Solder Connections with a Ni Barrier

Copper-tin intermetallic compounds readily form when tin-bearing solder connections are made to copper surfaces. These compounds continue to grow during the life of solder connections and represent potentially weak interfaces. An investigation was undertaken to determine if a diffusion barrier between the solder and copper would inhibit Cu-Sn intermetallic compound formation and thereby extend joint life. Hybrid integrated circuit solder connections formed between external clip-on terminals and Ti-Pd-Cu-Ni-Au (TPCNA) thin-film terminations were investigated. The terminals had 60wt%Sn-40wt%Pb solder clad on the circuit-gripping surfaces of phosphor-bronze clips. The diffusion barrier consisted of an electroplated Ni layer on the phosphorbronze underneath the solder. The mechanical strength of solder connections formed using terminals with and without a Ni barrier were compared by measuring joint strength both initially and after temperature cycling and accelerated temperature aging. Joint metallographic cross sections and fracture interfaces after pull testing were analyzed to determine how effectively the Ni layer isolated Sn in the solder from Cu in the phosphor-bronze. SEM analysis confirmed that 1-um-thick Ni provides an effective diffusion barrier. Consequently, strength of 60n40Pb solder connections formed between phosphor-bronze clip-on terminals and TPCNA thin-film terminations were markedly increased, particularly at elevated temperatures up to 150°C.