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Initial formation of CuSn intermetallic compounds between molten SnAgCu solder and Cu substrate
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Scripta Materialia 60 (2009) 333–335 www.elsevier.com/locate/scriptamat
Initial formation of CuSn intermetallic compounds between molten SnAgCu solder and Cu substrate Jicheng Gong,a,* Changqing Liu,b Paul P. Conwayb and Vadim V. Silberschmidtb a
b
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough LE11 3TU, UK Received 17 September 2008; revised 27 October 2008; accepted 28 October 2008 Available online 6 November 2008
The initial formation of SnCu intermetallic compounds (IMCs) between a molten SnAgCu alloy and Cu under-bump metallization is observed by removing the liquid solder from the substrate and rapidly cooling just as the interfacial IMCs are forming. The results show that a Cu3Sn layer is formed ahead of the liquid solder on the Cu substrate. The liquid solder subsequently spreads on this existing Cu3Sn layer, forming a Cu6Sn5 layer between the liquid phase and Cu3Sn. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Pb-free solder; Interfacial intermetallics; Wetting; Electronic packaging
In soldering processes, intermetallic compounds (IMCs) are the major reactants at the interface between the liquid solder and the metal substrate. For instance, interfacial Cu3Sn and Cu6Sn5 IMCs are formed between some Sn-based solder alloys and a Cu under-bump metallization (UBM) [1,2]. The formation of these interfacial IMCs is crucial since it is linked to several reliability issues [3,4]. Significant studies on the interfacial reaction between Cu and SnPb or Pb-free solders have been conducted [5–7]. However, little work is reported on the initial formation of these interfacial IMCs [8,9], which is essential to understand the wetting behaviour in the soldering process and the subsequent evolution of these IMCs. In this paper, an experiment [10] that is able to obtain interfacial reactants at an arbitrary moment of liquid/solid reaction is used. The SnCu IMCs between a molten SnAgCu solder and a Cu UBM can be observed at their initial stage of formation to understand their formation mechanisms. The basic set-up and experimental method was present in Ref. [10]. In addition, a rapid cooling system is engaged in the set-up. When the spinning is not applied, a rapid-solidification specimen can be obtained by this additional cooling system. Figure 1 shows the temperature profiles for tests. The spindle tests are conducted from the melting temperature (Tm) of the solder alloy * Corresponding author. E-mail: [email protected]
(490 K) to the maximum temperature of the reflow (point B in Fig. 1), with a temperature increment of approximately 1 K for each test following curve 1. For instance, the onset of spinning for the first specimen is at 490 K, with the second specimen following the same profile but terminating at 491 K. Interfacial reactants are found for the first time at a temperature around 505 K. Figure 2a provides an overview of interfacial reactants formed on Cu pads. It can be seen that there exists one main domain with several accompanying satellite secondary domains. The large area of the bare Cu surface suggests that the sample is experiencing a nucleation stage at this moment. The size variation of secondary domains shows that they are at different stages of growth, and their distribution exhibits nucleation sites. Based on these results, the growth of interfacial reactants in the tangential direction parallel to the Cu surface (TDS) can be predicted as follows: they nucleate at different locations on the Cu surface first, and then grow to form secondary domains. When these domains meet, they coalesce, finally forming the main domain. This is confirmed by the existence of small areas of bare Cu in the main domain due to incomplete coalescence. The details of the boundaries of both main and secondary domains are shown in Figure 2b and c. These areas inside the domains are clearly composed of a large number of coarse particles, which seem to have a particular crystal structure. Energy-dispersive X-ray (EDX) analysis shows that the composition of
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.10.029
334
J. Gong et al. / Scripta Materialia 60 (2009) 333–335
Figure 1. Temperature profiles for a solder bump during reflow. The cooling rate for curve 2 is about 40 K s 1.
these particles is close to that of Cu6Sn5 IMCs. These coarse particles inside the domains cannot represent the nucleation state of interfacial CuSn IMCs. The actual site is along the edge (or boundary) of the domain, which is composed of a large number of fine particles, as presented in Figure 2b and c, and named the ‘‘transition
Figure 2. Interfacial IMCs formed on the Cu substrate at point A in curve 1 (Fig. 1). (a) Low magnification; (b) the edge of a main domain; and (c) the edge of a secondary domain.
zone” in this paper. When reflow continues, the entire Cu substrate is covered with coarse particles [10], indicating that the edge of the domain moves towards the bare Cu if the liquid/solid reaction continues. In this process, the current transition zone will merge into a domain; the fine particles at the transition zone will become the coarse ones that are currently inside the domain; and new fine ones will be nucleated at the edge of a domain. Therefore, the growth of the domain in TDS is originated from new and fine particles at the transition zone. To identify and further characterize interfacial IMCs formed at the initial stage, a transmission electron microscopy (TEM) sample across the transition zone is prepared by means of focused ion beam (FIB) milling. The cross-section is perpendicular to the domain’s edge so that it can present the evolutionary trend of interfacial reactants. Figure 3a is the TEM image of the cross-section, which reveals the entire transition zone and a portion of the coarse particle zone. Five layers can be identified in the cross-section: (i) the copper substrate; (ii) a thin layer of fine particles; (iii) a thick layer of coarse particles, which are only formed on the top right-hand side of the section; (iv) a bright continuous layer; and (v) a dark continuous layer of platinum. The platinum layer is deposited to protect the sample’s surface during FIB milling. The bright continuous layer seems to have an amorphous structure, and is shown by EDX to contain a large portion of carbon. This layer is likely to form with the flux (rosin/C19H29COOH). This is further confirmed by the fact that it is transparent to optical microscope observation. Diffraction patterns of TEM and EDX results show that the coarse particles are Cu6Sn5 IMCs; and the fine particles underneath the Cu6Sn5 layer are Cu3Sn IMCs. The fine particles on the bottom left-hand side of the image (with no Cu6Sn5 layer on top) are also Cu3Sn IMCs. The Cu3Sn IMCs that are not covered by the Cu6Sn5 layer correspond to the transition zone of the investigated secondary domain. Therefore, these Cu3Sn IMCs are fine particles near the edge of a domain as presented in Figure 2b
Figure 3. TEM image of the transition zone of a domain formed at point A in Figure 1: (a) obtained by spinning at point A and (b) without spinning.
J. Gong et al. / Scripta Materialia 60 (2009) 333–335
and c. Since these Cu3Sn IMCs are closer to bare Cu, they are initially formed during the wetting process. A Cu6Sn5 layer is then formed on the existing fine Cu3Sn layer. Compared with the Cu3Sn IMCs underneath, they are much larger. In Figure 2b and c, they correspond to the coarse particles inside the domain. To understand the nucleation and evolution of these interfacial reactants, it is important to know the state of the surrounding solder materials. With this aim, a specimen is heated up to the point A shown inFigure 1 and then rapidly cooled down without spinning. Curve 2 in Figure 1 shows the temperature profile. A cross-section is prepared at the three-phase contact line. Figure 3b presents the corresponding TEM image, which shows that the solder ball is situated on the coarse Cu6Sn5 layer. This indicates that the layer of fine Cu3Sn-particles is formed prior to the liquid solder and on the Cu substrate, and that the liquid solder subsequently wets onto this existing interfacial reactant. To form Cu3Sn IMCs, both Sn and Cu are required. There are two sources of Cu supply: the SnAgCu solder and the Cu substrate. To clarify the Cu supply from the substrate, a Sn3.8Ag solder paste is used for the spindle test, by which a similar interfacial structure is observed at the transition zone of the domain, indicating that the Cu substrate itself can provide sufficient Cu to form the transition zone. Sn atoms in the transition zone come from the solder ball, which is the only source. There can be two transferring mechanisms: one is through the interfacial reactants; the other is through the solder/flux interface. The latter is demonstrated by
335
the presence of 3.94 at.% Sn in the flux (point E in Fig. 3b), found by means of EDX analysis on TEM. If the latter is the major mechanism, the formation of this fine Cu3Sn-particle layer can be assumed as follows: when a solder ball is placed on the Cu substrate with the flux, Sn in the solder ball and Cu in the substrate begin to move into the flux. The rate of this transport is low because the ball and the substrate are in solid state. So the transportation zones (TZs) for Sn and Cu are still very thin and close to the solder ball and the Cu substrate, as illustrated in Figure 4a. No obvious interfacial reactions are observed. As the temperature increases and the solder ball is molten, the transfer rate of Cu is still low, and its TZ is still thin and close to the substrate. In contrast, the transfer rate of Sn is considerably higher from the molten solder. This results in a much thicker layer of Sn-TZ around the molten solder ball, as illustrated in Figure 4b. When the Sn-TZ meets with the Cu-TZ, the formation of reactants is initiated. As shown in Figure 4b, the overlap area determines the morphology of fine Cu3Sn-particles layer. According to the concept of local nominal composition (LNC) [11], the specific interfacial SnCu reactants are influenced by the distance from the solder ball. At the transition zone, the initial reactant is Cu3Sn rather than Cu6Sn5 IMCs since the LNC is relatively high; Cu3Sn is more stable. After the layer of fine CuSn-particles is formed, the liquid solder spreads over the existing Cu3Sn IMCs. In this process, LNC decreases, enabling the formation of interfacial Cu6Sn5. Unlike in the cross-section in Figure 3a, no flux layer is found on the Cu6Sn5 layer in Figure 3b. This suggests that the flux layer may result from the spinning process. In summary, the surface of Cu UBM is modified by a layer of CuSn IMCs in front of the three-phase contact boundaries during the wetting process. The liquid solder then spreads and covers over the existing Cu3Sn layer, forming the Cu6Sn5/Cu3Sn/Cu sandwich structure at the interface. To form the Cu3Sn transition zone, both Cu and Sn are required. Cu can be mainly supplied by the Cu substrate. Sn may transport through the flux to the transition zone. The financial support from the Engineering and Physical Sciences Research Council’s Innovative Manufacturing and Construction Research Centre at Loughborough University under GR/R64483/01P is gratefully acknowledged.
Figure 4. Formation mechanism of the fine SnCu-particles layer near the three-phase contact line: (a) low temperature and (b) high temperature with the molten solder ball.
[1] K.N. Tu, K. Zeng, Mater. Sci. Eng. R 34 (2001) 1. [2] T. Laurila, V. Vuorinen, J.K. Kivilahti, Mater. Sci. Eng. R 49 (2005) 1. [3] D.R. Frear, JOM 48 (1996) 49. [4] K.N. Tu, K. Zeng, Mater. Sci. Eng. R 38 (2002) 55. [5] J.O. Suh, K.N. Tu, N. Tamura, Appl. Phys. Lett. 91 (2007) 051907. [6] R.A. Lord, A. Umantsev, J. Appl. Phys. 98 (2005) 063525. [7] A. Hayashi, C.R. Kao, Y.A. Chang, Scripta Mater. 37 (1997) 393. [8] C.H. Yu, K.L. Lin, Chem. Phys. Lett. 418 (2006) 433. [9] C.C. Pan, C.H. Yu, K.L. Lin, Appl. Phys. Lett. 93 (2008) 061912. [10] J. Gong, C. Liu, P.P. Conway, V.V. Silberschmidt, Acta Mater. 56 (2008) 4291. [11] K. Ro¨nka¨, F.J.J. van Loo, J.K. Kivilahti, Scripta Mater. 37 (1997) 1575.
Scripta Materialia 60 (2009) 333–335 www.elsevier.com/locate/scriptamat
Initial formation of CuSn intermetallic compounds between molten SnAgCu solder and Cu substrate Jicheng Gong,a,* Changqing Liu,b Paul P. Conwayb and Vadim V. Silberschmidtb a
b
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough LE11 3TU, UK Received 17 September 2008; revised 27 October 2008; accepted 28 October 2008 Available online 6 November 2008
The initial formation of SnCu intermetallic compounds (IMCs) between a molten SnAgCu alloy and Cu under-bump metallization is observed by removing the liquid solder from the substrate and rapidly cooling just as the interfacial IMCs are forming. The results show that a Cu3Sn layer is formed ahead of the liquid solder on the Cu substrate. The liquid solder subsequently spreads on this existing Cu3Sn layer, forming a Cu6Sn5 layer between the liquid phase and Cu3Sn. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Pb-free solder; Interfacial intermetallics; Wetting; Electronic packaging
In soldering processes, intermetallic compounds (IMCs) are the major reactants at the interface between the liquid solder and the metal substrate. For instance, interfacial Cu3Sn and Cu6Sn5 IMCs are formed between some Sn-based solder alloys and a Cu under-bump metallization (UBM) [1,2]. The formation of these interfacial IMCs is crucial since it is linked to several reliability issues [3,4]. Significant studies on the interfacial reaction between Cu and SnPb or Pb-free solders have been conducted [5–7]. However, little work is reported on the initial formation of these interfacial IMCs [8,9], which is essential to understand the wetting behaviour in the soldering process and the subsequent evolution of these IMCs. In this paper, an experiment [10] that is able to obtain interfacial reactants at an arbitrary moment of liquid/solid reaction is used. The SnCu IMCs between a molten SnAgCu solder and a Cu UBM can be observed at their initial stage of formation to understand their formation mechanisms. The basic set-up and experimental method was present in Ref. [10]. In addition, a rapid cooling system is engaged in the set-up. When the spinning is not applied, a rapid-solidification specimen can be obtained by this additional cooling system. Figure 1 shows the temperature profiles for tests. The spindle tests are conducted from the melting temperature (Tm) of the solder alloy * Corresponding author. E-mail: [email protected]
(490 K) to the maximum temperature of the reflow (point B in Fig. 1), with a temperature increment of approximately 1 K for each test following curve 1. For instance, the onset of spinning for the first specimen is at 490 K, with the second specimen following the same profile but terminating at 491 K. Interfacial reactants are found for the first time at a temperature around 505 K. Figure 2a provides an overview of interfacial reactants formed on Cu pads. It can be seen that there exists one main domain with several accompanying satellite secondary domains. The large area of the bare Cu surface suggests that the sample is experiencing a nucleation stage at this moment. The size variation of secondary domains shows that they are at different stages of growth, and their distribution exhibits nucleation sites. Based on these results, the growth of interfacial reactants in the tangential direction parallel to the Cu surface (TDS) can be predicted as follows: they nucleate at different locations on the Cu surface first, and then grow to form secondary domains. When these domains meet, they coalesce, finally forming the main domain. This is confirmed by the existence of small areas of bare Cu in the main domain due to incomplete coalescence. The details of the boundaries of both main and secondary domains are shown in Figure 2b and c. These areas inside the domains are clearly composed of a large number of coarse particles, which seem to have a particular crystal structure. Energy-dispersive X-ray (EDX) analysis shows that the composition of
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.10.029
334
J. Gong et al. / Scripta Materialia 60 (2009) 333–335
Figure 1. Temperature profiles for a solder bump during reflow. The cooling rate for curve 2 is about 40 K s 1.
these particles is close to that of Cu6Sn5 IMCs. These coarse particles inside the domains cannot represent the nucleation state of interfacial CuSn IMCs. The actual site is along the edge (or boundary) of the domain, which is composed of a large number of fine particles, as presented in Figure 2b and c, and named the ‘‘transition
Figure 2. Interfacial IMCs formed on the Cu substrate at point A in curve 1 (Fig. 1). (a) Low magnification; (b) the edge of a main domain; and (c) the edge of a secondary domain.
zone” in this paper. When reflow continues, the entire Cu substrate is covered with coarse particles [10], indicating that the edge of the domain moves towards the bare Cu if the liquid/solid reaction continues. In this process, the current transition zone will merge into a domain; the fine particles at the transition zone will become the coarse ones that are currently inside the domain; and new fine ones will be nucleated at the edge of a domain. Therefore, the growth of the domain in TDS is originated from new and fine particles at the transition zone. To identify and further characterize interfacial IMCs formed at the initial stage, a transmission electron microscopy (TEM) sample across the transition zone is prepared by means of focused ion beam (FIB) milling. The cross-section is perpendicular to the domain’s edge so that it can present the evolutionary trend of interfacial reactants. Figure 3a is the TEM image of the cross-section, which reveals the entire transition zone and a portion of the coarse particle zone. Five layers can be identified in the cross-section: (i) the copper substrate; (ii) a thin layer of fine particles; (iii) a thick layer of coarse particles, which are only formed on the top right-hand side of the section; (iv) a bright continuous layer; and (v) a dark continuous layer of platinum. The platinum layer is deposited to protect the sample’s surface during FIB milling. The bright continuous layer seems to have an amorphous structure, and is shown by EDX to contain a large portion of carbon. This layer is likely to form with the flux (rosin/C19H29COOH). This is further confirmed by the fact that it is transparent to optical microscope observation. Diffraction patterns of TEM and EDX results show that the coarse particles are Cu6Sn5 IMCs; and the fine particles underneath the Cu6Sn5 layer are Cu3Sn IMCs. The fine particles on the bottom left-hand side of the image (with no Cu6Sn5 layer on top) are also Cu3Sn IMCs. The Cu3Sn IMCs that are not covered by the Cu6Sn5 layer correspond to the transition zone of the investigated secondary domain. Therefore, these Cu3Sn IMCs are fine particles near the edge of a domain as presented in Figure 2b
Figure 3. TEM image of the transition zone of a domain formed at point A in Figure 1: (a) obtained by spinning at point A and (b) without spinning.
J. Gong et al. / Scripta Materialia 60 (2009) 333–335
and c. Since these Cu3Sn IMCs are closer to bare Cu, they are initially formed during the wetting process. A Cu6Sn5 layer is then formed on the existing fine Cu3Sn layer. Compared with the Cu3Sn IMCs underneath, they are much larger. In Figure 2b and c, they correspond to the coarse particles inside the domain. To understand the nucleation and evolution of these interfacial reactants, it is important to know the state of the surrounding solder materials. With this aim, a specimen is heated up to the point A shown inFigure 1 and then rapidly cooled down without spinning. Curve 2 in Figure 1 shows the temperature profile. A cross-section is prepared at the three-phase contact line. Figure 3b presents the corresponding TEM image, which shows that the solder ball is situated on the coarse Cu6Sn5 layer. This indicates that the layer of fine Cu3Sn-particles is formed prior to the liquid solder and on the Cu substrate, and that the liquid solder subsequently wets onto this existing interfacial reactant. To form Cu3Sn IMCs, both Sn and Cu are required. There are two sources of Cu supply: the SnAgCu solder and the Cu substrate. To clarify the Cu supply from the substrate, a Sn3.8Ag solder paste is used for the spindle test, by which a similar interfacial structure is observed at the transition zone of the domain, indicating that the Cu substrate itself can provide sufficient Cu to form the transition zone. Sn atoms in the transition zone come from the solder ball, which is the only source. There can be two transferring mechanisms: one is through the interfacial reactants; the other is through the solder/flux interface. The latter is demonstrated by
335
the presence of 3.94 at.% Sn in the flux (point E in Fig. 3b), found by means of EDX analysis on TEM. If the latter is the major mechanism, the formation of this fine Cu3Sn-particle layer can be assumed as follows: when a solder ball is placed on the Cu substrate with the flux, Sn in the solder ball and Cu in the substrate begin to move into the flux. The rate of this transport is low because the ball and the substrate are in solid state. So the transportation zones (TZs) for Sn and Cu are still very thin and close to the solder ball and the Cu substrate, as illustrated in Figure 4a. No obvious interfacial reactions are observed. As the temperature increases and the solder ball is molten, the transfer rate of Cu is still low, and its TZ is still thin and close to the substrate. In contrast, the transfer rate of Sn is considerably higher from the molten solder. This results in a much thicker layer of Sn-TZ around the molten solder ball, as illustrated in Figure 4b. When the Sn-TZ meets with the Cu-TZ, the formation of reactants is initiated. As shown in Figure 4b, the overlap area determines the morphology of fine Cu3Sn-particles layer. According to the concept of local nominal composition (LNC) [11], the specific interfacial SnCu reactants are influenced by the distance from the solder ball. At the transition zone, the initial reactant is Cu3Sn rather than Cu6Sn5 IMCs since the LNC is relatively high; Cu3Sn is more stable. After the layer of fine CuSn-particles is formed, the liquid solder spreads over the existing Cu3Sn IMCs. In this process, LNC decreases, enabling the formation of interfacial Cu6Sn5. Unlike in the cross-section in Figure 3a, no flux layer is found on the Cu6Sn5 layer in Figure 3b. This suggests that the flux layer may result from the spinning process. In summary, the surface of Cu UBM is modified by a layer of CuSn IMCs in front of the three-phase contact boundaries during the wetting process. The liquid solder then spreads and covers over the existing Cu3Sn layer, forming the Cu6Sn5/Cu3Sn/Cu sandwich structure at the interface. To form the Cu3Sn transition zone, both Cu and Sn are required. Cu can be mainly supplied by the Cu substrate. Sn may transport through the flux to the transition zone. The financial support from the Engineering and Physical Sciences Research Council’s Innovative Manufacturing and Construction Research Centre at Loughborough University under GR/R64483/01P is gratefully acknowledged.
Figure 4. Formation mechanism of the fine SnCu-particles layer near the three-phase contact line: (a) low temperature and (b) high temperature with the molten solder ball.
[1] K.N. Tu, K. Zeng, Mater. Sci. Eng. R 34 (2001) 1. [2] T. Laurila, V. Vuorinen, J.K. Kivilahti, Mater. Sci. Eng. R 49 (2005) 1. [3] D.R. Frear, JOM 48 (1996) 49. [4] K.N. Tu, K. Zeng, Mater. Sci. Eng. R 38 (2002) 55. [5] J.O. Suh, K.N. Tu, N. Tamura, Appl. Phys. Lett. 91 (2007) 051907. [6] R.A. Lord, A. Umantsev, J. Appl. Phys. 98 (2005) 063525. [7] A. Hayashi, C.R. Kao, Y.A. Chang, Scripta Mater. 37 (1997) 393. [8] C.H. Yu, K.L. Lin, Chem. Phys. Lett. 418 (2006) 433. [9] C.C. Pan, C.H. Yu, K.L. Lin, Appl. Phys. Lett. 93 (2008) 061912. [10] J. Gong, C. Liu, P.P. Conway, V.V. Silberschmidt, Acta Mater. 56 (2008) 4291. [11] K. Ro¨nka¨, F.J.J. van Loo, J.K. Kivilahti, Scripta Mater. 37 (1997) 1575.