Mechanism of the Electromigration in Ag-Pd Alloy Bonding Wires
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INTRODUCTION
THE current trend of minimizing interconnections in advanced electronic devices has led to electromigration in the solder joints of flip chip assembly[1,2] and ball grid array (BGA) packages.[3] It has been reported that not only do hillocks and voids form in the solder matrix, but also Pb-rich and Sn-rich phases separate. Although wire bonding, developed by Bell Labs in 1957, was used for interconnections in IC chips with substrates prior to the industrial application of flip chip and BGA products, less research has been conducted on electromigration in the wire materials of this traditional package than on electromigration in flip chip and BGA solder joints.[4] Tse and Lach analyzed the cause of failure of a 25.4 lm Al wire-bonded octal buffer inverter package after field exposure for a few years and found that was characterized by broken and melted wires.[5] The thinning and thickening of the wire material indicated that a mass redistribution of aluminum had occurred due to the electromigration mechanism. In addition, a ‘‘bamboo’’ structure was observed in the worn-out Al wire. For a Au wire-bonded package with an Al pad, Orchard et al.[6] also reported that electromigration accelerated the failure kinetics of this type of wire bonding joint. In contrast, Schepper et al.[7] and Krabbenborg[8] found that electromigration negligibly affected the contact failure. Furthermore, Zin et al. observed that, in a Au wire-bonded package, the intermetallic compounds grew
TUNG-HAN CHUANG and CHUN-HAO CHEN are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan ROC. Contact e-mail: [email protected] Manuscript submitted June 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
more rapidly when electrons flowed from the Au wire to the Al pad than when they flowed in the opposite direction.[9] They also reported that the electromigration-induced failure in this package was more severe than that caused by the thermal aging effect. Although Au wire has been used in the electronics industry for the interconnection of packages for more than three decades, the embrittlement of Au wire/Al pad interfaces is a concern due to the overgrowth of AuxAly intermetallic compounds.[10] Moreover, the increasing material cost of Au wire has created pressure on packaging companies. The application of Cu wire as a substitute for Au wire provides certain advantages, such as low material cost, excellent electrical conductivity, and high mechanical strength. However, the low intermetallic growth at as-bonded Cu/Al interfaces present the risk of insufficient adhesion between Cu wire and the Al pads.[11] In addition, the hardness of copper can cause chip cratering during the wire bonding process.[12] An innovative Ag-8Au-3Pd alloy bonding wire with a high percentage of annealing twinned grains has been developed and popularly applied in recent years in the IC and LED packaging industries.[13] It has been shown that such an annealing twinned Ag-alloy wire exhibits excellent thermal stability and
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