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Sn57Bi1Ag alloy system meets the basic solderability requirements while reliability of solder is less known. The presence of intermetallics that form at the solder-joint interface and in the bulk solder is the primary reason to cause joint failure on simple loading. Furthermore, dissolution of soldered surface (copper, gold, silver, nickel, etc.) into the solder during reflow alters the composition of the interface leading to the formation of intermetallic phases. The present paper addresses the characterization of low temperature Sn57Bi1Ag solder alloy, its joint interface metallurgy and integrity. Generally, formation of intermetallic phases (Cu3Sn, Cu6Sn5, Ag3Sn, AuSn2, Ni3Sn4, etc.) at the solder-pad interface is in the range of 0.3 to 1.0µm. On thermal ageing at 75°C (0.55Tm), the intermetallic phases grow further up to 2.3µm, particularly CuSn and AuSn phases. In addition, to distributed eutectic tin (Sn) and bismuth (Bi) phases, fine eutectic tin and bismuth phases and primary tin (β-Sn) are also observed. Coarsening of Bi and Sn phases is noticed on thermal ageing (75°C and 100°C). The rate of coarsening increases with the supply of thermal energy promoting Sn/Bi atomic diffusion and movement of eutectic boundaries (grain boundaries). However, the Sn-Bi interface is stable without any significant changes in eutectic structures and its morphologies. The mechanical drop test of the Sn57Bi1Ag solder joint passed 120 drops, characteristic life corresponds to the number of drops for a failure of 63.2%. Evaluated as per JESD22-B111 specification to a maximum peak acceleration of 1500G and half-sine shock pulse duration of 0.5 milliseconds, soldered the alloy to copper OSP surface. The solder joint also passed 1200cycles evaluated at -40 to +125°C. Electrical connectivity of the solder joints soldered to ENIG plated test boards is recorded continuously. Crystallographic orientation of Sn and Bi second phases is observed using electron back scattered diffraction (EBSD) of thermal aged solder joints and are also presented in this paper.

Developments in bonding wire focus on coated silver wire for stacked devices in memory sectors revealing near equivalent performances to 4N Au wire. In addition to stacked device applications, the wire is also examined for other conventional applications. The innovative wire exhibits excellent performance on biased Highly Accelerated Stress Test (bHAST) passing 192h and 504h at 130°C, 85%RH for +3.3V and +20V, respectively. Thus, the wire satisfied one of the important criteria required to pass automotive reliability test (2X stress test, AEC Q100 Rev-H, with limited test samples). The test is conducted using 0.8mil coated silver wire and molded with green epoxy molding compound. Another benefit of the wire is stitch bond bondability with high MTBA of greater than 2h.

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Formation of free air ball (FAB) within the range of certain processing parameters leads to excellent bondability (distributed intermetallic coverage (IMC) at the center and periphery of the bonded ball interface, bonded ball concentricity, etc.) on bonding to aluminum (Al) bond pad and Au/Ag plated surfaces. The wire revealed zero stoppage and 0.8M bond touchdowns when tested under laboratory conditions. Thermal aging of bonded interface showed absence of Kirkendall voids until 192h at 250°C where Al diffuses into Ag bonded ball up to a depth of 5.3µm, consuming the complete Al bond pad and thus transforming into silver-aluminide containing 5 to 12wt% of Al and rest silver (Ag). Traces of coated gold (Au) and alloyed palladium (Pd) are noticed at the bond interface.