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Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints

Zhao Ning Zhong Yi Huang Ming-Liang Ma Hai-Tao Liu Xiao-Ping

Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints

Zhao Ning, Zhong Yi, Huang Ming-Liang, Ma Hai-Tao, Liu Xiao-Ping
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  • With the continuous miniaturization of electronic packaging, micro bumps for chip interconnects are smaller in size, and thus the reliability of interconnects becomes more and more sensitive to the formation and growth of intermetallic compounds (IMCs) at liquid-solid interface during soldering. Thermomigration (TM) is one of the simultaneous heat and mass transfer phenomena, and occurs in a mixture under certain external temperature gradient. In the process of interconnection, micro bumps usually undergo multiple reflows during which nonuniform temperature distribution may occur, resulting in TM of metal atoms. Since the interdiffusion of atoms between solders and under bump metallization (UBM) dominates the formation of interfacial IMCs, TM which enhances the directional diffusion of metal atoms and induces the redistribution of elements, will markedly influence the growth behaviors of interfacial IMCs and consequently the reliability of solder joints. The diffusivity of atoms in liquid solder is significantly larger than that in solid solder and in consequence a small temperature gradient may induce the mass migration of atoms. As a result, the growth of interfacial IMCs becomes more sensitive to temperature difference between solder joints in soldering process. So far, however, few studies have focused on liquid state TM in solder joints, and the growth kinetics of interfacial IMCs under TM during soldering is still unknown to us. In this study, Cu/Sn/Cu solder joints are used to investigate the migration behavior of Cu atoms and its effect on the growth kinetics of interfacial Cu6Sn5 under temperature gradients of 35.33℃/cm at 250℃ and 40.0℃/cm at 280℃, respectively. TM experiments are carried out by reflowing the Cu/Sn/Cu interconnects on a hot plate at 250℃ and 280℃ for different durations. For comparison, isothermal aging experiments are conducted in a high temperature chamber under the same temperatures and reaction durations. During isothermal aging, the growth of interfacial Cu6Sn5 follows a parabolic law and is controlled by bulk diffusion. Under the temperature gradient, asymmetrical growth of interfacial Cu6Sn5 is observed between cold and hot ends. At the cold end, the growth of the interfacial Cu6Sn5 is significantly enhanced and follows a linear law, indicating a reaction-controlled growth mechanism; while at the hot end, the growth of the interfacial Cu6Sn5 is inhibited and follows a parabolic law, indicating a diffusion-controlled growth mechanism. The dissolved Cu atoms from the Cu substrate at the hot end are driven to migration toward the cold end by temperature gradient, providing the Cu atomic flux for the fast growth of the interfacial Cu6Sn5 at the cold end. With the variation of the measured thickness of Cu6Sn5 IMC at the cold end and the simulated temperature gradients, the molar heat of transport Q^* of Cu atoms in molten Sn is calculated to +14.11 kJ/mol at 250℃ and +14.44 kJ/mol at 280℃. Accordingly, the driving forces of thermomigration in molten solder FL are estimated to be 1.62×10-19 N and 1.70×10-19 N, respectively.
    • Funds: Project supported by the National Natural Science Foundation of China (Grant No. 51301030) and the Fundamental Research Funds for the Central Universities, China (Grant No. DUT14QY45).
    [1]

    Laurila T, Vuorinen V, Kivilahti J 2005 Mater. Sci. Eng. R 49 1

    [2]

    Jia S, Wang X S, Ren H H 2012 Chin. Phys. B 21 126201

    [3]

    Huang M L, Chen L D, Zhou S M, Zhao N 2012 Acta Phys. Sin. 61 198104 (in Chinese) [黄明亮, 陈雷达, 周少明, 赵宁 2012 物理学报 61 198104]

    [4]

    Hsiao H Y, Liu C M, Lin H W, Liu T C, Lu C L, Huang Y S, Chen C, Tu K N 2012 Science 336 1007

    [5]

    Zhang J S, Wu Y P, Wang Y G, Tao Y 2010 Acta Phys. Sin. 59 4395 (in Chinese) [张金松, 吴懿平, 王永国, 陶媛 2010 物理学报 59 4395]

    [6]

    Chen C, Hsiao H Y, Chang Y W, Ouyang F Y, Tu K N 2012 Mater. Sci. Eng. R 73 85

    [7]

    Ouyang F Y, Tu K N, Lai Y S, Gusak A M 2006 Appl. Phys. Lett. 89 221906

    [8]

    Huang A T, Gusak A M, Tu K N, Lai Y S 2006 Appl. Phys. Lett. 88 141911

    [9]

    Chuang Y C, Liu C Y 2006 Appl. Phys. Lett. 88 174105

    [10]

    Hsiao H Y, Chen C 2007 Appl. Phys. Lett. 90 152105

    [11]

    Ouyang F Y, Kao C L 2011 J. Appl. Phys. 110 123525

    [12]

    Chen H Y, Chen C, Tu K N 2008 Appl. Phys. Lett. 93 122103

    [13]

    Gu X, Chan Y C 2009 J. Appl. Phys. 105 093537

    [14]

    Ouyang F Y, Jhu W C, Chang T C 2013 J. Alloy. Compd. 580 114

    [15]

    Guo M Y, Lin C K, Chen C, Tu K N 2012 Intermetallics 29 155

    [16]

    Qu L, Zhao N, Ma H T, Zhao H J, Huang M L 2014 J. Appl. Phys. 115 204907

    [17]

    Zhao N, Pan X M, Ma H T, Wang L 2008 Acta Metall. Sin. 44 467 (in Chinese) [赵宁, 潘学民, 马海涛, 王来 2008 金属学报 44 467]

    [18]

    Zhao N, Huang M L, Ma H T, Pan X M, Liu X Y 2013 Acta Phys. Sin. 62 086601 (in Chinese) [赵宁, 黄明亮, 马海涛, 潘学民, 刘晓英 2013 物理学报 62 086601]

    [19]

    Gusak A M, Tu K N 2002 Phys. Rev. B 66 115403

    [20]

    Yu D Q, Wu C M L, Law C M T, Wang L, Lai J K L 2005 J. Alloy. Compd. 392 192

    [21]

    Dybkov V I 1998 Growth Kinetics of Chemical Compound Layers (Cambridge: Cambridge International Science Publishing) pp28-37

    [22]

    Frederikse H P R, Fields R J, Feldman A 1992 J. Appl. Phys. 72 2879

    [23]

    Cahoon J R 1997 Metall. Mater. T. A 28 583

    [24]

    Shim J H, Oh C S, Lee B J, Lee D N 1996 Z. Metallkd. 87 205

    [25]

    Dan Y, Wu B Y, Chan Y C, Tu K N 2007 J. Appl. Phys. 102 043502

  • [1]

    Laurila T, Vuorinen V, Kivilahti J 2005 Mater. Sci. Eng. R 49 1

    [2]

    Jia S, Wang X S, Ren H H 2012 Chin. Phys. B 21 126201

    [3]

    Huang M L, Chen L D, Zhou S M, Zhao N 2012 Acta Phys. Sin. 61 198104 (in Chinese) [黄明亮, 陈雷达, 周少明, 赵宁 2012 物理学报 61 198104]

    [4]

    Hsiao H Y, Liu C M, Lin H W, Liu T C, Lu C L, Huang Y S, Chen C, Tu K N 2012 Science 336 1007

    [5]

    Zhang J S, Wu Y P, Wang Y G, Tao Y 2010 Acta Phys. Sin. 59 4395 (in Chinese) [张金松, 吴懿平, 王永国, 陶媛 2010 物理学报 59 4395]

    [6]

    Chen C, Hsiao H Y, Chang Y W, Ouyang F Y, Tu K N 2012 Mater. Sci. Eng. R 73 85

    [7]

    Ouyang F Y, Tu K N, Lai Y S, Gusak A M 2006 Appl. Phys. Lett. 89 221906

    [8]

    Huang A T, Gusak A M, Tu K N, Lai Y S 2006 Appl. Phys. Lett. 88 141911

    [9]

    Chuang Y C, Liu C Y 2006 Appl. Phys. Lett. 88 174105

    [10]

    Hsiao H Y, Chen C 2007 Appl. Phys. Lett. 90 152105

    [11]

    Ouyang F Y, Kao C L 2011 J. Appl. Phys. 110 123525

    [12]

    Chen H Y, Chen C, Tu K N 2008 Appl. Phys. Lett. 93 122103

    [13]

    Gu X, Chan Y C 2009 J. Appl. Phys. 105 093537

    [14]

    Ouyang F Y, Jhu W C, Chang T C 2013 J. Alloy. Compd. 580 114

    [15]

    Guo M Y, Lin C K, Chen C, Tu K N 2012 Intermetallics 29 155

    [16]

    Qu L, Zhao N, Ma H T, Zhao H J, Huang M L 2014 J. Appl. Phys. 115 204907

    [17]

    Zhao N, Pan X M, Ma H T, Wang L 2008 Acta Metall. Sin. 44 467 (in Chinese) [赵宁, 潘学民, 马海涛, 王来 2008 金属学报 44 467]

    [18]

    Zhao N, Huang M L, Ma H T, Pan X M, Liu X Y 2013 Acta Phys. Sin. 62 086601 (in Chinese) [赵宁, 黄明亮, 马海涛, 潘学民, 刘晓英 2013 物理学报 62 086601]

    [19]

    Gusak A M, Tu K N 2002 Phys. Rev. B 66 115403

    [20]

    Yu D Q, Wu C M L, Law C M T, Wang L, Lai J K L 2005 J. Alloy. Compd. 392 192

    [21]

    Dybkov V I 1998 Growth Kinetics of Chemical Compound Layers (Cambridge: Cambridge International Science Publishing) pp28-37

    [22]

    Frederikse H P R, Fields R J, Feldman A 1992 J. Appl. Phys. 72 2879

    [23]

    Cahoon J R 1997 Metall. Mater. T. A 28 583

    [24]

    Shim J H, Oh C S, Lee B J, Lee D N 1996 Z. Metallkd. 87 205

    [25]

    Dan Y, Wu B Y, Chan Y C, Tu K N 2007 J. Appl. Phys. 102 043502

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  • Received Date:  05 January 2015
  • Accepted Date:  01 April 2015
  • Published Online:  20 August 2015

Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints

  • 1. School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
Fund Project:  Project supported by the National Natural Science Foundation of China (Grant No. 51301030) and the Fundamental Research Funds for the Central Universities, China (Grant No. DUT14QY45).

Abstract: With the continuous miniaturization of electronic packaging, micro bumps for chip interconnects are smaller in size, and thus the reliability of interconnects becomes more and more sensitive to the formation and growth of intermetallic compounds (IMCs) at liquid-solid interface during soldering. Thermomigration (TM) is one of the simultaneous heat and mass transfer phenomena, and occurs in a mixture under certain external temperature gradient. In the process of interconnection, micro bumps usually undergo multiple reflows during which nonuniform temperature distribution may occur, resulting in TM of metal atoms. Since the interdiffusion of atoms between solders and under bump metallization (UBM) dominates the formation of interfacial IMCs, TM which enhances the directional diffusion of metal atoms and induces the redistribution of elements, will markedly influence the growth behaviors of interfacial IMCs and consequently the reliability of solder joints. The diffusivity of atoms in liquid solder is significantly larger than that in solid solder and in consequence a small temperature gradient may induce the mass migration of atoms. As a result, the growth of interfacial IMCs becomes more sensitive to temperature difference between solder joints in soldering process. So far, however, few studies have focused on liquid state TM in solder joints, and the growth kinetics of interfacial IMCs under TM during soldering is still unknown to us. In this study, Cu/Sn/Cu solder joints are used to investigate the migration behavior of Cu atoms and its effect on the growth kinetics of interfacial Cu6Sn5 under temperature gradients of 35.33℃/cm at 250℃ and 40.0℃/cm at 280℃, respectively. TM experiments are carried out by reflowing the Cu/Sn/Cu interconnects on a hot plate at 250℃ and 280℃ for different durations. For comparison, isothermal aging experiments are conducted in a high temperature chamber under the same temperatures and reaction durations. During isothermal aging, the growth of interfacial Cu6Sn5 follows a parabolic law and is controlled by bulk diffusion. Under the temperature gradient, asymmetrical growth of interfacial Cu6Sn5 is observed between cold and hot ends. At the cold end, the growth of the interfacial Cu6Sn5 is significantly enhanced and follows a linear law, indicating a reaction-controlled growth mechanism; while at the hot end, the growth of the interfacial Cu6Sn5 is inhibited and follows a parabolic law, indicating a diffusion-controlled growth mechanism. The dissolved Cu atoms from the Cu substrate at the hot end are driven to migration toward the cold end by temperature gradient, providing the Cu atomic flux for the fast growth of the interfacial Cu6Sn5 at the cold end. With the variation of the measured thickness of Cu6Sn5 IMC at the cold end and the simulated temperature gradients, the molar heat of transport Q^* of Cu atoms in molten Sn is calculated to +14.11 kJ/mol at 250℃ and +14.44 kJ/mol at 280℃. Accordingly, the driving forces of thermomigration in molten solder FL are estimated to be 1.62×10-19 N and 1.70×10-19 N, respectively.

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