A technique for impeding Cu out-diffusion in Cu alloy based pre-plated Pb frames was developed by electroplating a 3 to 4nm thick Sn layer on a Cu alloy base prior to electroplating a Ni layer. A 10 to 14nm thick epitaxy-like and dense (Cu,Ni)3Sn intermetallic-compound layer was automatically formed during diffuse reaction, which led to a drastic reduction in Cu out-diffusion and hence significantly improved the protection of the lead frames against oxidation and corrosion attack. The estimated Cu diffusion coefficient in the intermetallic-compound interlayer was about 1.6 x 10–22m2/s at 250C.

In situ Formation of Cu–Sn–Ni Intermetallic Nanolayer as a Diffusion Barrier in Pre-Plated Lead Frames. R.Fu, L.Liu, D.Liu, T.Y.Zhang: Applied Physics Letters, 2006, 89[13], 131911 (3pp)