Elsevier

Journal of Alloys and Compounds

Volume 591, 5 April 2014, Pages 297-303
Journal of Alloys and Compounds

Enhanced growth of the Cu6Sn5 phase in the Sn/Ag/Cu and Sn/Cu multilayers subjected to applied strain

https://doi.org/10.1016/j.jallcom.2013.12.204Get rights and content

Highlights

  • Sn/Cu interfacial reaction subjected to applied strain is investigated.

  • Both tensile and compressive strain enhance the Cu6Sn5 growth.

  • Compressive strain especially results in remarkably enhanced growth of Cu6Sn5.

  • Enhanced Cu6Sn5 growth is attributed to enhanced grain boundary diffusion in Cu6Sn5.

Abstract

Thin Ag and Sn layers were deposited on flexible copper clad laminates (FCCLs) to form two types of multilayers, Sn/Ag/Cu and Sn/Cu. The Sn/Ag/Cu and Sn/Cu interfacial reactions subjected to applied strains were investigated by bending the FCCL and then placing the samples in an oven at 200 °C. Experimental results indicated that the growth rate of the Cu6Sn5 phase formed on the FCCL subjected to applied strain was enhanced regardless of the strain type, while the enhanced effect from compressive strain was more significant. Focused-ion-beam analysis indicated that the Cu6Sn5 phase formed on the FCCL subjected to compressive strain exhibited a multi-grain structure with smaller grain size, suggesting that the grain growth in the Cu6Sn5 phase was significantly retarded under compression. This multi-grain structure, however, provided high grain boundary density for atomic diffusion and thereby enhanced the growth rate of the Cu6Sn5 phase.

Introduction

Solders are joining materials widely used in microelectronics to electrically and mechanically connect a Si chip and a printed circuit board (PCB). To form a solder joint, reflow is performed, during which solder melts and wets the metallizations on both the Si and PCB sides. Simultaneously, an interfacial reaction accompanying the formation of intermetallic compounds (IMC) usually occurs at the solder/metallization interface [1], [2], [3], [4], [5]. The formation of IMCs can be used as an indicator of a successful joining. However, the IMCs continue to grow during subsequent usage of the microelectronics, inevitably consuming solder and metallization and changing the solder joint into a more complicated multilayer structure. Therefore, understanding the interfacial reactions and the growth of IMCs under any potential usage are essential for the evaluation of solder joints.

Sn is the primary material of solders, and Cu is widely used as the material for metallizations. Sn/Cu joints are the predominate joints in microelectronics. Additionally, a Ag immersion layer may be deposited on the Cu surface as a surface finish to protect the Cu from oxidation during storage [6], thereby forming a Sn/Ag/Cu joint. The interfacial reactions of Sn/Cu and Sn/Ag/Cu solder joints have been extensively studied to provide important information for a reliability evaluation [2], [7], [8], [9]. The Sn/Cu interfacial reaction is a well-known system with two IMCs, the Cu6Sn5 and Cu3Sn phases, forming at the Sn/Cu interface [7], [8], [9]. In the Sn/Ag/Cu joint, the Ag immersion layer might completely dissolve into the molten solder during initial reflow, and a subsequent interfacial reaction occurs between the Sn(Ag) alloy and Cu [10], [11], [12]. However, for flexible PCB application, solid-state bonding instead of high-temperature reflow was usually employed to fabricate the solder joint to avoid potential damage in the temperature-sensitive plastic substrate [13]. In this case, the solders did not melt, and consequently, the Ag layer remained at the interface and participated in the interfacial reaction. Lin and Chen found that a Ag3Sn phase layer was formed at the Sn/Cu interface first, and it significantly affected the growth behavior of the Cu–Sn IMCs subsequently formed [9]. Because flexible PCBs are usually bent to reduce the packaging volume, the solder joints on the bent PCBs may be subjected to strain. Lin et al. also found that applied strains (0.34%) did not affect the growth rate of the Cu–Sn IMCs but resulted in different growth morphologies of the Cu–Sn IMCs depending on the strain type [14].

In addition to flexible PCBs, the solder joints on rigid PCBs may also be subjected to applied strains. For example, in a flip-chip solder joint subjected to thermal cycling, the difference in the coefficient of thermal expansion (CTE) between the Si chip (2.6 ppm °C1) and the plastic substrate (15–18 ppm °C1) usually results in thermal stress and causes severe deformation (shear strain) of solder joints [15], [16]. Liao et al. designed a three-point bending apparatus to bend the Sn/Ni bilayer on a Si chip to investigate the strain effect on the Sn/Ni interfacial reaction [17]. The results revealed that both tensile and compressive strains (0.069%) enhanced the growth rate of the Ni3Sn4 phase. Even though the applied strain has been identified as an important factor when investigating the interfacial reactions of solder joints, relevant studies are limited [14], [17], [18], [19]. In this study, the Sn/Ag/Cu and Sn/Cu multilayers deposited on flexible copper clad laminate (FCCL) were subjected to strains by bending the FCCL. To gain a better understanding of the strain effect, the strain was enhanced by bending the FCCL to a higher degree, up to 2.5%. Experimental results indicated that the applied strains enhanced the growth rate of the Cu6Sn5 phase regardless of the strain type, while the enhanced effect was more significant for the compressive strain.

Section snippets

Experimental procedures

Commercial FCCL (DuPont) was used as the substrate. As seen in Fig. 1, the FCCL was a tri-layer structure composed of a Cu layer and a PI film bonded by a glue layer. The thicknesses of the PI film and the Cu layer were 25 μm and 30 μm, respectively. Two types of samples were prepared. The first one was prepared by successively depositing Ag and Sn on the Cu surface to form a Sn/Ag/Cu trilayer on the FCCL. First, the FCCL was immersed in a commercial Ag plating solution (Makin Technology, Taiwan)

Results and discussion

Fig. 2 shows the cross-sectional SEM micrographs of the Sn/Ag/Cu interface on strained FCCLs after reaction at 200 °C for 24–120 h. The samples in Fig. 2(a)–(f) were subjected to compressive and tensile strain, respectively. As seen in Fig. 2(a) and (d), the immersion Ag layer had been completely consumed and transformed into the Ag3Sn phase layer after 24 h of reaction regardless of strain type. In addition, another two IMCs, Cu6Sn5 and Cu3Sn, were also formed at the interface regardless of

Conclusions

Both the Cu6Sn5 and Cu3Sn phases were formed at the Sn/Ag/Cu and Sn/Cu interface reacted at 200 °C with and without applied strain. One additional Ag3Sn phase was formed at the Sn/Ag/Cu interface, and this phase layer acted as a diffusion barrier that retarded the Sn/Cu interdiffusion and reduced the growth rate of the Cu–Sn IMCs. The applied strain had an enhanced effect on the growth rate of the Cu6Sn5 phase formed at the Sn/Ag/Cu and Sn/Cu interface, regardless of the strain type. The

Acknowledgement

The authors thank the financial support of the National Science Council, Taiwan, ROC (NSC 100-2628-E-005-002).

References (22)

  • K. Zeng et al.

    Mater. Sci. Eng., R

    (2002)
  • K.S. Kim et al.

    J. Alloys Comp.

    (2003)
  • Y.T. Chen et al.

    J. Taiwan Inst. Chem. E

    (2012)
  • K.N. Tu et al.

    Acta Metall.

    (1982)
  • C.P. Lin et al.

    J. Alloys Comp.

    (2010)
  • W.K. Liao et al.

    Scr. Mater.

    (2011)
  • A. Paul

    Scr. Mater.

    (2013)
  • K.Z. Wang et al.

    J. Electron. Mater.

    (2005)
  • S.W. Chen et al.

    J. Mater. Res.

    (2007)
  • J.L. Fang et al.

    Circ. World

    (2007)
  • S.W. Chen et al.

    J. Electron. Mater.

    (1998)
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