Electroless Co–P for diffusion barrier in Pb-free soldering

Lead-free solders with high tin content and high melting temperature limit the reliability of electroless nickel/immersion gold finish on copper as diffusion barrier. Autocatalytic cobalt–phosphorus is proposed as a barrier metallization for copper in lead-free soldering. Autocatalytic deposition of cobalt is carried out in hypophosphite containing electrolytes at 95 °C and pH 8. Autocatalytic Co–P/Au finish with about 4 wt% 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 is comparable, while in the case of Sn–Ag–Cu alloy contact angle is 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 is evaluated with shear test on BGA coupons, obtaining higher joint strength values for autocatalytic cobalt.