Growth of Cu-Ni-Sn Alloys in Pb Free CuSnAg Solder/Au-Ni Metallization Reactions
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Growth of Cu-Ni-Sn Alloys in Pb Free CuSnAg Solder/Au-Ni Metallization Reactions Anis Zribi, Peter Borgesen,1 Lubov Zavalij, and Eric J. Cotts Physics Department, Binghamton University P.O. Box 6016; Binghamton, NY 13902 1 Universal Instruments Corp Binghamton, NY 13905 ABSTRACT Diffusion and phase formation were monitored in solder joints consisting of flip chips with Ni(V) under-bump metallizations bumped with Ag3.8Cu1.85Sn94.35 (atomic percentage) solder reflowed on printed circuit boards with Cu/Ni/Au metallizations. A ternary alloy, (CuNi)6Sn5 , was observed to form during reflow at solder/Ni interfaces in these Ag3.8Cu1.85Sn94.35/Au/Ni solder joints. After extended thermal aging at 150 oC, a second ternary compound, (CuNi)3Sn4 forms at the Ni/(CuNi)6Sn5 interface. The growth of these alloys depletes the solder of essentially all Cu, changing its mechanical properties and melting temperature. INTRODUCTION The movement away from solders containing Pb further increases the complexity of solder/metallization interactions in electronic packages. This paper concerns intermetallic alloy formation in Ag3.8Cu1.85Sn94.35/Au/Ni (all compositions are listed as atomic percentage or ratio) solder joints. The National Electronics Manufacturing Initiative (NEMI) has recommended a SnAg-Cu solder for Pb-free solder reflow in board assemblies [1], while the Soldering Technology Center at the International Tin Research Institute has recommended Sn-Ag-Cu solders as a replacement for Pb-Sn solders [2]. Ag-Cu-Sn solders have good mechanical properties and relatively low melting points [3-5]. However, the metal atoms added to Sn to lower the melting point and strengthen the solder can dramatically change the interaction between the solder and the metallization [6-10]. Since Cu, Ni and Au atoms all diffuse [11] at fast rates in Sn, all of these constituents may potentially combine with Sn and metallization atoms to form intermetallic alloys. A number of ternary alloys were observed to form in these solder joints, changing the kinetics of the intermetallic alloy formation, and affecting the final morphology and composition of the solder joints. EXPERIMENT AND RESULTS Diffusion and phase formation were monitored in solder joints (Figure 1) consisting of flip chips with Ni(V) under-bump metallizations bumped with Ag3.8Cu1.85Sn94.35 solder reflowed on printed circuit boards with Cu/Ni/Au metallizations. Solder joints were formed by annealing (reflow) in a nitrogen atmosphere (less than 50 ppm oxygen) with a peak temperature of 244 °C and a total time of 74 s above the liquidus. The assembled chips were then air-cooled and those to be annealed were sealed in glass tubes in an Ar gas atmosphere at a pressure of 10 psi. Thermal aging was conducted at a temperature of 150 °C for 4h, 16h, 32h, 64h and 261h.
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Figure 1 Backscattered electron image of a crosssectioned as prepared solder joint. The bottom interface is the solder/Ni interface while the top interface is the chip metallization/solder interface. The white precipitates within
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