Intermetallic formation induced substrate dissolution in electroless Ni(P)-solder interconnections
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Yi-Shao Lai and Ying-Ta Chiu Central Labs, Advanced Semiconductor Engineering, Inc., Nantze Export Processing Zone, 811 Nantze, Kaohsiung, Taiwan, Republic of China (Received 15 February 2008; accepted 25 June 2008)
The effect of minor transition metal (TM) additives of Ni, Co, or Zn on the interfacial reactions of the solder joints between Sn–Ag–Cu (SAC) solder and the Cu/Ni(P)/Au substrate was investigated, especially subsequent to multi-reflowing. (Cu,Ni)6Sn5 formed at the interface of all the joints except that of SAC–Ni, of which the interfacial compound was (Ni,Cu)6Sn5. The interfacial compounds of the SAC–Co and SAC–Zn contained a small amount of alloying elements of less than 3 at.%. Two P-rich layers, Ni3P and Ni–Sn–P emerged at the interface of the SAC joints. Nanoindentation analysis indicates that the hardness and Young’s modulus of these two phases were slightly higher than those of the Ni(P) substrate, which were in turn much greater than those of the Cu–Ni–Sn compounds. Worthy of notice is that with TM additions, the Ni–Sn–P phase between Ni3P and interfacial compounds was absent even after 10 reflows. For the SAC–Co joints, the growth of Ni-containing intermetallic compounds within the solder gave rise to the excess Ni dissolution, which caused a discrete Ni3P layer and over-consumed Ni(P) substrate underneath the grooves in-between (Cu, Ni)6Sn5 scallop grains at the interface. This phenomenon is presented for the first time, and the mechanism is proposed in this study.
I. INTRODUCTION
Mechanical and electronic interconnections in an electronic device can be achieved by solder joints. The move toward using lead-free solders for interconnections in the microelectronics industry is now an international trend because of the health and environmental concerns involved with lead usage. Among lead-free solder systems being developed, Sn–Ag–Cu alloys (SAC) are considered the new standard. However, some shortcomings still must be overcome, for example, a somewhat higher melting temperature,1 hot tearing due to coarse proeutectic Sn dendrites,2 large Ag3Sn plate and thus induced strain localization,3 and excess growth of interfacial intermetallic compounds (IMCs).4 Additions of transition metals (TM) have been proposed for modifying the microstructure, thermal behavior, and interfaces of Sn–Ag–Cu joints.5–12 With respect to interfacial reactions, it has been reported that for the joints of Ni-doped Sn–Ag sol-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0318 J. Mater. Res., Vol. 23, No. 9, Sep 2008
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ders (0.07 wt% Ni) and Cu, interfacial IMC was irregular (Cu,Ni)6Sn5 rather than scalloped Cu6Sn5 and did not show an observable change in thickness after long time aging.5 When co-doped with 0.2 wt% Co and 0.1 wt% Ni, the reaction product at Sn–3.5Ag/Cu interface after soldering was Cu6Sn5, in which Co and Ni were detected.6 Also, the growth of Cu6Sn5 during aging, as well as the Cu3Sn phase, was r
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