Durability to Electromigration of an Annealing-Twinned Ag-4Pd Alloy Wire Under Current Stressing
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INTRODUCTION
THE current trend of minimizing interconnections has made electromigration an important failure mode of electronic products. In fact, electromigration has been known in the IC industry for several decades to cause open circuits to form in connectors and devices to wear out.[1] Recently, electromigration has also been observed in electronic packages,[2] such as flip chip solder bumps[3] and ball grid array (BGA) solder joints.[4] However, electromigration in wire bonding materials has been scarcely reported. Tse and Lach[5] investigated the failure of Al bonding wires in an octal buffer inverter device after a few years of field usage. The worn-out packaging module was characterized by broken and melted wires. The thinning and thickening of the bonding wires indicated that a mass redistribution of Al atoms had occurred in association with current stressing. In addition, a special ‘‘bamboo’’ structure was observed in the worn-out wire material. Although Schepper et al.[6] and Krabbenborg[7] reported that electromigration negligibly affected the contact failure of a package with Au wire bonding on Al pad, Orchard et al.[8] indicated that electromigration indeed accelerated the failure kinetics of this type of wire bonding TUNG-HAN CHUANG, HSIN-JUNG LIN, and CHIEN-HSUN CHUANG, are with the Institute of Materials Science and Engineering, National Taiwan University, 106 Taipei, Taiwan ROC. Contact e-mail: [email protected] CHIH-HSIN TSAI, is with the Institute of Materials Science and Engineering, National Taiwan University, also with the Wire Technology Co., LTD, 432 Taichung, Taiwan ROC. JUN-DER LEE and HSING-HUA TSAI, are with the Wire Technology Co., LTD. Manuscript submitted May 14, 2014. Article published online August 29, 2014 5574—VOLUME 45A, NOVEMBER 2014
joint. Zin et al.[9] further clarified the effect of electromigration on the failure of such an Au/Al wire bonding package. They found that electromigration caused greater damage than thermal aging did. In addition, the growth of brittle intermetallics at the Au/Al interface was more rapid when electrons flowed from Au wire to Al pad than when they flowed in the opposite direction. Gold is a well-known material for wire bonding technology in electronic packaging. However, intermetallic compounds grow rapidly in a gold wire bonding package with an Al pad, causing serious embrittlement at the interface and failure of the electronic devices during reliability tests. In addition, the high cost of gold further restricts the application of Au wire as the bonding material in IC and LED packages. In recent years, copper wire has attracted attention as a substitute for gold wire. Although Cu wire has the advantages of low cost, high tensile strength, and good electrical and thermal conductivity, the inherent oxidation and sulfurization problems of Cu cause great concerns about reliability in electronic products. Oxidation and sulfurization cannot be completely prevented, not even by coatings of noble metal films such as Pd, Au, or Pt on the surface of Cu wire.
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