Nanoindentation study of Zn-based Pb free solders used in fine pitch interconnect applications

Abstract Because of their hazardous nature, conventional Sn–Pb solders are slowly being replaced by Pb free solders. Sn–Zn solder alloys have been identified as a potential alternative because of their low melting point close to that of Sn–Pb solder. However, the high Sn content and reflow temperature will accelerate intermetallic compound (IMC) formation at the solder/substrate interface, which in turn controls the reliability of the solders joints. A detailed study of the growth morphology of IMCs in solder joints will enhance our ability to control their influence on the mechanical and physical properties. Knowledge of the mechanical properties of individual phases makes predictions of the mechanical behavior of an overall joint more straightforward. The small length scale of intermetallic phase formation may lead to a behavior different from bulk samples. One would like to measure the mechanical properties of the phases in interesting realistic geometries. However, even low-load microhardness testing results in indentations that span multiple phases within a real joint. In the present work, the micromechanical properties of Pb-free solder joints such as Sn–8 wt.% Zn–3 wt.% Bi/Au/Ni/Cu were probed using nanoindentation. Nanoindentation is a reliable technique in testing small volumes of materials and complex microstructures manifested in the fine pitch interconnects employed in the flip-chip technologies. The IMCs formed between Zn and Au, and Ni, Zn were characterized and their hardness, elastic modulus were investigated in the present work. The IMCs are found to be ɛ-AuZn 8 ; δ-NiZn 8 compounds and formation of these compounds are found to depend on reflow temperature and time. It is also found that the hardness and modulus of the δ-NiZn 8 is higher than that of the ɛ-AuZn 8 compound.

[1]  Anne Lohrli Chapman and Hall , 1985 .

[2]  Haoran Ma,et al.  Microstructural evolution of Sn–9Zn–3Bi solder/Cu joint during long-term aging at 170 °C , 2004 .

[3]  Nikhilesh Chawla,et al.  Deformation behavior of (Cu, Ag)–Sn intermetallics by nanoindentation , 2004 .

[4]  King-Ning Tu,et al.  Ductile-to-brittle transition in Sn–Zn solder joints measured by impact test , 2004 .

[5]  J. Glazer Metallurgy of low temperature Pb-free solders for electronic assembly , 1995 .

[6]  Stephen A. Langer,et al.  OOF: an image-based finite-element analysis of material microstructures , 2001, Comput. Sci. Eng..

[7]  Nikhilesh Chawla,et al.  Young's modulus of (Cu, Ag)-Sn intermetallics measured by nanoindentation , 2004 .

[8]  K. Tu,et al.  Six cases of reliability study of Pb-free solder joints in electronic packaging technology , 2002 .

[9]  A. Tay,et al.  A study on microstructural and mechanical properties of nanocrystalline nickel , 2005 .

[10]  K. S. Kim,et al.  The formation and growth of intermetallic compounds and shear strength at Sn-Zn solder/Au-Ni-Cu interfaces , 2005, Microelectron. Reliab..

[11]  V. L. Dybkov,et al.  The homogeneity ranges of the delta (δ) and gamma (γ) phases of the Ni-Zn binary system grown by the reaction couple method , 1998 .

[12]  Subra Suresh,et al.  Some critical experiments on the strain-rate sensitivity of nanocrystalline nickel , 2003 .

[13]  M. Abtew,et al.  Lead-free Solders in Microelectronics , 2000 .

[14]  Jun-Mo Yang,et al.  Analysis on interfacial reactions between Sn–Zn solders and the Au/Ni electrolytic-plated Cu pad , 2004 .

[15]  G. Pharr,et al.  An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments , 1992 .

[16]  D. B. Knorr,et al.  The effect of aging on microstructure, room temperature deformation, and fracture of Sn-Bi/Cu solder joints , 1994 .

[17]  Richard P. Vinci,et al.  Nanoindentation measurements on Cu–Sn and Ag–Sn intermetallics formed in Pb-free solder joints , 2003 .

[18]  J. Glazer Microstructure and mechanical properties of Pb-free solder alloys for low-cost electronic assembly: A review , 1994 .

[19]  K. Chawla,et al.  Microstructure-based simulation of thermomechanical behavior of composite materials by object-oriented finite element analysis , 2002 .

[20]  K. Chawla,et al.  Mechanical Behavior of Materials , 1998 .

[21]  A. J. Rafanelli,et al.  Solders and soldering , 1964 .

[22]  Jyh-Wei Lee,et al.  The nanoindentation characteristics of Cu6Sn5, Cu3Sn, and Ni3Sn4 intermetallic compounds in the solder bump , 2004 .