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This study investigated the microstructure and properties of Bi-Ag alloys, a novel Pb-free solder for hightemperature applications. The results show that Bi-Ag alloys exhibited a nonequilibrium solidification feature and a considerably large undercooling. An increase in the Ag content effectively raised the tensile strength. However, the elongation in a decreasing order was hypereutectic Bi-11Ag, pure Bi, and then eutectic Bi-2.5Ag. The fracture mode was significantly affected by microstructural characteristics and the strain rate.Bismuth, rhombohedral in structure with both covalent and metallic bonds, is less commonly used as structural material due to its fragility and poor malleability. In the electronics industry, bismuth is usually applied as an alloying element for solder materials, for example, Sn-Bi, Sn-Ag-Bi, Sn-Zn-Bi, Sn-In-Bi, and SnPb-Bi. The addition of Bi can reduce the melting temperature and improve wetting properties. [1,2] It has been reported that Bi containing solders have the problem of embrittlement under a high strain rate. [3,4] That is, the ductility is drastically degraded when the deformation rate exceeds a critical value. In addition, lift off of Bi bearing surface-finished components from the Cu land of a printed wiring board may occur probably due to Bi segregation near the interface. [5,6] Recently, a new alloy system, Bi-Ag, has been considered as the replacement for high Pb solders for high-temperature applications (e.g., 95Pb-5Sn, with a melting range from 308°C to 312°C). [7][8][9][10] One of the basic requirements a high-temperature Pb-free solder should satisfy is the melting range. That is, the solidus should be higher than 260°C; thus, the solder is capable of surviving secondary reflow at 250°C, and the liquidus needs to be lower than 400°C, due to the limitation of the glass transition temperature of the polymeric substrate. Bi-Ag eutectic alloy exhibits an acceptable melting point (the eutectic temperature is 262.5°C at the eutectic composition, Bi-2.5 wt pct Ag), similar hardness to that of Pb-5Sn, and affordable cost. Thus, it has been developed into die attach solders for power devices and light-emitting diodes (LEDs). However, this solder alloy system still has some demerits of inferior thermal and electrical conductivity as well as poor workability. It has been demonstrated that raising the Ag content of Bi-Ag to 11 wt pct promotes an increase in thermal conductivity. [7] Our recent report [11] revealed that the electrical resistivity of the Bi-11 wt pct Ag alloy is 86.5 lW-cm, which is much lower than that of the Bi-2.5 wt pct Ag eutectic specimen, 116.5 lW-cm.In view of the processing parameters, the melting and solidification behavior of solders should be taken into consideration for alloy development. In addition, mechanical properties play an import role in the solder alloy and strongly affect the manufacturability and reliability. Due to a lack of relevant information, this study investigated thermal and tensile behavior of this Bi-based alloy, in particular t...

This study investigated microstructure, thermal behavior, and mechanical properties of Sn-3.3Ag-0.5Cu alloys (SAC) with the addition of transition metals (TM, Ni, Co, and Zn). Results show that alloying with TM elements was able to reduce the degree of undercooling and strengthen SAC alloys. Among these elements, only Zn can raise the ductility. CoSn and Cu-Ni-Sn intermetallics appeared, respectively, in the Co-containing and Ni-containing samples while coarse Sn dendrites and a large area of eutectic phases could be observed in the specimens with Zn. These microstructural changes led to an inferior vibration fracture resistance under resonant vibration with a constant pull force.

The present study investigated interfacial reactions between Cu substrates and Bi-Ag alloys during soldering. Without forming intermetallic compounds (IMCs), the molten solder grooved and further penetrated along the grain boundaries (GBs) of the Cu substrate. An increase in Ag content enhanced GB grooving, raised the dissolution rate and also the amount of dissolved Cu in molten Bi. A stoichiometric Cu-Bi phase formed isothermally in liquid solders and considerably affected the Cu dissolution kinetics. The results also show that Bi-Ag/Cu joints possessed a better shear strength than the Pb-Sn/Cu, which implies that mechanical bonding by grain-boundary grooves was strong enough to withstand shear deformation.