1157 The Growth of Intermetallic Compounds at Sn-Ag-Cu Solder/Cu and Sn-Ag-Cu Solder/Ni Interfaces and the Associated Evolution of the Solder Microstructure Journal of ELECTRONIC MATERIALS, Vol. 30, No. 6, 2001 Special Issue Paper 1157 INTRODUCTION Among the Pb-free solders, the Sn-Cu-Ag eutectic Sn 95.8 Ag 3.5 Cu 0.7 (Sn 94.9 Ag 3.8 Cu 1.3 in atomic percentages) has been recommended for general-purpose use as a substitute for Pb-Sn eutectic solder. 1 The mechanical properties of this alloy, in bulk geometries as well as in solder joints, are therefore of current interest to package designers. A full investigation of the me- chanical properties should include characterization of the microstructure, as the properties may depend upon this. The microstructure is observed to depend upon the preparation of the sample, including both reflow and subsequent thermal history. The present study examines the evolution of the microstructure of Sn-Ag-Cu samples during reflow, and subsequent (Received February 13, 2001; accepted May 15, 2001) The Growth of Intermetallic Compounds at Sn-Ag-Cu Solder/Cu and Sn-Ag-Cu Solder/Ni Interfaces and the Associated Evolution of the Solder Microstructure A. ZRIBI, 1 A. CLARK, 1 L. ZAVALIJ, 1 P. BORGESEN 2 , and E.J. COTTS 1 1.—State University of New York at Binghamton, Department of Physics and Astronomy. 2.—Universal Instruments Corp, Binghamton NY; ecotts@binghamton.edu The evolution of intermetallics at and near SnAgCu/Cu and SnAgCu/Ni inter- faces was examined, and compared to the behavior near PbSn/metal and Sn/ metal interfaces. Two different solder compositions were considered, Sn 93.6 Ag 4.7 Cu 1.7 and Sn 95.5 Ag 3.5 Cu 1.0 (Sn 91.8 Ag 5.1 Cu 3.1 and Sn 94.35 Ag 3.8 Cu 1.85 in atomic percent). In both cases, phase formation and growth at interfaces with Cu were very similar to those commonly observed for eutectic SnPb solder. However, the evolution of intermetallics at SnAgCu/Ni interfaces proved much more complex. The presence of the Cu in the solder dramatically altered the phase selectivity at the solder/Ni interface and affected the growth kinetics of intermetallics. As long as sufficient Cu was available, it would combine with Ni and Sn to form (Cu,Ni) 6 Sn 5 , which grew instead of the Ni 3 Sn 4 usually observed in PbSn/Ni and Sn/Ni diffusion couples. This growing phase would, however, eventually con- sume essentially all of the available Cu in the solder. Because the mechanical properties of Sn-Ag-Cu alloys depend upon the Cu content, this consumption can be expected to alter the mechanical properties of these Pb-free solder joints. After depletion of the Cu from the solder, further annealing then gradually trans- formed the (Cu,Ni) 6 Sn 5 phase into a (Ni,Cu) 3 Sn 4 phase. Key words: Pb-free solder, atomic diffusion, SnAgCu, AuNiSn, microstructure evolution, intermetallic growth kinetics, intermetallic growth mechanisms heat treatment below the liquidus temperature. The growth of intermetallics and the development of the microstructure in Pb-Sn solder joints has been investigated in considerable detail. 2–7 During reflow of Pb-Sn solder on Cu surfaces, Cu 6 Sn 5 was observed to grow first. Subsequent heat treatment eventually led to the growth of the Cu 3 Sn phase as well. Reflow and subsequent heat treatment of Pb-Sn solder/Ni joints led to the formation and growth of Ni 3 Sn 4 . 8–10 While Cu 6 Sn 5 grew at a faster rate than Ni 3 Sn 4 , both of these phases are the fastest growing in their respec- tive systems. In fact, kinetics, rather than energetics, dominate phase selection in these solder/metal sys- tems. The resulting mechanical properties and micro- structure of the solder joints have been carefully characterized. 11 A number of studies of the Sn-Ag-Cu system and of Sn-Ag-Cu solders have been conducted. 12–14 The melt-