The effect of liquid-solid electromigration (EM) on the interfacial reaction in Ni/Sn-9Zn/Ni interconnects was investigated under a current density of 5×103 A/cm2 at 230 ℃. A reverse polarity effect was revealed, i.e., the interfacial intermetallic compounds (IMC) at the cathode grew continuously and was remarkably thicker than those at the anode. This results from the directional migration of Zn atoms from the anode toward the cathode, which is induced by the positive effective charge number (Z*) of Zn atoms but not the back-stress. A thin Ni5Zn21 layer formed at each interface after soldering. The initial Ni5Zn21 interfacial IMC gradually transformed into [Ni5Zn21+(Ni, Zn)3Sn4] after liquid-solid interfacial reaction for 8 h, due to the local equilibrium at the interface changed with decreasing of Zn atoms content. The interfacial IMCs at both anode and cathode were identified as Ni5Zn21, and no IMC transformation occurred undergoing liquid-solid EM, because the Zn atoms content at the cathode was enough under electron current stressing, and the diffusion of Zn atoms toward anode was inhibited. The reverse proving was proposed to explain the positive value Z* of Zn atoms. The abnormal directional migration of Zn atoms toward the cathode prevented the dissolution of cathode substrate, which is beneficial to improving the EM reliability of micro-bump solder interconnects.
Fig.1 Schematic of the line-type Ni/Sn-9Zn/Ni solder interconnect
Fig.2 SEM images of microstructures of the Ni/Sn-9Zn/Ni solder interconnect after immersion soldering
Fig.3 SEM images of the Ni/Sn-9Zn (a, c, e) and Sn-9Zn/Ni (b, d, f) interconnects after reaction at 230 ℃ (without electromigration (EM)) for 1 h (a, b), 4 h (c, d) and 8 h (e, f)
Fig.4 SEM images of the Ni/Sn-9Zn (a, c, e) and Sn-9Zn/Ni (b, d, f) interconnects undergoing liquid-solid (L-S) EM under 5.0×103 A/cm2 at 230 ℃ for 1 h (a, b), 4 h (c, d) and 8 h (e, f)
Fig.5 Thicknesses of interfacial IMCs at both anode and cathode in Ni/Sn-9Zn/Ni interconnects as a function of L-S EM time (t)
Fig.6 SEM images of the whole Ni/Sn-9Zn/Ni interconnects undergoing L-S EM under 5.0 × 103 A/cm2 at 230 ℃ for 1 h (a), 4 h (b) and 8 h (c)
Fig.7 Schematic of Zn fluxes in Ni/Sn- 9Zn/Ni interconnectundergoing L-S EM ( Jchem and Jem are the Zn atomic fluxes induced by chemical potential gradient and EM, respectively)
[1]
Hsiao H Y, Liu C M, Lin H W, Liu T C, Lu C L, Huang Y S, Chen C, Tu K N. Science, 2012; 336: 1007
[2]
Huang M L, Zhou S M, Chen L D. J Electron Mater, 2012; 41: 730
[3]
Chen C, Tong H M, Tu K N. Annu Rev Mater Res, 2010; 40: 531
[4]
Huang M L, Ye S, Zhao N. J Mater Res, 2012; 26: 3009
[5]
Blech I A, Meieran E S. Appl Phys Lett, 1967; 11: 263
[6]
Chen S W, Chen C M, Liu W C. J Electron Mater, 1998; 27: 1193
[7]
Chen C M, Chen S W. J Electron Mater, 1999; 28: 902.
[8]
Gan H, Tu K N. In: Scott N ed., Proceedings IEEE 52nd Electronic Components and Technology Conference (ECTC 2002), San Diego: IEEE, 2002: 1206
[9]
Zhang X F, Guo J D, Shang J K. Scr Mater, 2007; 57: 513
[10]
Zhang X F, Guo J D, Shang J K. J Mater Res, 2008; 23: 3370
[11]
Huang M L, Zhou Q, Zhao N, Liu X Y. J Mater Sci, 2014; 49: 1755
[12]
Huang M L, Zhou Q, Zhao N, Zhang Z J. J Electron Mater, 2013; 42: 2975
[13]
Huang M L, Zhang Z J, Zhao N, Zhou Q. Scr Mater, 2013; 68: 853
[14]
Huang M L, Zhang Z J, Zhao N, Yang F. J Alloys Compd, 2014; doi: 10.1016/j.jallcom.2014.08.263
[15]
Liou W K, Yen Y W, Jao C C. J Electron Mater, 2009; 38: 2222
[16]
Wang C H, Chen H H. J Electron Mater, 2010; 39: 2375
[17]
Huntington H B, Grone A R. J Phys Chem Solids, 1961; 20: 76
[18]
Ho P S, Kwok T. Rep Prog Phys, 1989; 52: 301
[19]
Blech I A. J Appl Phys, 1976; 47: 1203
[20]
Tu K N. Phys Rev, 1992; 45B: 1409
[21]
Smolin M D, Frantsevich I N. Sov Phys-Sol, 1962; 3: 1536
