Fracture of SnBi/Ni(P) interfaces
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Fracture of SnBi/Ni(P) interfaces P.L. Liu and J.K. Shanga) Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 25 May 2004; accepted 4 November 2004)
Fracture resistance of the interface between electroless Ni(P) and the eutectic SnBi solder alloy was examined in the as-reflowed and aged conditions, to investigate the potential role of Ni in inhibiting interfacial segregation of Bi in SnBi–Cu interconnect. In the as-reflowed condition, the fracture resistance of the SnBi/Ni(P) interface was about the same as that of the SnBi/Cu interface. Upon aging at 120 °C for 7 days the fracture resistance of the SnBi/Ni(P) interface was much higher than that of the SnBi/Cu interface. Such a difference was shown to result from the difference in fracture mechanism as the crack remained along the solder–intermetallic interface in the aged SnBi–Ni interconnect but propagated along the intermetallic–substrate interface in the aged SnBi–Cu interconnect. While fracture of the intermetallic– substrate interface in SnBi–Cu interconnect was due to Bi segregation onto that interface, no Bi was detected at the intermetallic-substrate interface in SnBi–Ni interconnects, implying that Ni(P) was effective in inhibiting the interfacial segregation of Bi.
I. INTRODUCTION
Bismuth-containing solder alloys have emerged as one group of the leading Pb-free candidates for potential replacement of Pb-containing solder alloys in interconnects of surface mount microelectronic assemblies.1–4 Among them, the eutectic and near-eutectic Sn–Bi and Sn–Ag– Bi alloys are particularly attractive because of their relatively low melting temperatures and their narrow freezing range. Despite their lower melting temperatures, some of the Bi-containing solder alloys, such as the eutectic Sn–Bi alloy, have shown comparable or even superior strength and creep-resistance, compared to the eutectic Sn–Pb alloy.5–7 To make a reliable solder interconnect, Bi–Sn alloys must form a strong bond with device metallizations. Since Cu serves as one of the most common metallizations, the interfacial bond between Sn–Bi and Cu has been the subject of numerous studies.2–4,8,9 While the bond between Sn–Bi and Cu is considered strong after the reflow, recent work on isothermally aged Sn–Bi/Cu interconnects has exposed a fatal limitation of Sn–Bi/Cu system, namely severe interfacial embrittlement induced by segregation of Bi to the Cu/intermetallic interface.10,11 The condition for the segregation was not too
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0115 818
J. Mater. Res., Vol. 20, No. 4, Apr 2005
far from the thermal condition in manufacturing and service. The segregation was confined to a monolayer or so along the interface so that even a minute amount of Bi present in a solder alloy either as unwanted impurity or a desired alloying element could present a serious reliability problem. Therefore, it becomes necessary to find a solution to inhibit or prev
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