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Growth of Intermetallics at Coated Silver Wire Bond Interfaces
Keywords: bonding wire, ball and stitch bonding, bond interface characterization
At present, there is a high demand for a bonding wire alternate to gold and bond similar to it as well without purging inert or reactive gas during free air ball (FAB) formation. Currently, coated silver (C-Ag) 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: (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 bonds, the process window of C-Ag 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) Mean time between assist (MTBA) of the C-Ag wire meets the minimum industrial requirement of 7h bonding without any stoppage (f) Passed uHAST (110C, 85%RH, 1000h) and HTS (175C, 2000h) tests when bonded to leaded device and molded with green mold compound (<15ppm Cl, <10ppm S, pH 5 to 7, with ion-catcher) (g) Slower rate of growth of Ag dendrite is observed for coated Ag wire than alloyed Ag wire. The present paper discusses on the characterization of FAB and metallurgical reaction of bonded ball at the interface during thermal ageing at 130C and 175C. The growth and type of binary and/or ternary intermetallic phases (silver aluminide, gold aluminide, gold-silver aluminide) at the interface are to be identified using electron diffraction pattern analysis and Hanawalt search method. The rate of diffusion of aluminum (Al) from bond pad in to Ag bonded ball and vice-versa to be investigated and reported. Interface of Al and C-Ag wedge bonds on nickel (Ni) plated surfaces are also to be examined using electron diffraction pattern analysis for time zero bonding and thermal ageing conditions. The rate of diffusion of nickel from bond pad in to Ag bonded ball and vice-versa to be studied and discussed.
Sarangapani Murali, Senior Principal R&D Engineer
Heraeus Materials Singapore Pte Ltd
Singapore, Singapore

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