Effects of bismuth on growth of intermetallic compounds in Sn-Ag-Cu Pb-free solder joints

Effects of bismuth on growth of intermetallic compounds in Sn-Ag-Cu Pb-free solder joints

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Available online at www.sciencedirect.com OOICNQL

Science Press

@

DIRCEIT.

Transactions of Nonferrous Metals Society of China

Trans. Nonferrous Met. SOC.China 16(2006) s739-s743 www.csu.edu.cn/ysxb/

Effects of bismuth on growth of intermetallic compounds in Sn-Ag-Cu Pb-free solder joints LI Guo-yuan', SHI Xun-qing' 1School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798; 2. Intel Technology Development Ltd, Shanghai 20013 1, China Received 10 April 2006; accepted 25 April 2006 Abstract: The effects of Bi addition on the growth of intermetallic compound (IMC) formation in Sn-3.8Ag-0.7Cu solder joints were investigated. The test samples were prepared by conventional surface mounting technology. To investigate the element diffusion and the growth kinetics of intermetallics formation in solder joint, isothermal aging test was performed at temperatures of 100, 150, and 190 C , respectively. The optical microscope (OM) and scanning electron microscope (SEM) were used to observe microstructure evolution of solder joint and to estimate the thickness and the grain size of the intermetallic layers. The IMC phases were identified by energy dispersive X-ray (EDX) and X-ray diEactometer (XRD).The results clearly show that adding about 1.O% Bi in Sn-Ag-Cu solder alloy system can refine the grain size of the IMC and inhibit the excessive IMC growth in solder joints, and therefore improve the reliability of the Pb-free solder joints. Through observation of the microstructural evolution of the solder joints, the mechanism of inhibition of IMC growth due to Bi addition was proposed. Key words: Sn-Ag-Cu alloy; Pb-fkee solder; intermetallics; solder joint; bismuth; interconnects

1 Introduction The Sn-Ag-Cu alloy is a very attractive candidate for implementation of lead-free soldering because of its advantages in mechanical properties and solderability [ 1-31. However, the intermetallic compound (IMC) growth in Sn-Ag-Cu solder joints is faster than that in eutectic Sn-Pb solder joints [4, 51. It is well known that in Sn-containing solder joint, solder bonds with a Cu substrate through the formation of a dual Cu-Sn IMC layer consisting of Cu& and Cu3Sn that exist between the solder and Cu substrate. Because the IMC layers are more brittle than that of the solder matrix, they can be a site of mechanical weakness, causing failure of the joint with fractures in the IMC layer itself or along the interface between solder and IMC layer [6]. The excessive growth of the IMC in the Sn-Ag-Cu solder system will do harm to the long-term reliability of this alloy. Therefore, the prevention of excessive growth of IMC in solder joint becomes a challenging task for material researchers. Some researchers reported that a little amount of other elemental addition, such as Bi, In, Sb, and Zn improves solder strength, thermal resistance or fatigue

life due to its solid solution strengthening effect [7-91. Our previous work revealed that adding a small amount of Sb into Sn-Ag-Cu solder can visibly improve the wetting and mechanical properties of the solder. Elemental addition is also a possible method to inhibit the excessive growth of the IMC layer [lo, 111. This investigation aimed at exploring the effects of Bi addition on the formation and growth kinetics of the IMC layer between Sn-Ag-Cu lead-free solder and copper substrate.

2 Experimental Commercial Cu strips were used as substrates in this study. The Cu substrates were grounded and polished by using diamond paste with particle size of 0.25 km until a mirror surface was obtained. The prepared substrates were then dipped into 50% (by volume) nitric acid (HN03) to remove any oxide layer which may be present. Six different compositions of the Sb-containing solder Sn-3.5Ag- 0.7Cu-xBi (x = 0.0, 1.0, 2.0, 3.0, 4.0 and 5.0) were then coated on the substrates and solder joints were formed by means of a standard infrared reflow oven. The thickness of the coated solder paste was about 0.5 mm. To study the effects of Bi addition on

Corresponding author: LI Guoyuan; Tel: +65-67904583; Fax : +65-6790908 1; Email: [email protected]

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LI Guo-yuan, et aVTrans. Nonferrous Met. SOC.China 16(2006)

the formation and the growth kinetics of IMC layer between Sn-3.5Ag- 0.7Cu-xBi lead-free solder and Cu substrate, the specimens were reflowed at a peak temperature of 250 "C for 60 s, and then aged in the blinder M115 aging oven at temperatures of 120, 150, and 190 "C for 0, 200, 400, 600, 800, and 1 000 h, respectively. Metallographic preparation for all solder joints were done according to the method described in our previous work [lo]. Energy dispersive X-ray analysis (EDX) and XRD were used to characterize the composition of the IMCs. In order to determine the IMC thickness, two samples were prepared for each ageing condition. Eight SEM photographs were taken at difference positions for each sample. Software named "Image tool" was used to measure the area and the length of IMC for each photograph. The area divided by length was defined as thickness of IMC. Then sixteen thickness data were used to get an average thickness for one isothermal ageing condition.

3 Results and Discussion Backscattered SEM model was used to observe the cross-sectional microstructural evolution of the solder joints in this investigation as this model can show different IMC layers of C&Sn5 and Cu3Sn clearly. Figs. 1 and 2 show the typical SEM images of the solder/Cu interface of the Sn-3.5Ag-0.7Cu-xBi (x= 0, 1.0, 2.0, 3.0, 4.0, and 5.0) solder joints aged at the temperature of 190 'C for 0, 200, 400, 600, 800, and 1 000 h, respectively.

Observation reveals that the morphology of Cu6Sn5 phase gradually transforms from the scallop-type to layer-type during aging. Except as-soldered solder joint, it is clearly shown that layer structure of the CbSn, and Cu3Snphases exists at the interface. Fig.3 shows the relationship between average IMC thicknesses and ageing time at the aging temperature of 120 "C . The results shaw that the IMC thickness decreases with the increase of Bi composition and the downward trend persists until the mass fraction of Bi approaches 1.0%-2.0%. Beyond this amount, the IMC thickness increases again. The IMC growth at other two ageing temperatures has the similar trend. The relationship between the thickness of IMC layer and aging time is generally considered to follow the Fick's law:

[email protected] where X i s the average IMC thickness of the layer, r is the aging time and D is the interdiffision coeficient. The interdiffision coefficient can be determined by a linear regression analysis of X vs where the slope of the The IMC growth rate with different straight line is Bi compositions in Sn-3.5Ag-0.7Cu-xBi solder alloys for the aging temperature of 120, 150, and 190 "C are shown in Table 1. The results reveal that the lowest growth rate is located at the Bi composition of about 1.O%. A possible explanation for this phenomenon is that the mutual solubility of Sn and Bi increases with the increase in temperature leading to more fine particles

6.

4,

Fig.1 Backscattered SEM micrographs of cross-sectional view of Sn-3.5Ag-0.7Cu-2.OBi solder joints aged at 190 "C for different times: (a) 0 h; (b) 200 h; (c) 400 h; (d) 600 h; (e) 800 h; (f) 100 h

LI Guo-yuan, et al/Trans. Nonferrous Met. SOC.China 16(2006)

s74 1

Fig.2 Backscattered SEM micrographs of cross-sectional view of Sn-3.5Ag-0.7Cu-xBi solder joints aged at 190 "C for 400 h: (a)x=O; (b)x=l.O; (c)x=2.0; (d)x=3.0; ( e ) A . O ; ( 0 ~ 5 . 0

10.0% Bi 11 .O% Bi 02.0% Bi 1 3 . 0 % Bi 1 4 . 0 % Bi 1 5 . 0 % Bi

Fig.3 Average IMC thickness against aging time at aging temperature of 120 'C

precipitated on the grain boundary to inhibit the diffusion of Sn and Cu atoms across the IMC layer, which will discuss later on. In this study, Arrhenius relationship was used to determine the activation energy for intermetallic growth:

Table 1 Growth rate of 95.8Sn-3.5Ag-0.7Cu-xBi at 120, 150 and 190 "C

D/(cm*.s-')

w(Bi)/ %

120 'C

(2)

0.0

where D is the interdiffusion coefficient; Do is the pre-exponential constant of interdiffusion coefficient; Q is the activation energy, k is the Boltzmann constant; and T is the absolute temperature. The interdiffusion coefficient obtained from the thickness measurement was used to determine the activation energy for the growth of the intermetallics. The relationship between activation energies and Bi compositions are shown in Fig.4. The

D = D oexp(-)-Q kT

150 'C

190 'C

8 . 1 3 6 lo-'' ~

8.009~

4.956~

1.O

1.8 15x lo-''

5.153~

4 . 4 4 9 ~10-13

2.0

9 . 8 0 1 ~ 1 0 - ' ~ 2 . 9 2 4 ~ 1 0 - l ~ 5.242~10-l~

3.0

1.904~

4.0

2 . 0 7 4 ~ 1 0 - ' ~ 3.764~10-I~ 4.761~10-I~

5.0

1.277~

5.108~

2.890~

4.583~

4.173~IO-I~

LI Guo-yuan, et aVTrans. Nonferrous Met. SOC.China 16(2006)

s142

0

I

1

1.0

2.0

I

3.0 w(Bi)/Yo

1

I

4.0

5.0

Fig.4 Relationship between activation energy and Bi

composition results reveal that Bi-fiee solder joint has the lower activation energy of 88.23 kJ/mol. When 1.0% Bi is added, the activation energy raises to 117.87 kJ/mol. However, continuous increasing Bi to 4.0% causes the activation energy to decrease gradually and even lower than that of the Bi-free solder joint. When compositions of Bi further increase over 4.0%, the activation energy starts to rise again. Correlating IMC growth rates and activation energies, it can be concluded that adding of about 1.0% Bi in Sn-3.5Ag-0.7Cu solder alloy can increase the activation energy, and thus reduce the atomic diffusion rate so as to inhibit the excessive growth of IMC. The SEM micrographs of the top view of three types of solder joints aged at 190 'C for 600 h are shown in F i g 5 The results reveal that the grain size of IMCs in 1.O% Bi-containing solder joint is smaller, which may indicate that adding suitable amount of Bi can help to refine the grain size of the IMC and hence improves the mechanical properties of the solder joints. Until now, no phase diagram on Sn-Ag-Cu-Bi quarternary alloy can be found in publications. However it can be divided into four ternary systems to analyze the IMC phases [12, 131. Integrating four ternary systems together, the Sn-3.5Ag-0.7Cu-xBi (x is less than 5.0) alloy may consist of several phases, that is, the primary Sn-rich phase, the Ag3Sn phase, Bi compound, C&Sns phase and Cu3Sn phase. It was reported by some researchers that some Ag3Sn particles precipitated along the grain boundary of C&Sn5 IMC in Sn-Ag-Cu solder joints [ 10-1 1, 141, which may be an important factor that affects the growth of the interfacial IMC layer in solder joints. The results also confirm that many fine particles pin along the grain boundary in both the Bi-containing and Bi-free samples but the compositions of the particles in Bi-containing solder joints are somewhat different fiom that in Bi-free solder joints. Actually these small particles can precipitate in the channels between the IMC

Fig.5 SEM micrographs of top view of IMC layer of Sn-3.5Ag-0.7Cu-xSb solder joints aged at, 190 'C for 800 h: (a) FO; (b) ~ 2 . 0(c) ;~ 5 . 0

grains during reflow process. Fig.S(c) clearly reveals that the distinguishable particle pin along the grain boundaries of the IMC, which are suggested to be Ag3Sn, C&Sns and Bi-rich phases respectively by EDX analyses shown in Figs.6-8. It is also observed that the 1.0% Bi-containing solder joint has more fine particles precipitating along the grain boundary than that of other joints. From above results, it is clear that there is some correlation between the thickness of IMC and the amount of particles precipitated along the IMC grain boundary. As well known, grain boundary diffusion plays an important role in the growth kinetics of IMC in solder joints. IMC growth is controlled by diffusion of the constituent atomic species through the IMC layer to the reaction sites located at the solder/CusSns interface and the Cu3SdCu interface. Generally, elements diffuse through the grain boundaries more rapidly than through the grain to maintain continuous interfacial reaction [15, 161. The existence of Ag3Sn,

LI Guo-yuan,et ayTrans. Nonferrous Met. SOC.China 16(2006)

C%Sns and Bi-rich particles on the CySns grain boundary actually obstructs the grain boundary diffusion of Cu and Sn via the IMC layer leading to inhibit the growth of the IMCs. Element Ag Sn

8

wPh

xPh

1002 2998

14 14 2586

T-sn 1

- Zn

I

s743

morphologies of Sn-3.8Ag-0.7Cu solder joints have been changed as a result of the Bi addition. The IMC layer becomes thhner and the grain size becomes smaller with the l.Owt% Bi addition in ageing process. It is also observed that the fine particles precipitating could increase with the addition of the 1.OW% Bi. It is found that with about 1.Owt% Bi addition, the activation energy of Sn-3.8Ag-0.7Cu solder alloy system get the highest value, which leads to reduce the atomic diffusion rate so as to inhibit the excessive growth of the IMC. The effect of Bi addition on the IMC growth may be explained with grain boundary pinning mechanism.

References 0

2.00

4.00

6.00

8.00

10.00

EkeV

Fig.6 EDX analysis of small particle A pinning on grain boundary shown in Fig.S(c) [31

1

I Element

Cu Sn Total

w/%

xPh

41 26 5874

5614 4326 100

100

[41

0-c A

-cU

T

-Au

[71

ElkeV

Fig.7 EDX analysis of small particle B pinning on grain boundary shown in Fig.S(c)

w/%

x/%

Cu

804

Sn

5 69 8627

1652 625 1123

IW

100

Element

T

81 Total

'-C

T

A T

-cu - Bi

8-Au

- Sn

I

0

2.00

4.00

6.00

8.00

10.00

ElkeV Fig.8 EDX analysis of small particle C pinning on grain boundary shown in Fig.S(c)

4 Conclusions The effect of Bi addition on IMC growth in Sn-3.5Ag-0.7Cu solder joints has been studied in this work. SEM micrographs show that the IMC

WOOD E P, NIMMO K L. In search of new lead-t?ee electronic solders [J]. Journal of Electronic Materials, 1994, 23(8): 709-713. HARRISON M R,VINCENT J H, STEEN H AH. Lead-free reflow soldering for electronics assembly [J]. Soldering & Surface Mount Technology, 2001, 13(3): 21-38. GADAG S, PATRA S. Numerical prediction of mechanical properties of Pb-Sn solder alloys containing antimony, bismuth and or silver ternary trace elements [J]. Journal of Electronic Materials, 2000, 29( 12): 1392-1397. LEE T Y, CHOI W J, TU K N, JANG J W, KUO S M, LIN J K. Morphology, kinetics, and thermodynamics of solid-state aging of eutectic SnPb and Pb-fiee solders (Sn-3.5Ag, Sn-3.8Ag- 0.7Cu and Sn-0.7Cu) on Cu [J]. Journal of Materials Research, 2002, 17(2): 291-301. FREAR D R, JANG J W, LIN J K, ZHANG C. Pb-fiee solders for flip-chip interconnects [J]. Journal of Metals, 2001,53(6): 28-32. LEE Y G, DUH J G Interfacial morphology and concentration profile in the unleaded s o l d e r 0 joint assembly [J]. Journal of Materials Science: Materials in Electronics, 2000, 11: 33-43. KARIYA Y, OTSUKA M. Mechanical fatigue characteristics of SnJSAg-X (X= Bi, Cu, Zn, and In) solder alloys [J]. Journal of Electronic Materials, 1998,27(11): 1229-35. WADE N, WU K P, KLJNII J, YAMADA S, MIYAHARA K. Effect of Cu, Ag and Sb on the creep-rupture strength of lead-free solder alloys [J]. Journal of Electronic Materials, 2001,30(9): 1228-1231. LI C Y, CHEN B L, SHI S Q, WONG S C K, Effects of Sb addition on mechanical properties of Sn-3.5Ag-0.7Cu solder alloy and joints [J]. Thin Solid Films, 2006,504: 421-425. LI G Y, CHEN B L, TEY J N. Reaction of Sn-3.5Ag-0.7Cu-xSb solder with Cu metallization during reflow soldering [J]. IEEE Trans on EPM, 2004,27(1): 77-85. CHEN B L, LI G Y. An Investigation of effects of Sb on the intermetallic formation in Sn-3.5Ag-0.7Cu solder joints [J]. IEEE Trans on Comp & Pack Tech, 2005,28(3): 534-541. MIYASHITA 0 M, ANZAI K, LIU X J, OHANl H, KAINUMA R, ISHlDA K. Phase equilibria and the related properties of Sn-Ag-Cu based Pb-free solder alloys [J]. Journal of Electronic Materials, 2000, 29( 10): 1137-1 144. VILLARS P, PRINCE A, OKAMOTO H. Handbook of Ternary Alloy Phase Diagrams [MI. OH: ASM International, 1997. YE L, LA1 Z H, LIU J, THOBLEAN A. Microstructure investigation of Sn-0.5Cu-3.5Ag and Sn-3.5Ag-0.5Cu-OSB lead-free solders [J]. Soldering & Surface Mount Technology, 2001, 13(3): 16-20. HA J S, KANG C S, PARK J Y, JUNG J P. Effect of soldering and aging time on interfacial microstructure and growth of intermetallic compounds between Sn-3.5Ag solder alloy and Cu substrate [J]. Journal of Electronic Materials, 2000, 29( 10): 1207-1 2 13. KIM H K, LIOU H K, TU K N. Three-dimensional morphology of a very rough interface formed in the soldering reaction between eutectic SnPb and Cu [J]. Appl Phys Lett, 1995,66(18): 2337-2339. (Edited by LI Xiang-qun)