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.
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.
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