Solid-state interfacial reactions of Sn and Sn–Ag–Cu solders with an electroless Co(P) layer deposited on a Cu substrate

• Published in: Journal of alloys and compounds 2016-03, Vol.662, p.475-483
• Main Authors: Wang, Chao-hong, Wen, Chun-chieh, Lin, Che-yang
• Format: Article
• Language: English
• Subjects: Copper; Diffusion barrier; Electroless deposition; Electroless plating; Interface reactions; Interfacial reactions; Lead-free solders; Nanocrystals; Phase transformations; Solders; Substrates; Tin; Tin base alloys
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2015.12.060

In this study, an electroless Co(P) layer with 5 wt.% P was deposited as a diffusion barrier to prevent the rapid reaction of Cu with Pb-free solders. The electroless Co(P) layer exhibited a mixed amorphous/nanocrystalline structure. Solid-state interfacial reactions of a 6-μm-thick Co(P) layer deposited on a Cu substrate with pure Sn and Sn–3.5Ag–0.7Cu (wt.%) solders were examined at temperatures of 160, 180, and 200 °C. For reactions with Sn, the primary reaction product was identified as a metastable CoSn4 phase. In addition, a thinner uniform layer of Co–Sn–P phase with a nanocrystalline structure was formed at the CoSn4/Co(P) interface, which was likely a mixed phase of CoSn3, Co2P, and Sn4P3. Notably, the growth of the CoSn4 layer was linearly proportional to the aging time, implying that it was reaction-limited in the initial stage. Moreover, the growth kinetics of the Sn/Co(P)/Cu interfacial reactions were systematically studied. The growth rate constants and activation energy were determined. Furthermore, the reaction phases and microstructure in the reactions with the Sn–3.5Ag–0.7Cu solder were clearly different from those with Sn. The thin (Cu,Co)6Sn5 layer was first formed at the interface and the CoSn3 growth was considerably suppressed. After a specific aging period, the dominant phase changed to a CoSn3 phase with a fast growth rate. [Display omitted] •Electroless Co-5 wt.%P layer had a mixed amorphous/nanocrystalline structure.•Metastable CoSn4 phase was formed at the Sn/Co(P)/Cu interface.•CoSn3 phase was formed in the reaction of Co(P) with Sn–3.5Ag–0.7Cu solder.•The growth kinetics of the Sn/Co(P)/Cu reactions were systematically studied.