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Abstract. The C36 Laves phase Nb 1Àx Co 2þx has been reinvestigated in order to study phase stability and structure. Constitutional data have been obtained by investigating homogenized single-and two-phase samples and from diffusion couples. The C36 phase Nb 1Àx Co 2þx crystallizes with hexagonal MgNi 2 structure type (Z ¼ 8, space group P6 3 /mmc, a ¼ 4.7414(4) A and c ¼ 15.458(1) A at x ¼ 0.265(4)). The homogeneity range extends from 24.6(2) to 25.3(5) at% Nb. The temperature range of the phase field is limited by a eutectoid (C36 Nb 1Àx Co 2þx ¼ Nb 2 Co 7 + C15 NbCo 2 ) and a peritectic (L þ C15 NbCo 2 ¼ C36 Nb 1Àx Co 2þx ) reaction at %1050 C and 1264 C, respectively. In addition, the title phase is involved in the peritectoid reaction Co(Nb) þ C36 Nb 1Àx Co 2þx ¼ Nb 2 Co 7 at 1086 C and in the eutectic reaction L ¼ Co(Nb) + C36 Nb 1Àx Co 2þx at 1239 C. The C36 and C15 Laves phases of the Nb--Co system are separated by an extremely small two-phase field (<0.5 at%). The crystal structure exhibits pronounced deviations from ideal parameters obtained from a hard sphere model. The Co network displays a type of distortion known from many hexagonal Laves phases. Kagomé layers display an elongation of the Co--Co edges of the basal triangles of Co 5 trigonal bipyramids and a contraction of Co--Co edges of the uncapped triangles. The Nb atoms are also displaced from their idealized sites. In the crystal structure of C36 Nb 1Àx Co 2þx excess Co atoms randomly substitute Nb atoms --(Nb 1Àx Co x )Co 2 . The excess Co atoms occupy preferentially the Nb2 site approximately twice as much as Nb1. These positions differ mainly in the conformation of the corresponding Nb 6 Nb 2 fragments (Nb1--Nb1 eclipsed and Nb2-Nb2 staggered). In addition, Co atoms are displaced from the original Nb positions. The distortion of the Co and the Nb network is a consequence of the bonding situation of the defect-free crystal structure. The preferential site occupation of excess Co atoms is triggered by interactions with atoms beyond the first coordination shell. The C36 phase Nb 1Àx Co 2þx exhibits Pauli-paramagnetic behavior (c P ¼ þ1.31 Á 10 À3 emu mol À1 ). The temperature dependent part of the electrical resistivity r(300 K) -r 0 is only 17 mW cm whereas the residual resistivity is very large with r 0 ¼ 140 mW cm indicating strong structural disorder.

Recently, the research and development activities for replacing Pb-containing solders with Pb-free solders have been intensified due to both competitive market pressures and environmental issues. As a result of these activities, a few promising candidate solder alloys have been identified, mainly, Sn-based alloys. A key issue affecting the integrity and reliability of solder joints is the interfacial reactions between a molten solder and surface finishes in the solder joint structures. In this paper, a fundamental study of the interfacial reactions between several Pb-free candidate solders and surface finishes commonly used in printed-circuit cards is reported. The Pb-free solders investigated include Sn-3.5 Ag, Sn-3.8 Ag-0.7 Cu, and Sn-3.5 Ag-3.0 Bi. The surface finishes investigated include Cu, Au/Ni(P), Au/Pd/Ni(P), and Au/Ni (electroplated). The reaction kinetics of the dissolution of surface finishes and intermetallic compound growth have been measured as a function of reflow temperature and time. The intermetallic compounds formed during reflow reactions have been identified by SEM with energy dispersive x-ray spectroscopy.Index Terms-Dissolution, electroless Ni(P), interfacial reactions, intermetallics, morphology, Pb-free solders, Sn-3.5Ag, Sn-3.5Ag-3Bi, Sn-3.8Ag-0.7Cu.