Growth orientation of Cu–Sn IMC in Cu/Sn–3.5Ag/Cu–xZn microbumps and Zn-doped solder joints
Introduction
Cu6Sn5 intermetallic compounds (IMCs) commonly form during interfacial reactions at the Sn-based solder/Cu interface [1]. Due to the small size of microbumps, the joints are composed mostly of Cu–Sn IMCs [2], [3]. Therefore, the properties of IMCs, such as grain structure, toughness and voids, may dominate joint reliability in 3D-IC technologies [2], [3]. After multiple or long-time reflows in these microbumps, the IMCs join, causing an impingement phenomenon, and the crystals tend to grow in a homogenous direction [2], [3], [4]. This structure allows easy crack propagation through the IMCs under impacts.
Reducing this impingement phenomenon and randomizing the orientation of crystal growth therefore hinder crack propagation through the IMCs. Recently, a novel approach using Cu–Zn under bump metallurgy was investigated, revealing advantages such as (1) suppression of Cu6Sn5 compounds at the joint interface, (2) elimination of the formation of Cu3Sn and Kirkendall voids during thermal aging [5], and (3) exhibition of better mechanical reliability [6]. It has also been demonstrated that Zn in Cu6Sn5 can stabilize the hexagonal structure of the compound [7], [8]. However, the effect of Zn on Cu–Sn IMC grain structures is not well understood. It is thus crucial to investigate this effect.
In conventional-sized solder joints, the (001) axis of Cu6Sn5 is often perpendicular to the Cu substrate [9], [10], leading to easy crack propagation through the IMCs. Literature shows that replacing Sn with Zn in Cu6(Sn,Zn)5 could change the a axis and c axis of the lattice structure [11]. Distortion of the lattice may change the surface energy of the crystal planes and affect the grain orientation of Cu6(Sn,Zn)5.
In this study, grain orientation in Cu6Sn5 was compared to that in Cu6(Sn,Zn)5 in microbumps and conventional solder joints. The variation of grain structure in Cu6(Sn,Zn)5 may improve reliability in microbumps and solder joints.
Section snippets
Materials and methods
Six 600 μm diameter Sn–3.5Ag (wt%) solder balls were placed on Cu and Cu–15Zn (wt%) substrates and pressed into a disc shape with a bonding height of 20 um during reflow for 10 s. Cu substrates were then placed on the opposite sides and reflowed at 250 °C for 10 mins as illustrated in Fig. 1(a). Multiple Sn–3.0Ag–0.5Cu solder balls with washable flux were placed on Cu and Cu–15Zn (wt%) substrates (500 μm opening diameter) to reflow at 250 °C for 2 mins. Solders were then polished and the exposed Cu6Sn5
Results and discussion
As shown in Fig. 2(a) and (b), the IMC layers in Cu/Sn–3.5Ag/Cu grow together, which leads to an impingement phenomenon, and the grain structure in the joined Cu6Sn5 has a homogenous orientation. This preferred orientation in Cu6Sn5 leads to easy crack propagation through the IMCs, as indicated in literature [2]. In Cu/Sn–3.5Ag/Cu–15Zn, however, the Cu6(Sn,Zn)5 IMCs do not join, causing an inter-folding pattern as seen in Fig. 2(c) and (d). This inter-folding pattern in Cu/Sn–3.5Ag/Cu–15Zn may
Conclusions
In Cu/Sn–3.5Ag/Cu microbumps, impingement occurs which allows for easy crack propagation. In contrast, the IMC layers do not join easily in Cu/Sn–3.5Ag/Cu–15Zn and instead form an inter-folding pattern with more random grain orientations, hindering crack propagation. Tests on conventional solder joints confirm the tendency for Cu6Sn5 crystals to grow perpendicular to the substrate, and for Cu6(Sn,Zn)5 to grow more randomly. The possible mechanism for this is attributed to varied surface
Acknowledgments
Financial support from the MOST, Taiwan, under the Contract no. NSC-100-2221-E-007-050-MY3 is much appreciated.
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