g and the heat of fusion for melting the Znrich phase in the Sn-9Zn solder alloy is not necessary to consume for the one of Sn-9Zn-0.5Ag, which make the Sn9Zn-0.5Ag solder alloy have a heat of fusion close to pure Sn.

Results and Discussion 3.2

3.1 Thermal properties of the solder alloys Figures 2(a)–(d) show the DSC curves of the various solder alloys used in this study. It indicates that the melting points (Tonset ) of the Sn-37Pb, Sn-9Zn, and Sn-3.5Ag eutectic alloys are 179.5, 195.5 and 220.4 C respectively, which are very

Structure and morphology of the IMCs formed at the Sn-9Zn-0.5Ag/Cu interface Figures 4(a) and (b) are the XRD patterns of the Sn-9Zn/ Cu and Sn-Zn-0.5Ag/Cu interfaces, showing that the Cu6 Sn5 and Cu5 Zn8 are formed at the Sn-9Zn/Cu interface. Yu et al.27) have reported the same result and that the

608

T.-C. Chang, M.-H. Hon and M.-C. Wang

0

0

Sn-9Zn-0.5Ag

Sn-37Pb Tonset = 179.5°C

Tend = 208.0°C

-2

A/m=39.47 J/g

Fusion heat, H/mW

Fusion Heat, H/mW

-5

Tonset = 196.7°C

Tend = 191.0°C

-10

-15

A/m=28.31 J/g

-4

-6

-8

-20

120

160

Tpeak = 204.1°C

(c)

Tpeak = 185.5°C

(a)

-25 80

200

240

-10 80

280

120

160

200

240

280

Temperature, T/°C

Temperature, T/°C 0

Sn-9Zn

Sn-3.5Ag

0

Tonset = 220.4°C Tonset = 227.8°C Tonset = 195.5°C

Tonset = 208.1°C

-5 A/m=62.07 J/g

A/m=57.18 J/g

Fusion heat, H/mW

Fusion Heat, H/mW

-2

-4

-10

-15

-6

-20 Tpeak = 203.9°C

(b)

-8 80

Tpeak = 223.7°C

(d)

120

160

200

240

280

-25 80

120

Temperature, T/°C

160

200

240

280

Temperature, T/°C

0 Pure Sn Tonset = 229.5°C Tend = 238.0°C A/m=22.96 J/g

Fusion heat, H/mW

-4

-8

-12

-16 Tpeak = 233.6°C

(e) 80

120

160

200

240

280

Temperature, T/°C

Fig. 2

DSC curves for the various solder alloys of (a) Sn-37Pb, (b) Sn-9Zn, (c) Sn-9Zn-0.5Ag, (d) Sn-3.5Ag and (e) pure Sn.

structures of the -Cu6 Sn5 and Cu5 Zn8 are hexagonal and body centered cubic (bcc), respectively. Besides the Cu6 Sn5 and Cu5 Zn8 , the monoclinic 0 -Cu6 Sn5 is also found at the Sn-9Zn-0.5Ag/Cu interface, showing that the Cu6 Sn5 layer has two different structures. In the previous studies,28,29)

it was found that the IMCs formed at the Cu-Sn diffusion couple and Sn-37Pb/Cu interface are -Cu6 Sn5 and "-Cu3 Sn. Suganuma et al.30) have reported that the first IMC layer at the Sn-9Zn/Cu interface is Cu5 Zn8 . The IMC layers formed at the Sn-Zn-Al/Cu interface are Cu5 Zn8 and Cu9 Al4 as

Thermal Characteristics and Intermetallic Compounds Formed at Sn-9Zn-0.5Ag/Cu Interface 12000

Sn Zn

(a) 50000

609

(a)

Si Sn η−Cu6Sn5 Cu5Zn8 Ag3Sn Cu

10000

8000 Intensity (a. u.)

Intensity (a. u.)

40000

30000

20000

6000

4000

10000 2000

0 20

40

60

80

0 20

2 θ (degree)

30000

40

60

80

2θ

10000

Sn

(b)

Si Sn η'−Cu 6Sn5 η−Cu6Sn5 Cu5Zn8 Ag3Sn Cu

(b)

8000

Intensity (a. u.)

Intensity (a. u.)

20000

10000

6000

4000

2000

0 20

40

60

80

0 20

2θ

Fig. 3

XRD patterns of the (a) Sn-9Zn and (b) Sn-9Zn-0.5Ag bulk solders.

reported by Yu et al.31) In the present study, the bi-structural Cu6 Sn5 and bcc Cu5 Zn8 layers coexist at the Sn-9Zn-0.5Ag/ Cu interface, which are different from the result obtained in the Sn-9Zn solder system.30,31) It is caused by that Ag promotes the formation of the Cu6 Sn5 layer because Ag increases the solubility of Cu in Sn.32) The Ag3 Sn IMC forms at the solder alloy/Cu interface when the Ag content in solder alloy is above 0.1 mass% as reported by Huh et al.33) From the XRD pattern, the Ag3 Sn is found at the Sn-9Zn-0.5Ag/Cu interface because the Ag content in the solder alloy is higher than 0.1 mass%. The morphology of the Sn-9Zn-0.5Ag/Cu interface after dipping at 250 C for 10 s is shown in Fig. 5(a), which indicates that two different shaped IMCs layers are formed at the interface, namely, one is scallop-shaped and the other is planar. The chemical composition of the scallop-shaped IMC is determined as 51.9Cu-46.2Sn-1.2Zn-0.7Ag, showing that the IMC layer is Cu6 Sn5 and some Zn and Ag atoms dissolve in it. The planar IMC layer is determined as 42.8Cu-52.4Zn4.4Sn-0.4Ag, showing that the IMC layer is Cu5 Zn8 . Figures 5(b)–(e) show the mapping analysis of the Sn-9Zn0.5Ag/Cu interface, indicating that Sn concentrates at the scallop-shaped IMC layer, but Zn concentrates in the IMCs layers close to the Cu substrate, which agrees with the result

40

60

80

2θ

Fig. 4 XRD patterns of the (a) Sn-9Zn/Cu and (b) Sn-Zn-0.5Ag/Cu interfaces.

of EDS analysis. Besides, Ag also concentrates in the scallopshaped Cu6 Sn5 layer. The effect of Ag dissolution on the formation of bi-structural Cu6 Sn5 is delineated as follows. From the thermodynamic data listed in Table 2,34–36) the Gibbs free energies of the Cu5 Zn8 and Cu6 Sn5 are lower than those of the Ag3 Sn and Ag4 Sn. Therefore, the Cu5 Zn8 and Cu6 Sn5 layers tend to be formed at the Sn-9Zn-0.5Ag/Cu interface. Yu et al.31) have also reported the similar result. 3.3

Structures of the IMCs at the Sn-9Zn-0.5Ag/Cu interface Figures 6(a) and (b) show the bright field (BF) and dark field (DF) images of the IMCs at the Sn-9Zn-0.5Ag/Cu interface, indicating that the Cu6 Sn5 layer is formed at the interface close to the solder alloy and the Cu5 Zn8 layer is adjacent to the Cu6 Sn5 layer. The result agrees with the SEM micrograph. The Cu-Sn IMC has been determined as a monoclinic 0 -Cu6 Sn5 in our previous study.20) Figure 6(c) shows the EDS analysis of the 0 -Cu6 Sn5 , indicating that the dissolutions of Ag and Zn in the 0 -Cu6 Sn5 are 3.15 and 6.36 mass%, respectively. However, the Cu6 Sn5 layers with 0.33 mass% Ag and 0.80 mass% Zn are also found in the CuSn IMC as shown in Fig. 6(d). The ED pattern of the Cu-Sn

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Fig. 5 (a) SEM micrograph of the Sn-9Zn-0.5Ag/Cu interface after dipping at 250 C for 10 s, and mapping analyses of (b) Ag, (c) Zn, (d) Sn and (e) Cu.

Table 1 Solders Sn-37Pb Sn-3.5Ag

DSC analysis for various solder alloys. Tm ( C)


Hf (J/g)

179.5 220.4

104.2 151.0

Table 2 State

Intermetallic

eutectic eutectic

compounds

Thermodynamic data of intermetallic compounds.
G calculated

XCu
H (kJ/mol)
S (J/mol)

at 250 C

References

(kJ/mol)

Sn-9Zn

195.5

163.9

eutectic

-Cu5 Zn8

0.4

11:41

1.62

12:26

35)

Sn-Zn-0.5Ag

196.7

74.7

near-eutectic

-Cu6 Sn5

0.6

2:99

7.73

7:03

34)

Sn

229.5

60.6

pure metal

-Cu6 Sn5

0.5

1:99

8.05

6:20

34)

Ag3 Sn

0.7

2:68

4.89

5:24

36)

Ag4 Sn

0.8

3:20

3.34

4:95

36)

IMC with lower Ag and Zn contents is shown in Fig. 6(e), showing that it is a hexagonal -Cu6 Sn5 with zone axis (ZA) of [11 03 ]. Westgren and Phrahmen37) have reported that the Cu6 Sn5 transforms from  to 0 at a temperature around 170 C, where  is a high temperature phase with an ordered NiAs structure

(hexagonal) and the 0 -Cu6 Sn5 is a low temperature phase of a long periodic superlattice of -Cu6 Sn5 with a period of about 5 along both a and c directions.38) Gangulee et al.39) have demonstrated that quenching from about 170 C produc-

Thermal Characteristics and Intermetallic Compounds Formed at Sn-9Zn-0.5Ag/Cu Interface

611

Fig. 6 TEM micrographs of the IMCs formed at the Sn9Zn-0.5Ag/Cu interface, (a) BF image, (b) DF image, (c) EDS analyses of the 0 -Cu6 Sn5 , (d) EDS of the 0 Cu6 Sn5 and (e) ED pattern of the -Cu6 Sn5 with the [11 03] zone axis.

es an ordered NiAs structure, whereas slow cooling to room temperature shows extra superlattice spots. However, Larsson et al.40) have explicated that the 0 -Cu6 Sn5 has a monoclinic structure and is a new superstructure type belonging to the NiAs-Ni2 In structure group. In our previous study,20) the structure of the 0 -Cu6 Sn5 was found agreeing with the observation of Larsson et al.40) But it is found that the Cu6 Sn5 layer is bi-structural, namely,  and 0 phases coexisting at the Sn-9Zn-0.5Ag/Cu interface. The Ag atoms do not dissolve in the -Cu6 Sn5 at the Sn-3.5Ag/ Cu interface even after aging at 170 C for 4 days as reported by Vianco et al.41) Suganuma and Nakamura have deter-

mined the Cu6 Sn5 layer formed at the Sn-3.5Ag/Cu interface after aging being the  phase not the 0 one.42) Hence, the formation of the 0 -Cu6 Sn5 is caused by the dissolution of Ag and Zn in the -Cu6 Sn5 , and it expands the lattice of the Cu6 Sn5 . The BF and DF images of the Ag3 Sn IMC formed at the Sn-9Zn-0.5Ag/Cu interface are shown in Figs. 7(a) and (b), indicating that the Ag3 Sn particles form at the Cu5 Zn8 /Cu interface, not in the solder matrix for the Sn-3.5Ag/Cu system as reported by Vianco et al.41) Figs. 7(c) and (d) are the ED patterns of the Ag3 Sn with the ZA of [011] and [111], respectively, showing that the Ag3 Sn has an orthorhombic

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Fig. 7 TEM micrograph of the Ag3 Sn formed at the Sn-9Zn-0.5Ag/Cu interface, (a) BF image, (b) DF image, (c) ED pattern with the [011] zone axis and (d) ED pattern with the [111] zone axis.

structure.

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Conclusions

The results are summarized as follows: (1) The composition of the solder can be kept near-eutectic when 0.5 mass% of Ag is added to the Sn-9Zn solder alloy. The near-eutectic temperature is 196.7 C and the solidification range of the solder alloy is even lower than the Pb-Sn alloy. The sequence of the fusion heats for the various solders are: Sn-9Zn > Sn-3.5Ag > Sn37Pn > Sn-Zn-0.5Ag. (2) The planar Cu5 Zn8 and scallop-shaped Cu6 Sn5 are found at the Sn-9Zn-0.5Ag/Cu interface. The Cu6 Sn5 layer has two different structures, namely,  and 0 phases. Besides the Cu5 Zn8 and Cu6 Sn5 , the Ag3 Sn particles with an orthorhombic structure are also found at the Sn-9Zn-0.5Ag/Cu interface close to the Cu substrate. Acknowledgements The authors would like to thank National Science Council of Taiwan, ROC for the financial support under contract of NSC89-2216-E-151-011.

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