Reprinted with permission of Materials Science & Technology 2006

Conference and Exhibition Proceedings.

“Creep Behavior and Microstructure of Sn-Ag-Cu-Ni-Ge Lead-Free Solder Alloy”

N. HIDAKA, H. WATANABE, M. YOSHIBA, M. SHIMODA, T. ASAI, and M. ONO,

Materials Science andTechnology (2006), 185-197,

Creep Behavior and Microstructure of Sn-Ag-Cu-Ni-Ge Lead-free Solder Alloy

Noboru HIDAKA1, Hirohiko WATANABE1, Masayuki YOSHIBA2,Masayoshi SHIMODA1, Tatsuhiko ASAI1, and Masahiro ONO1

1Production Technology Laboratory, Fuji Electric Advanced Technology Co., Ltd. 1, Fuji-machi, Hino-city, Tokyo191-8502, Japan

2Department of Mechanical Engineering, Graduate School of Science and EngineeringTokyo Metropolitan University, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan

Keywords: Sn-Ag-Cu-Ni-Ge lead-free solders, Microstructure, Creep, IMCs.

Abstract

An advanced lead-free solder alloy, Sn-Ag-Cu-Ni-Ge, has been developed to improve the mechanical properties of the Sn-Ag-Cu base solders and prevent oxidation of those solders. In this paper, creep behavior of bulk and IMD of two lead-free solder alloys, Sn-3.5Ag-0.5Cu-Ni-Ge and Sn-3.0Ag-0.5Cu, were investigated under high temperature conditions. It was found that the creep rupture life of the Sn-3.5Ag-0.5Cu-Ni-Ge solder is over three times better than that of the Sn-3.0Ag-0.5Cu solder at 398K. Adding Ni into the solder can make the microstructure development to be finer and uniform. The Ni element added into the solder is easy to combine with Cu and can form a kind of stable intermetallic compounds (IMCs) of a (Cu, Ni)6Sn5 to improve the creep behavior of the solder. Moreover, microstructural characterizations using TEM analysis, were conducted to investigate in detail the creep behavior and cleared that dislocation moving are prevented effectively by the precipitates and stacking faults are observed in the Sn-3.5Ag-0.5Cu-Ni-Ge solder.

Introduction

According to the EU RoHS legislation, the use of lead-containing solders in electronic products, put on sale in Europe, is banned after 1 July 2006. A number of lead-free solder alloys have been proposed to replace Pb-Sn eutectic solder in electronic packaging. Consequently, Sn-Ag-Cu lead-free system solders have been regarded as the most promising lead-free substitutes for the Pb-Sn eutectic solder.1,2

Generally, solder joints undergo the temperature cycle and power cycle during practical service. Moreover, electronic devices, especially used in vehicles and industrial products, are much more frequently loaded into more severe environments, such as condition of higher temperature and fluctuating stresses and temperatures, which induces higher temperature creep and thermal fatigue in the joining materials of solder. Creep deformation is the most common and important deformation mechanism in solder joints because of its high homologous temperatures. As for the Sn-Ag-Cu solder, even at room temperature, the critical temperature for creep and recrystallization is easily reached. A small amount of Ni, and Ge, which is considered to improve the high temperature performance of the solder, was added into the Sn-Ag-Cu solder. Therefore, advanced solder alloys, Sn-Ag-Cu-Ni-Ge solder, have been developed.The effect of adding Ge, which can prevent the oxidation of Sn and suppress dross formation in wave soldering, has been reported.3In this paper, creep behavior of both bulk specimens and TH (through hole) joints of the Sn-3.5Ag-0.5Cu-Ni-Ge solder were investigated with comparing to the Sn-3.0Ag-0.5Cu solder alloy.

Lead-Free Soldering: It is Here to Stay - Applications, Alloy Development, and ImpactOrganized by M.A. Palmer, I.E. Anderson, and E.J. Cotts

Materials Science and Technology (MS&T) 2006: PRODUCT MANUFACTURING

185

Table . Compositions of two kinds of solders (mass%)

Alloy. No Sn Ag Cu Ni Ge

Sn-3.5Ag-0.5Cu-Ni-Ge Rem. 3.58 0.509 0.064 0.011

Sn-3.0Ag-0.5Cu Rem. 3.07 0.522

Table . Comparison of typical characteristics of solders

Sn-3.5Ag-0.5Cu-Ni-Ge Sn-3.0Ag-0.5Cu Solidus (K) 490 490

Liquidus (K) 494 492 Thermal expansion coefficient (ppm/K) 22.3 21.7

Thermal conductivity (W/m K) 62 64

Parallel part 3 0.01

6

(18.5)(18.5) 1518

( ) ( )

60

R3

Parallel part 3 0.01

6

(18.5)(18.5) 1518

( ) ( )

60

R3

Figure 1: Appearance of the bulk specimen.

Figure 2: Miniature creep testing machine for bulk specimen.

Table . Wave soldering condition Surrounding Atmosphere

Soldering temperature 523K Conveyor speed 16.7mm/s Conveyor speed 4

Dip time 6s

300

800

186

Flux EC-19S-A

Substrate

LoadClampingdevice

Substrate B-side

F-sidePin

LoadSolderjoints

(a) Schematic of the device (b) Appearance of the test device

Substrate

LoadClampingdevice

Substrate B-side

F-sidePin

LoadSolderjoints

(a) Schematic of the device (b) Appearance of the test deviceFigure 3: Schematic diagram of TH joints creep test.

Stress dependent and activation energy for creep were discussed in connection with microstructure observations to clarify the deformation mechanism during creep testing. Moreover, the growth behavior of precipitate particle was also investigated by transmission electron microscope (TEM) analysis.

Experimental Procedure

Chemical compositions of the materials used are shown in Table I. Sn-3.0Ag-0.5Cu solder is currently recommended by JEITA.2 Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge (abbr. Sn-3.5Ag-0.5Cu-Ni-Ge) solder, which has Ag content of 3.58mass%, Ni content of 0.064mass%, and Ge content of 0.01mass%, is an advanced solder developed. The compositions of two kinds of solders were analyzed by a Wavelength Dispersive X-ray Spectrometer (WDX) and Inductively Coupled Plasma (ICP). The thermal characteristics of the Sn-Ag-Cu-Ni-Ge solder and the Sn-Ag-Cu solders are summarized in Table II.

Bulk SampleIn order to make the creep specimen of the bulk, Cast ingots of Sn-3.5Ag-0.5Cu-Ni-Ge and Sn-3.0Ag-0.5Cu were manufactured. The solder bars, which were prepared from marker, were melted in air in an electric furnace at 603K for several hours. Then, the melt was chill-cast as an ingot in a stainless steel mold with geometry of 14mm diameter and 160mm length. The solder rods were then machined into round shape of creep specimen with 15mm gauge length and 3mm gauge diameter as shown in Fig.1. All specimens were heat-treated at 333K for 24 h to remove a residual stress and defects induced during specimen manufacturing. Tests were conducted by using a miniature creep test machine as shown in Fig.2. The creep testing was conducted at 313K, 348K and 398K, representing homologous temperatures (=T/Tm, where Tm is the melting point of the solders) of 0.63, 0.71, and 0.81 for Sn-3.5Ag-0.5Cu-Ni-Ge and Sn-3.0Ag-0.5Cu respectively. The stress range in the tests is within 5-20MPa, corresponding to the normalized stress of /E= 10-5- 10-3 (where E is Young’s modulus).

TH SampleTH test sample was shown in Fig.3. Because through-hole (pin-in-hole) technology is still widely used in the electronics industry, creep test of TH joints was conducted in this work. The substrate material used for the TH test also was standard FR-4 epoxy-grass laminate of which were 195 length, 120 width, and 1.6 thickness. The Cu pads on the FR-4 substrate were 0.85mm in diameter. The pin was made from Cu with Ni/Sn plating and its size was 0.5×0.55mm. The wave soldering conditions are shown in Table III. The wave soldering temperature was set at 523K, about 30K higher than the melting point of the solders. The creep test conditions were at temperatures of 313K, 348K and 398K, and loads of 300g,

187

400g, 2000g, and 3000g respectively. Moreover, because the part of the TH solder joints was too small, only the creep rupture time was recorded. Microstructural ObservationThe samples were prepared for scanning electron microscopy (SEM) by wet grinding to #2400-grit sandpaper, followed by diamond particle polishing down to 0.25 m, and a final polishing stage by using colloidal silica suspension. After polishing, the samples were etched by Ar+ ion milling and observed with SEM. Moreover, microstructural analysis of the initial and the creep samples were performed by using TEM at an acceleration voltage of 300kV. Element mapping by electron probe X-ray micro-analyzer (EPMA) was also investigated to determine the composition of particles.

Results and Discussion

Creep PropertiesFigure 4 (a), (b), (c) shows creep curves until rupture time of the Sn-3.5Ag-0.5Cu-Ni-Ge solder and the Sn-3.0Ag-0.5Cu solder at temperatures of 313K, 348K, and 398K under the applied stresses of 5MPa, 9.8MPa, 14.7MPa and 19.6MPa, respectively. Figure 5 shows the relationship between the creep stress and the creep-rupture life of the Sn-3.5Ag-0.5Cu-Ni-Ge solder and the Sn-3.0Ag-0.5Cu solder at each temperature. The creep rupture time of Sn-3.5Ag-0.5Cu-Ni-Ge solder is about 3 times longer than that of the Sn-3.0Ag-0.5Cu solder at 398K, although it is rather shorter at 313K and 348K. It’s considered that additing Ni is beneficial to improve the creep strength of the Sn-3.0Ag-0.5Cu solder in higher temperature regime.

0

10

20

30

0 100 200 300 400 500Time (h)

Cre

ep s

train

(%)

Sn-3.5Ag-0.5Cu-Ni-Ge (14.7MPa)Sn-3.0Ag-0.5Cu (14.7MPa)Sn-3.5Ag-0.5Cu-Ni-Ge (19.8MPa)Sn-3.0Ag-0.5Cu (19.8MPa)

0

10

20

30

0 200 400 600 800

Time (h)

Cree

p str

ain

(%)

Sn-3.5Ag-0.5Cu-Ni-Ge (9.8MPa)Sn-3.0Ag-0.5Cu (9.8MPa)Sn-3.5Ag-0.5Cu-Ni-Ge (14.7MPa)Sn-3.0Ag-0.5Cu (14.7MPa)

(a) 313K (b) 348K

0

10

20

30

40

50

0 100 200 300 400 500 600

Time (h)

Cre

ep st

rain

(%)

Sn-3.5Ag-0.5Cu-Ni-Ge (5MPa)Sn-3.0Ag-0.5Cu (5MPa)Sn-3.5Ag-0.5Cu-Ni-Ge (9.8MPa)Sn-3.0Ag-0.5Cu (9.8MPa)

188

(c) 398K Figure 4: Creep curves of the bulk of both solders at (a) 313K, (b) 348K and (c) 398K respectively.

0

5

10

15

20

25

30

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04

Rupture time (h)

Stre

ss (

MPa

)

Sn-3.5Ag-0.5Cu-Ni-Ge Sn-3.0Ag-0.5Cu

313K

348K

398K

Figure 5: Creep strength of the Sn-3.5Ag-0.5Cu-Ni-Ge and the Sn-3.0Ag-0.5Cu solders.

Sn-3.5Ag-0.5Cu-Ni-Ge

n=8.6

n=6.9n=11.3

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1 10 100Stress (MPa)

Stea

dy-s

tate

cre

ep ra

te (

%/s)

398K348K313K

Sn-3.0Ag-0.5Cu

n=9.4

n=5.1

n=12.7

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1 10 100 Stress (MPa)

Stea

dy-s

tate

cre

ep ra

re (

%/s)

398K348K313K

(a) (b) Figure 6: Relationship between the stress and minimum creep rate of the Sn-3.5Ag-0.5Cu-Ni-Geand the Sn-3.0Ag-0.5Cu solders.

Moreover, because some casting crack were observed in the fracture of the Sn-3.5Ag-0.5Cu-Ni-Ge solder at 348K, the shorter creep rupture life seems to be affected by the casting crack. Creep behavior can be normally characterized by the stress-strain rate relationship. The minimum creep rate is one of the most important creep parameters for fundamental and engineering studies. Its stress dependence is often described by the power-law equation:

=A n (1)

Here is the minimum creep rate, and is applied stress. n is the stress exponent, A is a material constant; both A and n depend upon temperature. As expected, the creep rate increases progressively

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with stress at a given temperature, and with temperature at a fixed stress; these relationships are readily observed in Fig.6.Figure 6 (a), (b) shows minimum creep strain rate as a function of applied stress of both solders at 313K, 348K, and 398K respectively. Figure 6 (a) indicates that the stress exponent of the Sn-3.5Ag-0.5Cu-Ni-Ge solder decreased with increasing temperatures (from n=11.3 at 313K to n=8.6-6.9 at 348-398K). On average, the stress exponent is approximately 8.9. Figure 6 (b), for Sn-3.0Ag-0.5Cu solder, also shows the tendency similar to Fig.6 (a). The stress exponents of Sn-3.0Ag-0.5Cu are 5.1 at 398K, 9.4 at 348K, and 12.3 at 313K, respectively. The creep mechanism of Sn-base solder alloys in the homologous temperature range of 0.63-0.81, and stress range of /E= 10-5- 10-3 is in the dislocation-creep regime.4-8 The higher n value of stress exponent, the better strengthening effect of the second phases in the matrix Sn. By comparing the both solders, the n value of stress exponent for Sn-3.5Ag-0.5Cu-Ni-Ge is larger than that of Sn-3.0Ag-0.5Cu at 398K, but smaller at 348K and 313K. The temperature dependence of creep deformation, and particularly the activation energy Q, can be calculated from the ln -1/T plot, according to Arrhenius type equation (2), and the results are shown in Fig.7.

=A ( /G)n exp (-Q/RT) (2)

where is the minimum creep rate, A is a constant, is applied stress, G is the temperature-dependent shear modulus, n is the stress exponent, Q is the creep activation energy, R is the universal gas constant, and T is the absolute temperature The apparent activation energy of creep for the Sn-3.5Ag-0.5Cu-Ni-Ge and the Sn-3.0Ag-0.5Cu solders is 48kJ/mol and 68kJ/mol under a constant stress of 9.8MPa respectively.The lattice self-diffusion activation energy of the pure tin is 102kJ/mol.9-12 The dislocation-pipe diffusion activation energy is about 0.6 times the self-diffusion activation energy, that is, 60kJ/mol.9-12

The creep-activation energies, for the both solders are lower and close to dislocation pipe diffusion in the test conditions. Moreover, the creep-activation energy for the Sn-3.0Ag-0.5Cu solder is higher, which implies that the microstructure of the Sn-3.0Ag-0.5Cu solder in this case is much more sensitive to the creep testing temperatures. 13, 14 It is considered that the microstructure of the Sn-3.5Ag-0.5Cu-Ni-Ge solder is more stable in the temperature range.The creep results of the TH joints of the both solders at 398K are shown in table IV. To show the relation of the applied stress, temperature, and creep rupture time of the bulk and the TH joints of the both solders, Sn-3.5Ag-0.5Cu-Ni-Ge and Sn-0.3Ag-0.5Cu, a Larson-Miller15,16 type plot ofall data is given in Fig. 8. The Larson-Miller Parameter (L.M.P) can be expressed in a form such as (3)

L.M.P = T (C+log(t)) (3)

where T is the absolute temperature, C is a constant (here 11), and t is the creep rupture time. Figure 8 shows that the creep strength of the Sn-3.5Ag-0.5Cu-Ni-Ge solder was higher than that of the Sn-3.0Ag-0.5Cu solder in relatively high temperature and low stress region. Moreover, the creep result of the bulk specimen is corresponding to the TH joints.

MicrostructureTo clarify the relation between creep strength and the microstructure, microstructural observation was conducted. Figure 9 is the SEM micrograph of bulk specimen of the both solders at initial state.

190

The microstructure of the Sn-3.5Ag-0.5Cu-Ni-Ge solder shows two types of regions. The black-gray colored regions are dendritic -Sn phase, and the light-colored regions are eutectic network band of

dispersed Ag3Sn, and (Cu, Ni)6Sn5 intermetallic within the -Sn phase. It is finer and uniform, and the hardness of the eutectic regions is about 0.45GPa by nano-indentation testing.

24 26 28 30 32 34-25

-20

-15

-10

-5

Sn-3.0Ag-0.5Cu: Q = 68kJ/mol Sn-3.5Ag-0.5Cu-Ni-Ge: Q = 48kJ/mol

ln (c

reep

rate

)

1/T 104(K-1)

398K 313K

=9.8MPa

Figure 7: Arrhenius plots of strain and reciprocal temperature of the Sn-3.5Ag-0.5Cu-Ni-Ge and the Sn-3.0Ag-0.5Cu solders.

1

10

100

3.5 4.0 4.5 5.0 5.5 6.0

L.M.P = T(11+log(t))×10 3

Stre

ss (

MPa

)

Sn-3.5Ag-0.5Cu-Ni-Ge(IMD)Sn-3.5Ag-0.5Cu-Ni-Ge ( 3)Sn-3.0Ag-0.5Cu (IMD)Sn-3.5Ag-0.5Cu ( 3)

Figure 8: Correlation of creep strength with Larson-Miller parameter.

Table IV. Creep results of the TH joints of the both solders at 398K.

Creep rupture time (h) Load (g)

Sn-3.5Ag-0.5Cu-Ni-Ge Sn-3.0Ag-0.5Cu

300 500(stopped) 500(stopped)

191

400 500(stopped) 500(stopped)

2000 740 170(stopped) 168 153

3000 7 16 6 7

(a) Sn-3.5Ag-0.5Cu-Ni-Ge solder (b) Sn-3.0Ag-0.5Cu solder

40 m

(Ag3Sn+(Cu, Ni)6Sn5)

-Sn

(Ag3Sn+Cu6Sn5)

-Sn

40 m

Figure 9: Microstructure of the bulk specimen of the solders.

B side

100 m

F side

100 m

F side

B side

100 m

100 m

(a) Sn-3.5Ag-0.5Cu-Ni-Ge solder (b) Sn-3.0Ag-0.5Cu solder Figure 10: Microstructure of cross-section of the TH joints of the both solders.

On the other hand, segregated (larger block) eutectic regions of dispersed Ag3Sn, and Cu6Sn5

intermetallic within the -Sn phase, which are dense microstructure, are observed in the Sn-3.0Ag-

192

0.5Cu solder from Fig.9 (b), and the hardness of the segregated eutectic region is about 0.7GPa. It was easily thought that the deformation was unbalanced under an applied stress. Figure 10 shows the microstructure of the TH joints for the both solders after wave soldering. The microstructure of TH joints for Sn-3.0Ag-0.5Cu was also segregated as similar to the bulk sample. Moreover, shrinkage cracking was easily observed from the surface and cross-section of the TH joints. Whereas, the TH joint of the Sn-3.5Ag-0.5Cu-Ni-Ge solder does indeed show an apparently finer and

(a) Initial state of eutectic region (b) Initial state of eutectic region

(c) After crept at 398K,5MPa for 200h

(d) After crept at 398K,5MPa for 180h

Sn-3.5Ag-0.5Cu-Ni-Ge solder Sn-3.0Ag-0.5Cu solder

Figure 11: SEM microstructure of eutectic region of the both solders.

uniform microstructure than that of the Sn-3.0Ag-0.5Cu solder. Figure 11 shows a SEM image of a eutectic region of the Sn-3.5Ag-0.5Cu-Ni-Ge solder and the Sn-3.0Ag-0.5Cu solder at initial state and after creep test at 398K, 5MPa.Before creep test, although the microstructure of Sn-3.0Ag-0.5Cu solder has a higher density of Ag3Snparticles compared with the Sn-3.5Ag-0.5Cu-Ni-Ge solder, both of the solders have fine microstructure in the eutectic regions. After a creep test at 398K, 5MPa, the particles size for both solders coarsened.It should be especially noted that precipitations of Cu6Sn5 appeared and significantly grew up in the Sn-3.0Ag-0.5Cu solder.17 The apparent difference in creep rupture strength between the Sn-3.5Ag-0.5Cu-Ni-Ge solder and the Sn-3.0Ag-0.5Cu solder at high temperature (398K) and low stress (5MPa) depends on the size and the stability of the precipitation particles in the eutectic region.In the case of high temperature and low stress, precipitation of Cu6Sn5 grows rapidly in eutectic region of the Sn-3.0Ag-0.5Cu solder, and the cavity and/or the crack, generated by the coarsened precipitation, propagated, and the creep rupture occurred in a shorter time. On the other hand, the precipitation, which

(Cu6Sn5+Ag3Sn)

2 m2 m

2 m 2 m

193

looks like coarsened Cu6Sn5, was not observed in the Sn-3.5Ag-0.5Cu-Ni-Ge solder. This seems to be affected by adding Ni into the Sn-3.0Ag-0.5Cu solder. Figure 12 shows a similar comparison for TH joints of the both solders after creep testing at 398K, 2000g for 170h. Significant microstructural changes were also observed in the TH joints of the Sn-3.0Ag-0.5Cu solder.To analyze the elemental composition of the precipitate particles, EPMA mapping analysis was performed at the same areas of the TH joints of the Sn-3.0Ag-0.5Cu solder.

(a) After crept at 398K, 2000g for170h (b) After crept at 398K, 2000g for170h

Sn-3.5Ag-0.5Cu-Ni-Ge solder Sn-3.0Ag-0.5Cu solder Figure 12: BS microstructure of the TH joints of the both solders.

194

SEM

BEI

Analyzed area

Cu

AgNi

Sn

Figure 13: EPMA mapping analysis of the TH joints of the Sn-3.0Ag-0.5Cu solder after creep testing at 398K, 2000g for 170h.

Sn phase

Ag3Sn / Sn Eutectic region Dislocations

Stacking Fault

(a) Initial state (b) After creep testing at 398K, 5MPa for 550h

Figure 14: TEM micrograph of the Sn-3.5Ag-0.5Cu-Ni-Ge solder.18

Figure 13 shows the result of EPMA analysis. Cu, Sn elements were detected from the strong coarsened precipitates, but the Ni element cannot be confirmed. The precipitate particles were IMCs of the Cu6Sn5.17

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To investigate the relation between the creep strength property and precipitates in detail, TEM analysis was also executed. Figure 14 is a TEM image of Sn-3.5Ag-0.5Cu-Ni-Ge solder in initial state and crept at 398K. In the initial state, the size of precipitates is 0.1 0.2 m. The precipitates grow to 0.5-1.0 mafter the creep test at 398K and 5MPa for 550h, and the size is several times larger compared to that of the initial state. Moreover, dislocations are blocked by the precipitates and stacking faults are observed. It is believed that the smaller precipitates contribute to improve creep strength. Moreover, the precipitate of the (Cu, Ni)6Sn5, diameter of which is about 0.5 m, was confirmed in the Sn-3.5Ag-0.5Cu-Ni-Ge solder. 18

Concluding Remarks

Relationships between microstructural and creep behaviors of the bulk specimen and the TH joints of Sn-3.5Ag-0.5Cu-Ni-Ge and Sn-3.0Ag-0.5Cu solder are reported and compared. The following conclusions are summarized.

The creep behavior of the both solders is investigated at three temperature ranging form 313K- 398K, and the stress range of /E= 10-5- 10-3 . The creep strength of Sn-3.5Ag-0.5Cu-Ni-Ge solder is about 3 times better than that of the Sn-3.0Ag-0.5Cu solder at 398K. Moreover, it is possible to correspond with the creep strength of the bulk specimen and the creep strength of the TH joints sample by L.M.P plot. In this study, the stress exponent of the Sn-3.5Ag-0.5Cu-Ni-Ge and the Sn-3.5Ag-0.5Cu were 6.9, 5.1, and their apparent activation energy were 48kJ/mol and 68kJ/mol respectively at 9.8MPa. It is considered that the microstructure of the Sn-3.0Ag-0.5Cu solder in this case is much more sensitive to the creep testing temperatures. The initial microstructure of the Sn-3.5Ag-0.5Cu-Ni-Ge is finer and uniform than that of the Sn-3.0Ag-0.5 solder. The creep characteristic was apparently affected by the IMCs growth in eutectic region. The stable precipitates of the (Cu, Ni) 6Sn5 formed by adding Ni is effectively to improve creep strength at higher temperature (398K) and lower stress (5MPa) region.

References

1. NCMS, “Lead-Free Solder Project”, Final Report, 0401RE96(1997). 2. JEITA Lead-free Roadmap 2002 ver.1.0 (2002).3. M. Nagano et al.,“Effect of Germanium content on Oxidation Prevention of Sn-Ag-Cu Lead-

Free Solder”, Proc. of New Frontiers of Process Science and Engineering in Advanced Materials (High Temperature Society of Japan, 2004), 256-261.

4. V.I.Igoshev and J.I.Kleiman, “Creep Phenomena in Lead-Free Solders ”, J. Electron. Mater. 29, (2000), 244-250.

5. C.M.L.Wu and M.L.Huang, “Creep Behavior of Eutectic Sn-Cu Lead-Free Solder Alloy”, J. Electron. Mater. 31, (2002), 442-448.

6. M.Kerr and N.Chawla, “Creep deformation behavior of Sn-3.5Ag solder/Cu couple at small length scales”, Acta. Metall. 52, (2004), 4527-4535.

7. R. J. Mccabe, M. E. Fine, “Creep of Tin, Sb-Solution-Strengthened Tin, and SbSn-Precipitate-Strengthened Tin”, Metallurgical and Materials Transactions. 33A, (2002), 1531-1538.

8. K. Atsumi, Y. Kariya, and M. Otsuka, “Creep Properties of Sn-3.5Ag-xBi and Sn-3.5Ag-xCu Solder Alloy”, Proc. of 6th Symposium on Microjoining and Assembly Technology in Electronics (The Japan Welding Society, 2000), 281-286.

196

9. Y.Kariya, M. Otsuka, and W.J.Plumbride, “The Constitutive Creep Equation for a Eutectic Sn-Ag Alloy Using the Modified Theta-Projection Concept”, J. Electron. Mater. 32, (2003), 1398-1402.

10. I.Shohji et al.,“Mechanical Properties of Low-melting Lead-Free Solders”, Proc. of 9thSymposium on Microjoining and Assembly Technology in Electronics, (The Japan Welding Society, 2003), 229-234.

11. Z.Chen, Y.Shi, and Z.Xia, “Constitutive Relations on Creep for SnAgCuRE Lead-Free Solder Joints”, J. Electron. Mater. 33, (2004), 964-971.

12. F.Ochoa, X.Deng, and N.Chawla, “Effects of Cooling Rate on Creep Behavior of a Sn-3.5Ag Alloy”, J. Electron. Mater. 33, (2004), 1596-1607.

13. P.T.Vianco, J.Rejent, and A.C.Kilgo, “Creep Behavior of the Ternary 95.5Sn-3.9Ag-0.6Cu Solder-Part I: As-Cast Condition”, J. Electron. Mater. 33, (2004), 1389-1400.

14. Q.Xiao and W.D.Armstrong, “Tensile Creep and Microstructure Characterization of Bulk Sn3.9Ag0.6Cu Lead-Free Solder”, J. Electron. Mater. 34, (2005), 196-211.

15. D.Mitlin, C.H.Raeder, and R.W.Messler,Jr., “Solid Solution Creep Behavior of Sn-xBi Alloy”, Metall. Mater.Trans.30A,(1999),115-122.

16. R.Ninomaya, Y.Nakahara, and T.Takemoto, “Microstructure and Mechanical Property of Sn-Ag-Bi-In Solder”, Proc. of 9th Symposium on Microjoining and Assembly Technology in Electronics (The Japan Welding Society, 1998), 249-252.

17. M.Shimoda et al., “Effect of Aging on the Strength and Microstructure of Lead Free Solders”,Proc. of 9th Symposium on Microjoining and Assembly Technology in Electronics, (The Japan Welding Society, 2003), 325-330.

18. N.Hidaka et al., “Creep Properties and Microstructure of the Sn-Ag-Cu-Ni-Ge Lead-Free Solder Alloy”, Proc. of ASME/Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems. (ASME, InterPACK, 2005), 73148.

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