Morphology, kinetics, and thermodynamics of solid state aging of eutectic SnPb and Pb-free solders (SnAg_3.5, SnAg_3.8Cu_0.7 and SnCu_0.7) on Cu

T. Y. Lee, W. J. Choi, and K. N. Tu

Department of Materials Science & Engineering, UCLA, Los Angeles, CA 90095-1595

J. W. Jang, S. M. Kuo, J. K. Lin and D. R. Frear

Interconnect System Laboratory, Motorola, Tempe, AZ 85284

K. Zeng, and J. K. Kivilahti

Laboratory of Electronics Production Technology, Helsinki University of Technology,

FIN-02015, TKK, Finland

Abstract

Intermetallic compound (IMC) growth during solid-state aging at 125 °C, 150 °C and 170 °C up to for 1500 hrs for four solder alloys (eutectic SnPb, SnAg_3.5, SnAg_3.8Cu_0.7 and SnCu_0.7) on Cu Under Bump Metallization (UBM) was investigated. The samples were reflowed before aging. Cu₆Sn₅ formed during the reflow of eutectic SnPb and SnAg_3.5 had a rounded scallop-type morphology, but in SnCu_0.7 and SnAg_3.8Cu_0.7 the scallops of Cu₆Sn₅ were faceted. In solid-state aging, all these scallops changed to a layered-type compound. In addition to the layered Cu₆Sn₅, Cu₃Sn also grew as a layer and was as thick as the Cu₆Sn₅. The activation energy of intermetallic growth in solid- state aging is 0.94 eV for eutectic SnPb, 1.03 eV for SnAg_3.5, 1.01 eV for SnAg_3.8Cu_0.7 and 1.05 eV for SnCu_0.7. The rate of intermetallic growth in solid-state aging is about four orders of magnitude slower than during reflow. Ternary phase diagrams of Sn-Ag-Cu are presented in discussing the reactions. These diagrams predict intermetallic formation in the wetting reaction during the reflow and the other intermetallics formed in solid-state aging. The morphological change and the large difference in growth rates between the two kinds of reactions cannot be predicted.

[Published in J. Mater. Res., 17, 291-301 (2002).]