Reaction Diffusion in the Au-Cu-Pb-Sn System at Low Temperature
The microstructure of a Cu 3 Au v. Pb-5wt%Sn diffusion couple was examined in the as-dipped and annealed conditions. The as-dipped couple showed no reaction between the solder and the base metal. The couple developed intermediate layers of two intermetallic phases upon annealing for 289 hours at 169°C. These phases were identified as phi and eta’ which are contained in the Au-Cu-Sn ternary system. No phases containing significant amounts of Pb formed, indicating that even at low concentrations in a Pb-Sn solder, Sn is the active chemical specie in the solder for the Au-Cu-Pb-Sn system. The microhardness of the eta’ phase was found to be equal to that of Cu 3 Sn, which forms in the absence of Au in the Cu base metal. However, the the phi phase was harder, and therefore the presence of Au in the base metal may adversely affect the thermal fatigue resistance of the solder joint.
Solder Connections with a Ni Barrier
Copper-tin intermetallic compounds readily form when tin-bearing solder connections are made to copper surfaces. These compounds continue to grow during the life of solder connections and represent potentially weak interfaces. An investigation was undertaken to determine if a diffusion barrier between the solder and copper would inhibit Cu-Sn intermetallic compound formation and thereby extend joint life. Hybrid integrated circuit solder connections formed between external clip-on terminals and Ti-Pd-Cu-Ni-Au (TPCNA) thin-film terminations were investigated. The terminals had 60wt%Sn-40wt%Pb solder clad on the circuit-gripping surfaces of phosphor-bronze clips. The diffusion barrier consisted of an electroplated Ni layer on the phosphorbronze underneath the solder. The mechanical strength of solder connections formed using terminals with and without a Ni barrier were compared by measuring joint strength both initially and after temperature cycling and accelerated temperature aging. Joint metallographic cross sections and fracture interfaces after pull testing were analyzed to determine how effectively the Ni layer isolated Sn in the solder from Cu in the phosphor-bronze. SEM analysis confirmed that 1-um-thick Ni provides an effective diffusion barrier. Consequently, strength of 60n40Pb solder connections formed between phosphor-bronze clip-on terminals and TPCNA thin-film terminations were markedly increased, particularly at elevated temperatures up to 150°C.
Comparative studies of α-Lactalbumin and lysozyme: Echidna lysozyme
Summaryα-Lactalbumin is a modifier protein for galactosyl transferase in the biosynthesis of lactose and lysozyme catalyses the cleavage of β(1–4) glycosidic linkages. It is now generally believed that α-lactalbumin and lysozyme have diverged from a common ancestor. Comparative studies of these proteins from several species are reported here.The presence of lactose and lactose synthetase activity is shown in mature milk samples from a variety of placental mammals (eutheria) and a pouch bearing mammal (marsupialia). The isolation of the α-lactalbumins involved and their properties are compared.The position is different in the milk samples of monotremes. The α-lactalbumin, lactose and lactose synthetase activity levels in the two platypus samples studied are very low. The levels of lactose and lactose synthetase activity in some echidna(Tachyglossus aculeatus) samples are low, but appreciable in others. No α-lactalbumin is present. The lysozyme activity is appreciable, some samples containing a different lysozyme from that present in others. One of these (designated echidna lysozyme I) is capable of lactose synthetase modifier activity in the presence of bovine “A protein”.The properties of this lysozyme are compared with those of lysozymes and α-lactalbumins from a variety of species and their significance is discussed.