J Mater Sci: Mater Electron (2012) 23:56 60 DOI 10.1007/s10854-011-0412-z Suppression of Cu 3 Sn and Kirkendall voids at Cu/Sn-3.5Ag solder joints by adding a small amount of Ge Chun Yu Yang Yang Peilin Li Junmei Chen Hao Lu Received: 10 March 2011 / Accepted: 14 May 2011 / Published online: 28 May 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Kirkendall voids (KVs) have disastrous effects on the properties of the solder joints in the integrated circuits, which are formed after the occurrence of the Cu 3 Sn intermetallic compound (IMC) layer at the Sn-based solder/cu interface. In this paper, 0.1 and 0.3 wt% Ge additions were separately added into the Sn-3.5 wt%ag eutectic solder, to investigate the effects of Ge on the interfacial reaction under thermal aging at 150 C. It is found that the Cu 6 Sn 5 layer was still the original product, regardless of the concentration of Ge. Moreover, Ge was identified to dissolve into the IMC layer. As the aging time was prolonged to 10 days, the concentration of Ge increased to about 3.0 at%, but the Cu 3 Sn IMC layer was not obvious. The single Cu 6 Sn 5 IMC layer became flat little by little. Meantime, the thickness of the IMC layer increased slowly. And more significant finding is that the KVs were also not obvious at the interface. 1 Introduction Up to date, a lot of lead-free solders have been developed to replace the lead containing solders, especially Sn-37 wt% Pb [1]. Meantime, SnCu, SnAg, and SnAgCu (for simplification, we call them SnTM) eutectic or near eutectic alloys have been become the prospective candidates. Copper is a common substrate, and a spontaneous C. Yu Y. Yang P. Li J. Chen H. Lu (&) School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People s Republic of China e-mail: shweld@sjtu.edu.cn C. Yu J. Chen H. Lu Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai 200240, People s Republic of China reaction can be occurred between the copper and the tin even at room temperature. Since the SnTM solder alloys contain more than 90 wt% Sn, the reaction between the SnTM and the Cu would be more acute than that between the Sn-37Pb and the Cu. It is well known that the properties of the solder joints, as well as the reliability of the whole package, are highly sensitive to the thickness and morphology of the intermetallic compound (IMC) layers [2, 3]. An IMC layer is necessary to ensure a good joining between the solder and the substrate, while too thick one is sensitive to stress and sometimes induces crack initiation and propagation. Recent investigations indicate that the Kirkendall voids (KVs) usually form at the Cu/Cu 3 Sn interface due to acute reaction between the Sn-based solder and the copper [4 9]. The density of the KVs increases with the aging time, and the KVs even coalesce into bigger voids after a long aging period. Since the occurrence of the KVs decreases the contact area of the interface, and therefore weakens the mechanical and electrical properties, this reliability issue has been attracting a lot of concerns. Sn-3.5 wt% Ag (SA) eutectic solder is regarded as a prospective substitution for the Sn-37 wt% Pb alloy in certain applications, due to its good mechanical properties and high reliability [10, 11]. Germanium (Ge) is an IVA group element, which is just above the Sn in the element periodic table. Recently, many works have been done to investigate the effects of Ge on the lead free solders [12 14]. The results show that Ge can refine the grain size of the solder, and prevent the solder from oxidation [12]. Meantime, the mechanical properties of the solder can be improved [13]. Moreover, quite a few efforts have been performed to investigate the reaction interface of the Ge-containing solder/cu joints [15 17]. Since 0.01 0.05 wt% Ge was found to have little positive effects on the interfacial
J Mater Sci: Mater Electron (2012) 23:56 60 57 reaction [15, 16], further investigations, considering different concentration of Ge in solder, should be performed to exploit the impacts of Ge on the interfacial reaction. In this work, 0.1 and 0.3 wt% Ge was added into the SA eutectic solder. And the effects of Ge addition on the formation, evolution and growth behaviors of the IMCs and the KVs at the SA/Cu interface were investigated. 2 Experimental materials and methods The as cast samples of bulk alloy were prepared by melting the high purity tin, silver, and germanium at 600 C for several hours, in a crucible protected by the eutectic KCl? LiCl solution. The chemical components are listed in Table 1. The liquid solders were casted into a stainless steel mold of 6 mm in diameter, then cooled in air to room temperature with a rate of 10 K/s. The solders were made into a disk shape with dimensions of 6 mm in diameter and 2 mm in height, and slightly polished and then dipped into the rosin mildly activated (RMA) flux for soldering. The DSC profiles for the SA-xGe solders were shown in Fig. 1. It is found that the onset temperature for SA, SA-0.1Ge and SA-0.3Ge are 220.7, 220.0, and 220.2 C, respectively. Ge slightly decreases the melting temperature of SA solder. The Cu plate (25 mm 9 25 mm 9 0.2 mm) with 99.9% purity was used as the substrate. Before soldering, the Cu surface was polished with grade no.3 metallographic sandpaper and burnished, and then degreased with acetone using ultrasonic vibration. The joint samples were obtained by melting the solder on the top face of the Cu substrate in an IR-reflow machine under atmosphere environment. The peak reflow temperatures were 270 C for the SA-Ge solder joints. The time above the melting point of the solders was about 60 s. Immediately after the reflow process, the solder joints were subjected to a high-temperature aging at 150 C for 0 20 days. The as-soldered and as-aged samples were mounted with cold epoxy, ground by using SiC paper, polished with 0.05 lm Al 2 O 3 powder and then lightly etched for the cross-sectional observation. The microstructures of the joints were observed in a scanning electron microscopy (SEM), equipped with an energy dispersive spectrometer (EDS) and an electron probe micro analyzer (EPMA), to analyze the chemical compositions of each phase at the interface. The average thickness of the Table 1 Chemical components of solders (wt%) Solder Sn Ag Ge Sn-3.5Ag Bal. 3.51 Sn-3.5Ag-0.1Ge Bal. 3.43 0.13 Sn-3.5Ag-0.3Ge Bal. 3.40 0.29 Fig. 1 Heating curves of SA and SA-Ge lead free solder alloys IMC layer at the interface was determined by dividing the overall area of the IMCs by the horizontal length of the interface. 3 Experimental results and discussions 3.1 Cu/SA-xGe interfacial microstructure In our previous investigation, we studied the interfacial evolution process of the SA/Cu solder joints under isothermal aging [18]. It is found that a continuous scallop like Cu 6 Sn 5 IMC layer was formed at the SA/Cu interface after the reflowing process. At the aging state, a thin Cu 3 Sn layer was found between the Cu substrate and the Cu 6 Sn 5 layer. Meantime, the thickness of the whole IMC layer became thicker with the aging time. Interestingly, we also found a few microvoids both at the Cu/Cu 3 Sn interface and inside the Cu 3 Sn layer, while the interface between Cu 6 Sn 5 and SA was perfect. Since these voids were induced by Kirkendall effect, they were called as Kirkendall voids (KVs) [6 9]. The micro-voids were found to coalesce into big voids after a long time aging. Therefore, KVs would decrease the properties of the solder joints. KVs were found by many researchers, and how to suppress the
58 J Mater Sci: Mater Electron (2012) 23:56 60 Fig. 2 Evolution of the IMCs at SA-0.1Ge/Cu interface aged at 150 C. a As soldered. b 5 days and c 10 days Fig. 3 Evolution of the IMCs at SA-0.3Ge/Cu interface aged at 150 C. a As soldered. b 5 days and c 10 days formation and evolution of the KVs would become a hot topic due to minimization of solder joints. The interfacial microstructures of the SA-0.1Ge/Cu and SA-0.3Ge/Cu joints were presented in Figs. 2 and 3. It is seen that, the original IMC layers at the Ge-containing solder joints were also scallop-like Cu 6 Sn 5 IMC. Ge did not seem to change the liquid reaction at the interface. However, the Cu 3 Sn IMC layer was not observed even after 15 days aging isothermally at 150 C, the single Cu 6 Sn 5 layer became flatter and flatter, and the growth rate was decreased. Importantly, we did not find KVs at the interface near Cu side, as show in Figs. 2c and 3c, which reflect the morphologies of the SA-Ge/Cu solder joints after aging for 10 days. EDS analysis indicates that there is a small amount of Ge existing in the Cu 6 Sn 5 IMC layer, the contents of Ge are 0.2 and 0.3 at% for SA-0.1Ge/Cu and SA-0.3Ge/Cu joints as soldered, respectively. And the amount increased to 3.1 and 3.8 at% for SA-0.1Ge/Cu and SA-0.3Ge/Cu joints, respectively, after aging for 10 days. However, Amagai [15] found that Ge has no positive effect on suppressing the growth rate of the IMC layer after several reflowings, by adding 0.05 wt% Ge into SA eutectic solder. Moreover, Ge was not observed in the IMC layer. And in another research, Meng et al. [17] found that the thickness of the original IMC layer become thicker after adding 0.25 1.0 wt% Ge into Sn-2.5Ag-0.7Cu, but the addition of Ge restrained the transformation of Cu 6 Sn 5 to Cu 3 Sn and the growth rate of the IMC layer. And this conclusion is in good agreement with our results. 3.2 Growth of IMC layer The thickness of IMC layer has significant effect on the reliability of solder joint. A small amount of Zn addition was found to be beneficial in suppressing the growth of IMC layer at the SA/Cu interface [18]. Similarly, as shown in Fig. 4, the growth rate of IMC was also decreased by Ge addition. For instance, the thickness of the whole IMC layer of SA/Cu joint was about 5 lm after aged at 150 C for 10 days, while it was about 3.5 lm for the SA-0.1Ge/ Cu and the SA-0.3Ge/Cu joints. Fig. 4 Change of the thickness of IMC layer with isothermal aging time
J Mater Sci: Mater Electron (2012) 23:56 60 59 Fig. 5 Sketch map of interfacial evolution of SA/Cu solder joint 3.3 Discussions Generally, the scallop-like Cu 6 Sn 5 is the original phase at the SA/Cu interface. And the Cu 3 Sn IMC layer forms at the isothermal aging state, as shown in Fig. 5. While, Gao et al. [19] found that the Cu 3 Sn layer was suppressed by the formation of (Cu,Co) 6 Sn 5 and (Cu,Ni) 6 Sn 5 after adding a small amount of Co and Ni into the SA solder. Wang et al. [20] also found that the Zn addition could suppress the formation of the Cu 3 Sn layer, as the Zn addition was 0.2 wt%, the single Cu 6 Sn 5 layer could be kept for at least 20 days at 150 C. After adding the trace element, the Cu/ Cu 6 Sn 5 interface may be stabilized or some other more stable IMC layer replaces the Cu 6 Sn 5 [9, 18, 20]. According to our previous calculations, (Cu,Ni) 6 Sn 5 is more stable than Cu 6 Sn 5 [21]. Since EDS identified that there existed Ge in the Cu 6 Sn 5 layer, meantime, the Ge and Sn belong to the same group, we can suppose that the Cu 6 (Sn,Ge) 5 IMC layer was formed at the SA-Ge/Cu interface replacing of the Cu 6 Sn 5. The stability of the former was better than the latter, and therefore the formation of the Cu 3 Sn layer was suppressed. And this assumption would be verified by an atomistic simulation, this is under performing. KVs are formed due to a non-equilibrium diffusion, which were usually found at the Cu 3 Sn/Cu interface. Gao also found the KVs at the Ni 3 P/Cu interface [9]. Notice that, most research found that the KVs did not form at the original phase/substrate interface, but at the second phase/ substrate. Since the concentration of Sn in the Sn-based lead-free solder is as high as 90 wt%, and Ag does not participate in the interfacial reaction, we can regard the Cu/ solder joint as a binary diffusion couple. In this couple, the formation of the KVs is controlled by a vacancy mediated diffusion mechanism. And the vacancy flux (J V ) is determined by the atom flux at the IMC/Cu interface, it is, J V ¼ J Cu J Sn ð1þ where J Cu is the Cu flux drifted out of the Cu/IMC interface, and J Sn presents the Sn flux migrated into the Cu/ IMC interface. In addition, the atom flux is also controlled by the diffusion equation, J ¼ D oc ð2þ ox where D is diffusion coefficient, C is concentration, and x is the thickness of the IMC layer. Therefore, the vacancy flux can be described according to Eqs. (1) and (2) as, J V ¼ ðd Sn D Cu Þ oc ð3þ ox where, D Sn and D Cu are the diffusion coefficient of Sn and Cu in the IMC layer. Equation (3) indicates that the net vacancy flux is induced by the different diffusion coefficients of Sn and Cu. However, the diffusion coefficients of Sn in the Cu 6 Sn 5 and Cu 3 Sn layers are closely equal to that of Cu in the both IMC layer, respectively [22]. Based on the above theory, there should not form KVs at the Cu/IMC interface. It is worth noting that the KVs always occur at the Cu 6 Sn 5 -Cu 3 Sn/Cu interface, but not at the Cu 6 Sn 5 /Cu (here, Cu 6 Sn 5 -Cu 3 Sn presents double IMC layer). As the Cu or Sn diffuses into the Cu 6 Sn 5 /Cu 3 Sn interface, many atoms would be consumed due to the interfacial reaction, this is also the reason why the Cu 6 Sn5 and Cu 3 Sn layers have different growth rate. The mechanism in details can be found in Ref. [23]. However, if there was only a single IMC layer, all the Sn atoms would diffuse into the Cu/IMC layer, no voids or only a few vacancies would be formed. 4 Conclusions In this paper, a small amount of Ge was added into the eutectic SA lead-free solder. It is found that Ge had little effect on the melting point of the SA. However, the morphologies of the SA/Cu interface were affected remarkably by the Ge. A single Ge-contained Cu 6 Sn 5 IMC layer existed at the interface for a very long isothermal aging time, and the growth rate of the IMC layer at the SA-Ge/Cu joints was much lower than that at the SA/Cu joint.
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