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.