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

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-mu m-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 degrees 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) Sn-6(5) 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. (C) 2015 Elsevier B.V. All rights reserved.