Effect of microstructural evolution on electrical property of the Sn-Ag-Cu solder balls joined with Sn-Zn-Bi paste
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–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously on Cu/Ni/Au metallized ball grid array (BGA) substrates. The correlation between microstructural evolution and the electrical resistance of the joints under various testing conditions of reflow cycles and heat treatment was investigated. The electrical resistance of the Sn–Ag–Cu joints without Sn–Zn–Bi was also conducted for comparison. The average resistance values of Sn–Ag–Cu and Sn–Ag–Cu/Sn–Zn–Bi samples changed, respectively, from 7.1 (single reflow) to 7.3 (10 cycles) m⍀ and from 7.2 (single reflow) to 7.6 (10 cycles) m⍀. Furthermore, the average resistance values of Sn–Ag–Cu and Sn–Ag–Cu/Sn–Zn–Bi samples changed, respectively, from 7.1 (aging 0 h) to 7.8 (aging 1000 h) m⍀ and from 7.2 (aging 0 h) to 7.9 (aging 1000 h) m⍀. It was also noticeable that the average resistance values of Sn–Ag–Cu/Sn–Zn–Bi samples were higher than those of Sn–Ag–Cu samples in each specified testing condition. The possible reasons for the greater resistance exhibited by the Sn–Zn–Bi incorporated joints were discussed.
I. INTRODUCTION
The ball grid array (BGA) technology has become a preferable interconnect technology for electronic packaging for the features of high input–output terminal densities, small footprints, and better electrical performances than some other conventional packages such as the pin through-hole or the quad flat package.1–7 As for the packaging materials, Sn–Pb (tin-lead) solder ball has been used to joint the BGA packages to printed circuit boards (PCBs) due to its low eutectic temperature (around 183 °C) and good wetting behavior on several substrate metallizations such as Cu, Ag, Pd, and Au.8–10 However, the elimination of toxic lead from electronic products is a global tendency actively driven by legislation. The European Union has set July 2006 as the deadline for inhibiting the tin-lead solder from electronic products.11 To replace the lead-containing solder, a large number of studies on lead-free solder for device and product interconnections are being conducted worldwide. Among the profitable lead-free solders are Sn–Ag based alloys, holding promise because of their good resistance to thermal fatigue, high ductility,12 and better solderability on copper than Sn–Pb solder.13 Investigations have
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0352 2854
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 10, Oct 2005 Downloaded: 16 Mar 2015
been conducted to modify the thermal and mechanical characteristics of eutectic Sn–3.5Ag solder with the addition of Bi, Cu, In, Sb, and Zn.14–19 A small addition of Cu to Sn–Ag eutectic solder (eutectic point 221 °C) not only decreases the melting point by four degrees Celsius (217 °C), but also enhances mechanical properties.13,20 Moreover, an addition of Bi to the ternary Sn–Ag–Cu solder can further lower the melting point of the corresponding ternary solders.7,13 In spite of the compositional modification, the melting points of t
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