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Curriculum of Coated Silver Bonding Wire
Keywords: Coated silver wire, bondability and reliability, wire bond characterization
Wire bonding of copper, palladium coated copper and alloyed silver are in production, where, reactive forming gas (5% hydrogen and 95% nitrogen) is purged during formation of free air ball (FAB). At present, coated silver wire with alloyed core is developed to form FAB under atmosphere similar to gold wire that demonstrates good bondability and pass the basic reliability tests. The basic research conducted and discussed are on two aspects: coating design and alloy design of the core. The test wires are primarily evaluated for bonding (FAB formation, 1st and 2nd bond bondability) and pass minimum reliability performances. Coated silver wire revealed potential benefits such as: (a) Formation of axi-symmetrical spherical FAB under atmosphere with good spread of coated element along the periphery of FAB (b) For both ball and stitch bond, the process window of coated silver wire is similar to gold wire and performs better than copper wire (c) Easy to achieve good intermetallic coverage (IMC) of about 80 to 85% at the interface, distributed at the center and periphery of the bonded area (d) Satisfactory behavior is observed for low loop, high loop and SSB bonding (e) Passed uHAST (110C, 85%RH, 1008h) and HTS (175C, 2000h) tests when bonded to leaded device and molded with green mold compound (f) Slower rate of growth of Ag dendrite is observed for coated Ag compared to alloyed Ag wire (g) Mean time between assist (MTBA) of the coated silver wire meets the minimum industrial requirement of 7h bonding without any stoppage (h) Dynamics of the formation of intermetallic phases at bond interface are characterized using electron diffraction pattern analysis
Sarangapani Murali, Senior Principal R&D Engineer
Heraeus Materials Singapore Pte Ltd
Singapore, Singapore

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