Lead-free solders with high tin content and high melting temperature limit the reliability of electroless Ni/immersion Au finish on Cu as diffusion barrier. Autocatalytic Co–P was proposed as a barrier metallization for Cu in lead-free soldering. Autocatalytic deposition of Co was carried out in hypophosphite-containing electrolytes at 95C and pH 8. Autocatalytic Co–P/Au finish with about 4wt% P content strongly limits interdiffusion and intermetallic compounds formation with respect to the Ni–P/Au finish with Sn–Pb and Sn–Ag–Cu solder alloys. Contact angle of Sn–Pb solder alloy with Ni–P/Au and Co–P/Au layers was comparable, while in the case of Sn–Ag–Cu alloy contact angle was much lower for Co–P/Au than for Ni–P/Au layers. Mechanical strength of lead-free joints for Ni–P/Au and Co–P/Au finishes was evaluated with shear test on BGA coupons, obtaining higher joint strength values for autocatalytic Co.

Electroless Co–P for Diffusion Barrier in Pb-Free Soldering. L.Magagnin, V.Sirtori, S.Seregni, A.Origo, P.L.Cavallotti: Electrochimica Acta, 2005, 50[23], 4621-5